|
Digital Camera Patent Abstract
A digital camera comprises an image sensor for capturing an image,
a lens arrangement arranged to focus light onto the image sensor
and providing a variable focus, and a memory for storing images
captured by the image sensor. Focusing is achieved by a series of
images having differing focus provided by the lens arrangement being
captured by the image sensor and stored in the memory. Analysis
of the images stored in the memory to determine the quality of the
focus of the images is used to derive an in-focus image from the
series of images. This avoids the complication of employing autofocusing
control of the lens arrangement. Movement of the lens arrangement
may be driven by movement of a button operable by a user which avoids
the need for an actuator for the lens arrangement.
Digital Camera Patent Claims
1. A digital camera comprising: an image sensor for capturing an
image; a lens arrangement arranged to focus light onto the image
sensor and providing a variable focus; a memory for storing images
captured by the image sensor; and a controller arranged to control
the operation of the digital camera, the controller being arranged
to perform an image capture operation comprising: causing a series
of images, each consisting of the entire image area and having differing
focus provided by the lens arrangement, to be captured by the image
sensor and stored in the memory; and analyzing the images stored
in the memory to determine the quality of the focus of the images
and on the basis of the analysis, selecting one of the series of
images determined to have the best focus as an in-focus image: and
in respect of the in-focus image performing either one or both of:
(a) displaying the in-focus image on a display of the digital camera;
and (b) retaining the infocus image in the memory in a manner allowing
the user subsequently to retrieve the in-focus image from the memory.
2. A digital camera according to claim 1, wherein the lens arrangement
is movable to vary the focus.
3. A digital camera according to claim 2, wherein the digital camera
further comprises: a button operable by a user; and a mechanical
linkage connecting the button to the lens arrangement and adapted
to move the lens arrangement on operation of the button, the controller
being arranged to perform said image capture operation in response
to operation of the button with the series of images being captured
as the lens arrangement is moved on operation of the button.
4. A digital camera according to claim 3, wherein the linkage mechanism
is arranged to moved the lens arrangement from its rest position
by depression of the button and further comprises: a resilient element
arranged to bias the lens arrangement back towards its rest position
after depression of the button; and a damper arranged to control
the speed of movement of the lens arrangement back towards its rest
position, the controller being arranged to perform said image capture
operation with the series of images being captured as the lens arrangement
is moved back towards its rest position after depression of the
button.
5. A digital camera according to claim 2, wherein the digital camera
further comprises an actuator arranged to move the lens arrangement,
and the image capture operation further comprises controlling the
actuator to move the lens arrangement to vary the focus, said capture
of the series of images being performed as the actuator is thus
moved.
6. A digital camera according to claim 5, wherein the actuator
is a piezoelectric actuator or an electric motor.
8. A digital camera according to claim 6, wherein the quality of
the focus of the images is determined on the basis of an area of
analysis which is a partial area of the entire image area.
15. A digital camera according to claim 1, wherein said step of
said image capture operation which said controller is arranged to
perform of analyzing the images stored in the memory to determine
the quality of the focus of the images and, on the basis of the
analysis, selecting one of the series of images determined to have
the best focus as an in-focus image is performed after all the series
of images have been stored in the memory.
16. A digital camera according to claim 1, wherein said step of
said image capture operation which said controller is arranged to
perform of analyzing the images stored in the memory to determine
the quality of the focus of the images and, on the basis of the
analysis, selecting one of the series of images determined to have
the best focus as an in-focus image is performed as successive images
of the series are captured by initially storing the first image
of the series as said in-focus image and in respect of each successive
image in the series analysing the image to determine the quality
of the focus of the image in comparison with the image stored as
said in-focus image and on the basis of the analysis updating the
image stored as said in-focus image.
19. A focus method for a digital camera having an image sensor
for capturing an image, a lens arrangement arranged to focus light
onto the image sensor and having a variable focus, and a memory
for storing images captured by the image sensor, the autofocus method
comprising: capturing a series of images on the image sensor, each
captured image consisting of the entire image area, and storing
the captured images in the memory; and analysing the images stored
in the memory to determine the quality of the focus of the images
and, on the basis of the analysis, selecting one of the series of
images determined to have the best focus as an in-focus image and
in respect of the in-focus image performing either one or both of:
(a) displaying the in-focus image on a display of the digital camera;
and (b) retaining the in-focus image in the memory in a manner allowing
the user subsequently to retrieve the in-focus image from the memory.
Digital Camera Patent Description
[0001] This invention relates to digital cameras, for example miniature
cameras for use in portable electronic equipment such as a mobile
telephone, a Personal Digital Assistant (PDA), a portable computer,
or a digital camera per se. Such digital cameras have an image sensor
which captures images and a lens arrangement which focuses light
onto the image sensor.
[0002] The invention is particularly concerned with focusing of
a digital camera in which the lens arrangement has a variable focus,
typically by the lens arrangement being movable.
[0003] Many digital cameras are furnished with an autofocus facility.
In general the autofocus algorithm may be closed-loop or open-loop.
Typically, in known closed-loop autofocus algorithms an actuator
moves the lens arrangement and a series of sample images are captured
at positions of the lens arrangement providing differing focus.
The sample images usually cover only a small area of the picture,
typically the centre. The sample images are then analysed to compare
the quality of the focus of the sample images to determine which
of the positions of the lens arrangement provides the best focus.
The actuator is then used to move the lens to that position so that
a focussed photograph can be taken. Typically, in known closed-loop
autofocus algorithms sample images are repeatedly captured and analysed
to determine the quality of the focus, this being used to derive
a feedback signal which controls an actuator to move the lens arrangement
to optimise the focus
[0004] Such autofocus algorithms, whether closed-loop or open-loop,
require an actuator to move the lens arrangement. The actuator is
necessarily a precision device of some complexity, typically an
electromechanical actuator such as an electromagnetic motor, for
example a stepper motor, or a piezoelectric actuator. For example
in the case of open-loop control, the actuator must allow precise
control to return to the position determined to provide the best
focus. Such precision motors and actuators are relatively costly
to manufacture. In addition, the actuator adds significant bulk
and mass to the camera, which is undesirable in portable devices
such as mobile phones. Further, actuators draw power during operation,
using up battery life.
[0005] It would be desirable to reduce these problems arising from
the need to provide an actuator capable of precise and repeatable
control.
[0006] In accordance with a first aspect of the present invention,
there is provided a digital camera comprising:
[0007] an image sensor for capturing an image;
[0008] a lens arrangement arranged to focus light onto the image
sensor and providing a variable focus;
[0009] a memory for storing images captured by the image sensor;
and
[0010] a controller arranged to control the operation of the digital
camera, the controller being arranged to perform an image capture
operation comprising:
[0011] causing a series of images, each consisting of the entire
image area and having differing focus provided by the lens arrangement
to be captured by the image sensor and stored in the memory; and
[0012] analyzing the images stored in the memory to determine the
quality of the focus of the images and on the basis of the analysis
deriving an in-focus image from the series of images.
[0013] In accordance with a second aspect of the present invention,
there is provided a focusing method for a digital camera having
an image sensor for capturing an image, a lens arrangement arranged
to focus light onto the image sensor and having a variable focus,
and a memory for storing images captured by the image sensor, the
autofocus method comprising:
[0014] capturing a series of images, each consisting of the entire
image area, and storing them in the memory; and
[0015] analysing the images stored in the memory to determine the
quality of the focus of the images and on the basis of the analysis
deriving an in-focus image from the series of images.
[0016] Thus the focus of the lens arrangement is varied and images
are captured with differing focus. The captured images are not the
sample images comprising part of the entire image area, as in some
prior art techniques summarised above, but consist of the entire
image area required by the user. Analysis of the images is then
carried out to determine the quality of the focus. On the basis
of the analysis, an in-focus image is then derived for use as the
photographic shot, for example by being displayed on a display of
the camera and/or stored in the memory of the camera. In the simplest
application of the invention, the in-focus image is derived by selecting
one of the images of the series determined to have the best focus,
but in more complex applications, the in-focus image is synthesized
from the series of images, as described in more detail below.
[0017] The advantage of the invention is that less precise control
of the lens arrangement is needed. For example, in contrast to open-loop
autofocus technique summarised above, the lens arrangement does
not need to be physically returned to the best in-focus position
to take the photographic shot, as the appropriate image is derived
from the series of images available in storage. An actuator capable
of accurate or reproducible positioning is therefore not required.
In one type of embodiment described further below no actuator is
necessary at all which is a significant advantage. Even if an actuator
is used, there is an important advantage that is not necessary to
provide the same degree of precise accurate control as with the
known autofocus techniques. This can reduce some or all of the complexity,
cost and bulk of the actuator used.
[0018] Another advantage of the invention is that the time required
to obtain a focussed image is reduced as compared to the open-loop
autofocus algorithm described above as there is no need to perform
the final step of returning the lens arrangement to the position
of best focus before capturing the output image.
[0019] In the case that an actuator is employed to move the lens
arrangement, the digital camera further comprises an actuator arranged
to move the lens arrangement, and the image capture operation further
comprises controlling the actuator to move the lens arrangement
to vary the focus, said capture of the series of images being performed
as the actuator is thus moved.
[0020] The invention may be applied to a piezoelectric actuator.
Piezoelectric actuators provide many advantages, notably small size
and low power consumption. However, many piezoelectric actuators
suffer from hysteresis which makes the position of the lens arrangement
unpredictable from the control signal and hence renders it difficult
to apply an open-loop autofocus algorithm requiring return to a
position previously determined to provide the best focus. However
the present invention provides an in-focus image without the need
for such return to a previously identified position. This allows
use of a piezoelectric actuator with the associated advantages.
[0021] The invention may be applied to an actuator in the form
of an electrical motor. In this case, instead of requiring a precision
stepper motor as commonly used in cameras providing autofocus, it
is possible to use a simpler and cheaper motor such as a DC motor
as precise control or knowledge of the position is not needed.
[0022] In the type of embodiment in which no actuator is necessary,
the digital camera further comprises:
[0023] a button operable by a user; and
[0024] a mechanical linkage connecting the button to the lens arrangement
and adapted to move the lens arrangement on operation of the button,
the controller being arranged to perform said image capture operation
in response to operation of the button with the series of images
being captured as the lens arrangement is moved on operation of
the button.
[0025] Thus, the movement of the lens arrangement is driven mechanically
through the mechanical linkage by operation of the button. That
is, the motive force for movement of the lens arrangement originates
from the operation of the button and hence from the user. Hereinafter
the button will be referred to as the "shutter button"
or "shutter release button" to refer to the button the
user operates to capture an image. It is noted that in general in
digital cameras there is no mechanical "shutter" and this
terminology does not imply the presence of any shutter but is simply
derived from previous functionality of film cameras.
[0026] One option is that when the user depresses the button, a
simple mechanical linkage causes the lens to move the requisite
distance. This may be a direct connection of a simple mechanism
such as a lever to change the direction of the applied force or
the gearing. In a miniature camera, the lens diameter is a few millimetres
and the corresponding mass of the lens assembly a few grams or less
so the required force is hardly noticable to the user. Typically,
the lens needs to move about 0.2 mm to cover the range of possible
focus positions. Thus a direct connection is possible. If the operator
depresses the button further, say by 1-2 mm, a simple lever mechanism
or other geared mechanism suffices to effect movement. The mechanical
linkage may be of any suitable form. Preferably it comprises one
or a few components formed as plastic mouldings.
[0027] Another option is that the linkage mechanism is arranged
to move the lens arrangement from its rest position by depression
of the button and further comprises: a resilient element (most simply
a compression spring) arranged to bias the lens arrangement back
towards its rest position after depression of the button; and a
damper arranged to control the speed of movement of the lens arrangement
back towards its rest position, the controller being arranged to
perform said image capture operation with the series of images being
captured as the lens arrangement is moved back towards its rest
position after depression of the button. Thus, the action of depressing
the button stresses the resilient element which then causes the
lens arrangement to move back towards its rest position under the
control of the damper which controls the movement in a predetermined
manner. Such a damper could be implemented with a classic "dash-pot"
using a viscous liquid, or more preferably could be a lossy/mechanically
resistive plastic material. The resilient element and the damper
could be fabricated from one and the same plastic moulding (possibly
multi-shot) by suitable choice of geometry and material combination.
[0028] This provision of automatic return mechanism removes operator
dependency from the lens dynamics during the picture-capture sequence.
Lens travel is therefore known and repeatable so that timings of
image capture (lens position) can be accurately pre-selected.
[0029] An optional feature is to provide an optical sensor to sense
dark and light marks on the lens barrel assembly, such optical marks
representing positions (or transition points between positions)
of various focus positions at which it is desired to capture the
sequence of images. The signals from the optical sensor then may
be used to trigger the image capture process, independently of any
reliance on actuator motion, accuracy or repeatability, or of lens
velocity during the sequence.
[0030] The present invention may be used in any size of digital
camera, but advantageously the digital camera is a miniature one,
that is, one in which the lens diameter is a few millimetres, say
in the range 2 mm to 20 mm. At this small size, the mechanical load
on the linkage is slight, as the mass of the lens elements is small
(a few grams or less) so that depression of the button by the user
is straightforward, that is, depression of the button does not meet
with great resistance and can be engineered to have a good `feel`
to the user.
[0031] As to the number of images in the series, increasing the
number improves the approximation to perfect focus. For some applications,
two or three focus positions suffice to provide one image approximately
in focus. For best focus when used with high resolution image sensors,
say 3 megapixel or more, better results are obtained when more lens
positions are used, say 10 or more. In practice, capturing images
at 5 to 7 lens positions generally provides one image which is adequately
focussed.
[0032] In a first type of embodiment, the series of images are
all stored for subsequent analysis and determination of an in-focus
image. In this case the memory requirement is relatively high. Typical
memory requirements are of the order of 3.times. megabytes for an
image at .times. megapixel resolution. Thus, for example, a single
frame of a 3 megapixel camera requires of the order of 9 megabytes
of storage space. However, alternative formats and compressions
are available which reduce the memory required to the order of 1-2
megabytes for a 3 megapixel camera. Thus sufficient temporary memory
must be provided to allow storage of the number of images in the
series. After the analysis, the determined in-focus image is available
for display and further storage, while the remaining images in the
series can be erased, freeing up the memory, or can be simply overwritten
when the memory is next required.
[0033] In a second type of embodiment, the images are analysed
in real time by:
[0034] initially storing the first image of the series as said
in-focus image and
[0035] in respect of each successive image in the series analysing
the image to determine the quality of the focus of the image in
comparison with the image stored as said in-focus image and on the
basis of the analysis updating the image stored as said in-focus
image.
[0036] This second type of embodiment requires less memory than
the first type of embodiment, since the most images required to
be stored at any one time is two, ie the most recently captured
image in the series and the in-focus image being updated, rather
than the total number in the series. On the other hand, the second
type of embodiment needs a sufficiently high processing speed, or
a low rate of capture of the series of images, in the sense that
one image must be fully analysed before the start of the readout
of the next from the image sensor into memory. Typical frame rates
in digital cameras are 30 per second, in which case the time available
for image comparison is of the order of 33 ms.
[0037] There are several ways to derive the in-focus image from
the series of images.
[0038] One option for deriving the in-focus image is to select
one of the images having the best focus. The analysis of the quality
of focus may be performed on the basis of an area of analysis which
is a partial area of the entire images, for example a central area,
or on the basis of the entire image area.
[0039] Another option for deriving the in-focus image is to synthesise
the in-focus image from the series of images, for example as a composite
of more than one of the images of the series. This may be achieved
by determining the quality of the focus of the images in each of
a plurality of parts of the image and selecting, in respect of each
of said plurality of parts of the image area, the part of the image
area determined to have the best focus from one of the series of
images. Thus different parts of the in-focus image may originate
from different images captured at different focus positions, allowing
all areas of the picture to appear in focus. This can increase the
apparent depth-of-field of the camera. In this embodiment, the selections
are made on a part-by-part basis. In general, the quality of the
focus of the images may be determined in each of a plurality of
parts of the image on the basis of an area of analysis which is
any of (a) a partial area of the part of the image area, (b) the
entire area of each part of the image area, or (c) the entire area
of that part of the image area and an adjacent area.
[0040] The parts of the image area may be regions of a plurality
of pixels. In this case, it is possible to select the part of the
image area from one of the series of images determined to have the
best focus in that part of the image area. For best effect, the
size of the regions needs to be relatively small and the number
of lens positions relatively large. Simulations indicate that for
a 3 megapixel sensor, a high quality picture can be obtained with
between 9 and 25 regions of roughly equal area and between 3 and
10 lens positions. The regions may have any shape and arrangement.
The boundaries of the regions may be chosen to be "ragged"
rather than straight lines. Also the regions may usefully have a
dominantly hexagonal perimeter rather than rectangular. Both these
features make the region boundaries far less noticeable to the human
eye.
[0041] Alternatively, the parts of the image may each comprise
a single pixel. In this case, the quality of the focus of the images
is determined for each pixel on the basis of an area of analysis
consisting of the pixel and an adjacent area of the image. The great
advantage of this scheme over the previously described process,
is that there are no artificially introduced boundaries between
different parts of the final composite in-focus image, across which
boundaries significant focus error might be visible. Instead, this
process effectively makes every pixel a region in its own so the
resultant composite will have no region boundaries visible whatsoever.
[0042] To allow better understanding, an embodiment of the present
invention will now be described by way of non-limitative example
with reference to the accompanying drawings, in which:
[0043] FIG. 1 is a front view of a mobile telephone including a
camera;
[0044] FIG. 2 is a perspective, rear view of the lens arrangement
of the camera;
[0045] FIG. 3 is a cross-sectional view of the arrangement of the
optical components of the camera, the cross-section being taken
along the line AA' in FIG. 2;
[0046] FIG. 4 is a diagram of the electronic components of the
camera;
[0047] FIG. 5 is a flow chart of the analysis performed by the
camera to determine the quality of the focus of an image;
[0048] FIG. 6 is a flow chart of a first image capture operation
of the camera;
[0049] FIG. 7 is a schematic view of a series of images captured
by the camera at successive positions of the lens arrangement;
[0050] FIG. 8 is a side view of a modified form of linkage mechanism
for the shutter release button of the camera;
[0051] FIG. 9 is a flow chart of a first image capture operation
of the camera;
[0052] FIG. 10 is a schematic view of an example of the images
processed by the first image capture operation of FIG. 9;
[0053] FIG. 11 is a schematic view of another example of images
processed by the first image capture operation of FIG. 9;
[0054] FIG. 12 is a diagram of the camera in an alternative form
employing an actuator.
[0055] FIG. 1 shows a mobile phone 1 in which a camera 5 in accordance
with the present invention is provided. The mobile phone 1 has on
its front surface a keypad 2 and a display screen 3, as well as
a shutter release button 4 of the camera 5.
[0056] As best seen in FIG. 2, the camera 5 has a housing 7 in
which is mounted a lens assembly 6 arranged towards the rear of
the mobile phone 1 to receive light from the exterior of the mobile
phone 1. As shown in FIG. 2, the lens assembly 6 comprises a fixed
lens 9 and a movable lens 10. The lens assembly 6 is arranged in
front of an image sensor 11 to focus the received light onto the
image sensor 11. The lens assembly 6 is movable, in particular by
movement of the movable lens 10 to vary the focus of the light on
the image sensor 11. For clarity, the fixed and movable lenses 9
and 10 are depicted as simple lenses, whereas in reality they are
generally formed by lens groups.
[0057] As shown in dotted outline in FIG. 2 and in detail in FIG.
3, the camera 5 has a mechanical linkage 8 connecting the shutter
release button 4 to the lens assembly 6, in particular to the movable
lens 10. In this case the mechanical linkage 8 is a simple rod.
On depression of the shutter release button 4 by the user, the button
4 moves to the position shown by dotted lines 4a, the mechanical
linkage 8 moves together with the button and drives the movable
lens 10 to move to the position indicated by dotted lines 10a, thereby
varying the focus of light on the image sensor 11.
[0058] In addition, the camera 5 has electrical components of the
camera 5 as shown in FIG. 4 and arranged as follows.
[0059] The image sensor 11 is connected to supply the output image
signal of captured images through a signal processor 12 to a memory
13. As discussed further below, in operation images consisting of
the entire image area are stored in the memory 13. The operation
of the image sensor 11, the signal processor 12 and the memory 13,
as well as other components of the camera 5 are controlled by a
controller 14. The controller 14 is also responsive to operation
of the shutter release button 4. The controller 14 is typically
implemented by a microprocessor running an appropriate program.
Alternatively some or all of the functions of the controller 14,
for example the analysis of the captured images to determine the
focus quality as described below, may be implemented by dedicated
hardware.
[0060] The controller 14 analyses the quality of the focus of images
stored in the memory 13 using an algorithm shown in FIG. 5. In step
S1, an area of analysis of the image is selected. This area of analysis
may be the entire image area or may be a partial area of the entire
area, for example a central portion or a plurality of portions of
the entire area.
[0061] In step S2, the selected area is filtered by a high-pass
filter. The high-pass filter is used on the basis that the high
spatial frequency components increase with better focus, so the
output of the high-pass filter is representative of the focus quality.
The high-pass filter is designed accordingly. The following can
be said about the requirements for this filter:
[0062] The DC coefficient must be zero as the DC signal never conveys
useful focus information
[0063] Very high frequencies are likely to be dominated by pixel
noise (if this can be proved by analysis of the circle of confusion
of a particular system, that would be very helpful information).
These frequencies should also be attenuated.
[0064] Intermediate frequencies will contain the useful focus information
[0065] The transition bands between these zones should not be too
abrupt, otherwise they could act as a threshold, and prevent the
algorithm working under some circumstances.
[0066] Designing frequency domain filters from spatial prototypes
is one way to get satisfactory results. Knowing what convolution
operation is needed in the spatial domain, this can be transformed
into a frequency domain multiplication.
[0067] One possible high-pass filter is the Laplacian of a Gaussian
filter.
[0068] The high-pass filter may be implemented in the frequency
domain. One possibility is to perform a discrete cosine transform,
eg on 8.times.8 pixel blocks. Then the measure of focus quality
might be derived by multiplying the spatial frequency components
by the frequency domain filter coefficients.
[0069] In step S3 the absolute values of the output of step S2
are taken and in step S4 the absolute values are summed. As an alternative
to taking the absolute value in step S3, the power could be calculated,
but the absolute value calculation is computationally cheaper than
a power calculation and is nearly as useful.
[0070] Thus the output of step S4 gives a measure of the quality
of the image focus. This algorithm shown in FIG. 5 produces quite
satisfactory results and compares well in simulation with other
methods (some frequency based, some spatial based). However it will
be appreciated that other algorithms for determining focus quality
could alternatively be applied.
[0071] A first image capture operation performed by the controller
is shown in FIG. 6 and will now be described.
[0072] In step S10, depression of the button 4 is detected. In
response to this, the operation proceeds to step S11 in which the
controller 14 causes a series of images to be captured by the image
sensor 11 and stored in the memory 13. Each stored image consists
of the entire image area. This may correspond to the entire area
of the image sensor 11, but in some cases it may be that some of
the peripheral pixels of the image sensor 11 are discarded. These
images are stored at predetermined times after initial depression
of the button 4 so that each stored image is an image captured at
a different position of the lens arrangement 6 and having a different
focus.
[0073] This is shown for example in FIG. 7 which shows a schematic
cross-section of the part of the camera 5 housing the lens assembly
6. The movable lens 10 is supported in a lens holder 15 which may
be a barrel, both of which are circularly symmetric. The lens holder
15 is attached to the mechanical linkage 8 capable of moving the
lens holder 15 in a direction parallel to the optic axis (horizontal
in the drawing). The lens holder 15, movable lens 10 and mechanical
linkage 8, together with other components such as suspension, fixed
lenses and image sensor (not shown) are housed in the housing 7.
During the depression of the button 4, the mechanical linkage 8
moves the lens holder 15 and thereby the movable lens 10 to the
positions shown by dotted lines and denoted 8a, 15a and 10a, as
indicated by the horizontal arrows. During the movement of the lens
assembly 6, full images are captured and stored in the memory 13
at several positions of the movable lens 10, indicated by the fine
vertical lines labelled 1-6, position 1 corresponding to near focus
and position 6 to far focus. In this example, 6 lens positions are
used but fewer or more lens positions could be used. A full image
is captured at position 1 at the start of travel and position 6
at the end of travel and at four intermediate positions, 2-5. The
six images captured by the image sensor during lens travel are indicated
schematically in the lower part of FIG. 7.
[0074] Although the number of images in the series is shown as
being six in FIG. 7, in general it may be any plural number.
[0075] In step S12 of FIG. 6 which is performed after all the images
have been stored in the memory 13, the focus quality of each image
is determined using the algorithm shown in FIG. 5. Then in step
S13, the image having the best focus quality is selected as the
in-focus image. This in-focus image is displayed on the display
screen 3 and retained in the memory 13.
[0076] As will be apparent to those skilled in the art, the mechanical
linkage 8 may in general be readily adapted to connect a shutter
release button 4 and a lens assembly 6 whatever their positions
in the phone, and further, may be designed to produce the desired
extent and speed profile of movement of the lens assembly 6. For
example, the linkage mechanism 8 may incorporate a spring and damper
system, arranged so that no matter how fast the button 4 is depressed,
the movement of the lens arrangement 6 is essentially controlled
by the spring stiffness and damper resistance. Return of the lens
assembly 6 to its starting position may be readily incorporated,
for example using a return spring.
[0077] Similarly, the capture and storage of the series of images
may occur during the return of the lens assembly 6 to its original
position instead of during the depression of the button 4. In this
case, the movement of the lens assembly 4 is still driven by the
operation of the button 4 by the user, but there is the advantage
that the movement of the lens assembly 6 may be better controlled
as it is less dependent on the action of the user. All such designs
are included in the scope of the invention.
[0078] A modified form of the linkage mechanism 8 which facilitates
the capture and storage of the series of images during the return
of the lens assembly 6 to its original position is shown in FIG.
8. In FIG. 8, two opposing walls 21 and 22 of the housing of the
mobile phone 1 are shown, these walls being nominally fixed and
the linkage mechanism being arranged therebetween. The shutter release
button 4 protrudes through one of the walls 21 and connects via
stiff linkage 23 to an over-travel-disconnect mechanism 24, which
in turn connects via a stiff linkage 25 to one end of a spring 26.
The other end of the spring 26 reacts with the wall 22 of the housing
of the mobile phone 1. The spring 26 may be replaced by any resilient
element. The linkage 25 also connects to a damping mechanism 27
(e.g. a dashpot, or other viscous-characteristic damping device)
which also reacts with wall 22 of the housing of the mobile phone
1. Lastly, the linkage 25 connects mechanically with the movable
lens 10 of the lens assembly 6, this being the primary object to
be moved by the linkage mechanism 8.
[0079] Over-travel-disconnect mechanism 24 acts in such a way as
to transmit any compressive force applied to the shutter release
button 4, until such time as a certain depression (to the right
in FIG. 8) is reached. After that the shutter release button 4 is
effectively disconnected until such time as the linkage 25 (under
reverse drive from compressed spring 26) has returned to its rest
position, as shown in FIG. 8 and as limited for example by the wall
21. Any suitable conventionally known mechanism will suffice here.
[0080] Operation of the linkage mechanism 8 is as follows. Initially
the spring 7 is largely uncompressed and shutter release button
4 is in its rest position (to the left in FIG. 8). The user depresses
the shutter release button 4 (to the right in FIG. 8), the user's
compressive force being transmitted to linkage 25 via the over-travel-disconnect
mechanism 24. This causes the linkage 25 to follow the movement
of shutter release button 4, in so doing compressing spring 26 and
depressing damper 27, and driving the movable lens 10 to an extreme
position. When shutter release button 4 gets close to its end of
travel, the over-travel-disconnect mechanism 24 trips in, effectively
disconnecting the shutter release button 4 from the linkage 25.
Thereafter, the linkage 25 and its connected components (the spring
26, the damper 27 and the movable lens 10) are free to move back
towards their rest positions (to the left in FIG. 8) under the reaction
force of compressed spring 26 with velocity controlled by friction
and predominantly by damper action from the damper 27. These together
produce smooth traversal of the movable lens 10 across its operating
range at essentially constant velocity (and if desired, through
different velocity profiles are possible by careful design and profiling
of the damper 27).
[0081] The controller 14 is operative to cause capture and storage
of the images during the return movement of the linkage 25 and the
movable lens 10.
[0082] Advantageously, the normal rest position of the lens assembly
6 is set to the hyperfocal distance for the lens assembly, so that
as much of the scene as possible is in focus all the time when the
camera is being panned around. This would be a factory pre-set position.
In this case, the linkage mechanism 8 could be arranged to allow
operation as follows. On depression of the button 4, the lens assembly
6 is pushed back to one end of its range (say the minimum focal
distance) and stays there until button 4 reaches the end of its
travel, ie without the need for the button 4 to be released. The
button 4 might usefully emit a noise when this end of travel position
is reached, by, for example, pushing back an arm that is released
at end of travel, the arm then returning and striking another element
to produce a noise. Once end of travel has been reached, the focus
image sequence occurs as already described, powered by a spring
that was compressed by the user on the downstroke of the button
4. Once the lens assembly 6 reaches its other end of travel, it
trips another lever (or perhaps electronic switch) which then decouples
the lens assembly 6 from the return-stroke spring, after which the
position of the lens assembly 6 is under the control of a weaker
spring that simply returns the lens assembly 6 to the hyperfocal
distance. In this context, the "spring" may be any resilient
element but is probably just implemented by a piece of bent plastic,
metal or a bent wire. This is likely to work well because the hyperfocal
distance (HFD) return mechanism only needs to be strong enough to
move the lens assembly 6 which is light, whereas the button powered
return stroke system can be much more powerful (enough to completely
override the HFD return system) because it is powered by the user
who is relatively strong and is geared down, say by the order of
ten times.
[0083] A second image capture operation alternatively performed
by the controller is shown in FIG. 9 and will now be described.
Whereas in the first image capture operation, analysis of the series
of images is performed after all the images have been stored in
the memory 13, in the second image capture operation the images
are analysed on-the-fly, thereby reducing the memory requirments.
[0084] In step S20, depression of the button 4 is detected. In
response to this, the operation proceeds to step S21 in which the
controller 14 causes the first image in the series to be captured
by the image sensor 11 and stored in the memory 13 as the in-focus
image, this image consists of the entire image area. Next in step
S22, the controller 14 causes the next image in the series to be
captured by the image sensor 11 and stored in the memory 13 separately
from the in-focus image, the stored image consisting of the entire
image area . . . Each image is stored in steps S21 and S22 at the
same predetermined times after initial depression of the button
4 as in the first image capture operation so that each stored image
is an image captured at a different position of the lens arrangement
6 and having a different focus.
[0085] After that, in step S23 the focus quality of that next image
is determined using the algorithm shown in FIG. 5, and the focus
quality of the in-focus image is also so determined (if not already
determined in a previous iteration of step S23). In step S24, the
focus qualities of the next image and the in-focus image are compared
and the image having the best focus quality is stored as the in-focus
image, for example by overwriting the previous in-focus image if
the next image has a better focus quality.
[0086] In step S25, it is determined if all the images in the series
have been stored and analysed. If not, the operation returns to
step S22. Once all the images in the series have been stored and
analysed, the process finishes in step S26 in which case the image
of the series having the best focus quality has been retained as
the in-focus image. Thus the result is the same as the first image
capture operation but less space of the memory 13 has been used
albeit with the requirement of speedy analysis in steps S23 and
S24.
[0087] An example of the second image capture operation in which
the fourth image is found to have the best focus quality is shown
in FIG. 10. The images are identified by their numbers as in FIG.
7 and the area of analysis 19 is shown as being a partial area of
the entire image area. Each row indicates a comparison performed
in step S24 by a question mark, the first column of images being
the stored in-focus images and the second column being each successive
new image. The final column indicates the image stored as the in-focus
image as a result of the comparison. Thus in the first three comparisons,
the in-focus image is updated each time to give the fourth image
as the in-focus image, whereafter there is no change of the in-focus
image.
[0088] As described above, the first and second image capture operations
result in selection of an entire one of the images in the series
as the in-focus image. As an alternative, step S13 of the first
image capture operation and step S24 of the second image capture
operation may be altered by notionally dividing the image area into
a plurality of parts and selecting each part from one of the images
in the series. The result is that the in-focus image may be a composite
image formed from more than one of the images in the series.
[0089] Division into any number of parts of the image is possible.
As the number increases, the overall focus quality improves but
an increasing processing power is needed. The simplest variant may
divide the image into two parts, which could usefully be arranged
as a single circular pixel-block in the centre of the image (for
focusing an object of interest) surrounded by a second pixel-block
(for focusing the background).
[0090] The parts may in general have any shape and size. To reduce
the required processing the parts may comprise a region of a plurality
of pixels in any shape, for example rectangles, triangles or hexagons,
which may have boundaries which are straight or wavy to allow adjacent
regions to interlock and thereby reduce the visibility of boundary
artefacts. The regions may be regularly or irregularly arranged
and may have the same or different sizes.
[0091] To increase the resolution, the parts of the image could
be very small, for example a single pixel or a single pixel and
its nearest neighbours, say 5 or 9 pixels. This gives the highest
resolution of all but may be prone to interference from noise in
the image since the signal to noise ratio at the pixel level may
be high. However, the focusing process can be modified to allow
for noise if the noise level is known. The noise level can be estimated
for example from: known characteristics of the sensor chip; overall
or local brightness of the scene (a dim scene will have more noise);
and the ambient temperature (noise increases at higher temperatures),
which can be measured by measuring the voltage on a single transistor.
[0092] Where regions are used, the selection of each part of the
image area is preferably performed on the basis of a determination
of the focus quality of the image in the area in question. However
where smaller parts of the image are used it may be desirable to
select each part of the image on the basis of a determination of
the focus quality of the image in an analysis area consisting of
the part of the image in question and an adjacent area of the image.
[0093] An example of the second image capture operation applied
with selection of parts of the image area independently is shown
in FIG. 11, for the case of using nine rectangular regions as the
parts of the image. In FIG. 11, the upper drawing denotes the in-focus
image composition at the start of the process, that is at lens position
1; the middle drawing denotes the image composition after 3 comparison
process steps at lens position 4; and the lower drawing shows the
final image composition after the last processing step at lens position
6. In this example, the central and lower-central regions are best
in-focus at lens position 1, corresponding to a near or foreground
object; the regions to the right are best in-focus at lens position
3, corresponding to intermediate distance; and the remaining regions
are best in-focus at lens position 6, corresponding to infinity.
[0094] As described above, the autofocus operation may be linked
to operation of the shutter release button 4, that is when the user
desires to take a photograph. Alternatively, the autofocus operation
can be caused to occur at other times also. This is useful if the
user wants to view an in-focus image on the display 3 before taking
a photograph. For this purpose, a focus button can be provided in
addition to the shutter release button 4. Alternatively, the shutter
release button 4 can be arranged to trigger the autofocus operation
separately from the photograph-taking operation. However, since
at a minimum, to be useful an autofocus operation will either capture
and store a focussed image, or, capture and display a focussed image,
or both, it can be equally useful to simply provide for two modes
of camera operation; in mode 1, depression of the shutter release
button 4 causes the entire multi-image capture, focus selection
process, and final best-focussed image display only (with an option
to subsequently store more permanently that displayed image); and
in mode 2, all of the mode 1 operations occur with the best-focussed
image automatically being transferred to more permanent storage.
So Mode 1 is a "look and see" mode, while mode 2 is most
similar to conventional point-and-shoot. Alternatively, the shutter
release button 4 can be designed such that the first part of the
travel of the button 4 causes the autofocus mechanism to operate
and the second part of the travel of the button 4 causes a photograph
to be taken. Thus the first part results in an in-focus image being
displayed but not stored as a photograph, while in the second part,
the in-focus image is both displayed and stored.
[0095] The camera 5 described above could be adapted as shown in
FIG. 12 to use an actuator 15 to drive movement of the lens assembly
6 instead of the linkage mechanism. In this case, the controller
14 controls the actuator 15 to move the lens assembly 6 (or more
specifically the movable lens 10) in response to operation of the
shutter release button 4. Thus the first or second image capture
algorithms may be applied but with the steps S11, S21 and S22 being
modified to include control of the actuator 15 and to cause capture
and storage of the images at the appropriate values of the control
signal applied to the actuator 15. The actuator may be a piezoelectric
actuator, for example of the type disclosed in WO-01/47041 which
may be used in a camera as disclosed in WO-02/102451.In this case,
the lens arrangement 6 may be suspended using a suspension system
incorporating the actuator 15 as disclosed in WO-2005/003834. Alternatively,
the actuator 15 may be an electric motor such as a DC motor.
[0096] The camera 5 described above is a still-picture camera 5
but could easily be adapted to be a video camera employing the same
focussing method. |