Digital Camera Patent AbstractA digital camera maintains a consistent effective focal length upon
switching from a first focus mode to a second focus mode, the focus
modes having different maximum focal lengths. Maintaining a consistent
effective focal length across focus mode changes obviates the need
for a user to reframe a scene upon changing focus modes.
Digital Camera Patent ClaimsWhat is claimed is:
1. A method for maintaining a consistent effective focal length
in a digital camera, the digital camera having a lens system, the
lens system having a first focus mode and a second focus mode, the
first focus mode having a first maximum focal length, the second
focus mode having a second maximum focal length, the method comprising:
determining an effective focal length of the digital camera while
the lens system is in the first focus mode; and applying sufficient
digital zoom to maintain the effective focal length, when the effective
focal length exceeds the second maximum focal length, the second
maximum focal length is smaller than the first maximum focal length,
and the lens system is switched from the first focus mode to the
second focus mode.
2. The method of claim 1, wherein, while the lens system is in
the first focus mode, the effective focal length is achieved by
a combination of optical zoom and digital zoom.
3. The method of claim 1, wherein the effective focal length is
maintained by a combination of optical zoom and digital zoom.
4. The method of claim 1, wherein the first focus mode is a macro
focus mode and the second focus mode is a normal focus mode.
5. The method of claim 1, wherein the first focus mode is a normal
focus mode and the second focus mode is a macro focus mode.
6. A method for maintaining a consistent effective focal length
in a digital camera, the digital camera having a lens system, the
lens system having a first focus mode and a second focus mode, the
first focus mode having a first maximum focal length, the second
focus mode having a second maximum focal length, the method comprising:
determining an effective focal length of the digital camera while
the lens system is in the first focus mode; and applying as much
optical zoom as possible to maintain the effective focal length,
when the effective focal length exceeds the first maximum focal
length, the first maximum focal length is smaller than the second
maximum focal length, and the lens system is switched from the first
focus mode to the second focus mode.
7. The method of claim 6, wherein, while the lens system is in
the first focus mode, the effective focal length is achieved by
a combination of optical zoom and digital zoom.
8. The method of claim 6, wherein the effective focal length is
maintained by a combination of optical zoom and digital zoom.
9. The method of claim 6, wherein the first focus mode is a macro
focus mode and the second focus mode is a normal focus mode.
10. The method of claim 6, wherein the first focus mode is a normal
focus mode and the second focus mode is a macro focus mode.
11. A digital camera, comprising: a lens system to produce an optical
image, the lens system having a first focus mode and a second focus
mode, the first focus mode having a first maximum focal length,
the second focus mode having a second maximum focal length; a function
to select between the first and second focus modes; an imaging module
to convert the optical image to a digital image; and a focal length
compensation module configured to apply sufficient digital zoom
to the digital image to maintain an effective focal length of the
digital camera, when the effective focal length exceeds the second
maximum focal length, the second maximum focal length is smaller
than the first maximum focal length, and the lens system is switched
from the first focus mode to the second focus mode.
12. The digital camera of claim 11, wherein, while the lens system
is in the first focus mode, the digital camera is configured to
achieve the effective focal length by a combination of optical zoom
and digital zoom.
13. The digital camera of claim 11, wherein the focal length compensation
module is configured to maintain the effective focal length by a
combination of optical zoom and digital zoom.
14. The digital camera of claim 11, wherein the first focus mode
is a macro focus mode and the second focus mode is a normal focus
mode.
15. The digital camera of claim 11, wherein the first focus mode
is a normal focus mode and the second focus mode is a macro focus
mode.
16. A digital camera, comprising: a lens system to produce an optical
image, the lens system having a first focus mode and a second focus
mode, the first focus mode having a first maximum focal length,
the second focus mode having a second maximum focal length; a function
to select between the first and second focus modes; an imaging module
to convert the optical image to a digital image; and a focal length
compensation module configured to apply as much optical zoom as
possible to the optical image to maintain an effective focal length
of the digital camera, when the effective focal length exceeds the
first maximum focal length, the first maximum focal length is smaller
than the second maximum focal length, and the lens system is switched
from the first focus mode to the second focus mode.
17. The digital camera of claim 16, wherein, while the lens system
is in the first focus mode, the digital camera is configured to
achieve the effective focal length by a combination of optical zoom
and digital zoom.
18. The digital camera of claim 16, wherein the focal length compensation
module is configured to maintain the effective focal length by a
combination of optical zoom and digital zoom.
19. The digital camera of claim 16, wherein the first focus mode
is a macro focus mode and the second focus mode is a normal focus
mode.
20. The digital camera of claim 16, wherein the first focus mode
is a normal focus mode and the second focus mode is a macro focus
mode.
21. A digital camera, comprising: optical means for producing an
optical image, the optical means having a first focus mode and a
second focus mode, the first focus mode having a first maximum focal
length, the second focus mode having a second maximum focal length;
means for selecting between the first and second focus modes; means
for converting the optical image to a digital image; and focal length
compensation means for applying sufficient digital zoom to the digital
image to maintain an effective focal length of the digital camera,
when the effective focal length exceeds the second maximum focal
length, the second maximum focal length is smaller than the first
maximum focal length, and the optical means is switched from the
first focus mode to the second focus mode.
22. A digital camera, comprising: optical means for producing an
optical image, the optical means having a first focus mode and a
second focus mode, the first focus mode having a first maximum focal
length, the second focus mode having a second maximum focal length;
means for selecting between the first and second focus modes; means
for converting the optical image to a digital image; and focal length
compensation means for applying as much optical zoom as possible
to the optical image to maintain an effective focal length of the
digital camera, when the effective focal length exceeds the first
maximum focal length, the first maximum focal length is smaller
than the second maximum focal length, and the optical means is switched
from the first focus mode to the second focus mode. Digital Camera Patent DescriptionFIELD OF THE INVENTION
The present invention relates generally to digital cameras and
more specifically to digital cameras with multiple focus modes having
different focusing ranges.
BACKGROUND OF THE INVENTION
Some digital cameras, like their film counterparts, include a zoom
lens with multiple focus modes, each focus mode having a different
focusing range. For example, a zoom lens may have a macro focus
mode and a normal focus mode. A macro focus mode allows the lens
to focus on objects at a shorter minimum distance for a given focal
length than the normal focus mode. This mode is convenient for taking
extreme close ups.
Depending on the lens design, the range of available focal lengths
in each focus mode may be different. In particular, the maximum
focal length may be smaller in one focus mode than in the other.
For example, a zoom lens may be designed to provide focal lengths
of 35 mm to 105 mm in normal focus mode and 35 mm to 75 mm in macro
focus mode. A user may zoom out to 105 mm while framing a subject
in normal focus mode, only to decide that macro focus mode is the
better choice. Upon switching to macro focus mode, however, the
digital camera necessarily drops to a smaller focal length within
the available range (e.g., 75 mm) for macro focus mode, forcing
the user to reframe the picture.
It is thus apparent that there is a need in the art for an improved
method and apparatus for maintaining a consistent effective focal
length in a digital camera.
SUMMARY OF THE INVENTION
A method for maintaining a consistent effective focal length in
a digital camera is provided. An apparatus for carrying out the
method is also provided.
Other aspects and advantages of the present invention will become
apparent from the following detailed description, taken in conjunction
with the accompanying drawings, illustrating by way of example the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are functional block diagrams of a digital camera
in accordance with an illustrative embodiment of the invention.
FIGS. 2A and 2B are a flowchart of the operation of the digital
camera shown in FIGS. 1A and 1B in accordance with an illustrative
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In a digital camera having multiple focus modes, each focus mode
having a different maximum optical focal length, a consistent effective
focal length may be maintained across focus-mode changes by combining
optical and digital zoom. Digital zoom is typically accomplished
by cropping a digital image and, optionally, applying pixel interpolation
techniques well known in the image processing art to rescale the
image to its original size. In some implementations, interpolation
is applied to the previewed version of a digital image shown on
the display of the digital camera but not on the final image that
is captured. As those skilled in the art will recognize, digital
zoom results in some loss of image quality. Therefore, maximum optical
zoom may be employed, whenever possible, to achieve the highest
possible image quality. Two introductory examples will serve to
illustrate the principles of the invention. In both examples, the
digital camera is assumed to have a normal focus mode with a focal
length range of 35 mm to 105 mm and a macro focus mode with a focal
length range of 35 mm to 75 mm. The macro focus mode may focus at
distances ranging from, for example, 0.1 m to 0.7 m. The normal
focus mode may focus at distances ranging from, for example, 0.5
m to infinity.
In the first example, a user attempts to frame a subject at an
optical focal length of 105 mm in normal focus mode. The user then
decides to switch to macro focus mode to focus at a shorter distance.
Consequently, the zoom lens must drop to a maximum focal length
of 75 mm. By applying digital zoom at 1.4.times., however, the same
effective focal length of 105 mm may be maintained. This allows
the user to continue composing the shot without having to reframe
the subject.
In the second example, a user attempts to frame a subject using
a combination of digital zoom at 1.33.times. and optical zoom at
75 mm for an effective focal length of 100 mm while the digital
camera is in macro focus mode. The user then decides to switch to
normal focus mode. In this case, as much optical zoom as possible
may be applied to maintain the same effective focal length of 100
mm. In fact, since 100 mm is less than the maximum focal length
of 105 mm in normal focus mode, the effective focal length may be
maintained entirely optically. Using as much optical zoom as possible
in this situation maximizes the quality of the captured image.
Some digital cameras may have a zoom lens in which the normal focus
mode has a smaller maximum focal length than the macro focus mode.
The same techniques illustrated in the above examples still apply,
however, depending on the effective focal length prior to a focus
mode change and whether the focus mode change is to a focus mode
having a smaller or larger maximum optical focal length.
An illustrative embodiment of the invention may be summarized in
general terms as follows. A digital camera has at least a first
focus mode and a second focus mode, each of which is designed to
focus within a predetermined distance range. In some implementations,
the digital camera may have more than two focus modes. For example,
the first focus mode may be a normal focus mode, and the second
focus mode may be a macro focus mode, or vice versa. The first focus
mode has a first maximum focal length ("MAX. FL1"). The
second focus mode has a second maximum focal length ("MAX.
FL2") that is different from MAX. FL1. The "effective
focal length" ("EFL") of the digital camera throughout
this description is the overall magnification factor at which a
digital image is captured, whether produced entirely by an optical
zoom lens or by a combination of an optical zoom lens and digital
zoom.
Table 1 summarizes the operation of this illustrative embodiment
of the invention, in each of two situations, when the focus mode
is switched from the first focus mode to the second focus mode.
TABLE-US-00001 TABLE 1 EFL > MAX. FL2, EFL > MAX. FL1, MAX.
FL1 > MAX. FL2 MAX. FL1 < MAX. FL2 Apply sufficient digital
zoom to Apply as much optical zoom as possible maintain effective
focal length. to maintain effective focal length.
As shown in Table 1, digital zoom may be applied to maintain the
EFL in the second focus mode whenever the EFL in the first focus
mode exceeds MAX. FL2 and MAX. FL1 is greater than MAX. FL2. Additionally,
the maximum amount of available optical zoom may be applied to maintain
the EFL in the second focus mode whenever the EFL in the first focus
mode exceeds MAX. FL1 and MAX. FL1 is less than MAX. FL2.
FIGS. 1A and 1B are functional block diagrams of a digital camera
100 in accordance with an illustrative embodiment of the invention.
Digital camera 100 may be a digital still camera, a digital video
camera, a combination digital still/video camera, or any other device
capable of focusing and digitizing an optical image. In FIG. 1A,
controller 105 communicates over data bus 110 with imaging module
115, memory 120, display 125, focus-mode selection function 130,
and focal length compensation module 135. Lens system 140 produces
optical images that are converted to digital images by imaging module
115. In a typical implementation, lens system 140 may comprise a
zoom lens having multiple focus modes, each focus mode having a
different focusing range (e.g., normal mode and macro mode). Focus-mode
selection function 130 selects among the different focus modes.
Focus-mode selection function 130 may be actuated by a user input
device such as a button or menu option. Imaging module 115, in a
typical implementation, may comprise an array of photosensors based
on charge-coupled-device (CCD) or CMOS technology, an analog-to-digital
converter (A/D), a gain control, and a digital signal processor
(DSP) (not shown in FIG. 1A). Memory 120 further comprises random
access memory (RAM) 145 and non-volatile memory 150.
Referring to FIG. 1B, Focal Length Compensation Module 135 may
further comprise Modules Focal Length Compensation Control 155,
Determine Effective Focal Length 160, Digital Zoom 165, and Select
Optical Focal Length 170. These modules may, for example, be executed
by controller 105. Module Focal Length Compensation Control 155
manages the process of maintaining a consistent effective focal
length across focus mode changes by invoking the other modules as
needed. Module Determine Effective Focal Length 160 retrieves, measures,
or computes the effective focal length achieved by lens system 140
or by lens system 140 in combination with digital zoom. Since lens
system 140 is typically controlled electromechanically with high
precision, the optical focal length of lens system 140 may be set
to a specific value by controller 105 (executing module Select Optical
Focal Length 170) and tracked in, for example, a register or other
memory location. Therefore, Module Determine Effective Focal Length
160 may determine the effective focal length by simply reading the
current optical focal length of lens system 140 and, if applicable,
multiplying the optical focal length by the digital zoom factor.
Module Digital Zoom 165 crops digital images and optionally rescales
them to their original size as explained earlier in this description.
Module Select Optical Focal Length 170 sets lens system 140 to a
particular optical focal length in accordance with the allowable
focal length range of the applicable focus mode. The functional
boundaries characterizing Modules Focal Length Compensation Control
155, Determine Effective Focal Length 160, Digital Zoom 165, and
Select Optical Focal Length 170 are purely conceptual. For example,
these functions may be implemented as four separate software or
firmware modules, as fewer than four software or firmware modules,
or in any other fashion that is advantageous, including custom hardware
or a combination of hardware and software/firmware.
FIGS. 2A and 2B are a flowchart of the operation of digital camera
100 shown in FIGS. 1A and 1B in accordance with an illustrative
embodiment of the invention. At 205 in FIG. 2A, lens system 140
of digital camera 100 is in the first focus mode. As explained above,
whether the first focus mode is a macro focus mode or a normal focus
mode is arbitrary. The same principles apply in either case. If
a request to switch to the second focus mode is received from focus-mode
selection function 130 at 210, Module Determine Effective Focal
Length 160 acquires the current effective focal length (EFL) at
215. At 220, Module Focal Length Compensation Control 155 determines
whether MAX. FL1 is greater than MAX. FL2. If so, control proceeds
to 225. Otherwise, control proceeds to 245 in FIG. 2B. (Note that,
since it is assumed that MAX. FL1 and MAX. FL2 are not equal, MAX.
FL1 must be less than MAX. FL2 if the test at 220 is not true.)
At 225, Module Focal Length Compensation Control 155 determines
whether the EFL exceeds MAX. FL2. If not, lens system 140 is switched
to the second focus mode at 230, and the process terminates at 240.
Otherwise, control proceeds to 235, where Module Focal Length Compensation
Control 155 determines what digital zoom factor is needed to maintain
the EFL, Module Select Optical Focal Length 170 sets the focal length
of lens system 140 to a focal length less than or equal to MAX.
FL2, Module Digital Zoom 165 applies sufficient digital zoom to
maintain the EFL, and the lens system 140 is switched to the second
focus mode.
If control proceeds to 245 in FIG. 2B after the test at 220, Module
Focal Length Compensation Control 155 determines whether the EFL
exceeds MAX. FL1. If not, control proceeds to 250, where lens system
140 is switched to the second focus mode, and the process terminates
at 240. Otherwise, control proceeds to 255, where Module Focal Length
Compensation Control 155 determines whether the EFL exceeds MAX.
FL2. If not, the focal length of lens system 140 may be set to the
EFL to maximize image quality. If the EFL exceeds MAX. FL2, the
EFL may be maintained by setting the focal length of lens system
140 to MAX. FL2 using Module Select Optical Focal Length 170 and
adding digital zoom using Module Digital Zoom 165.
The foregoing description of the present invention has been presented
for the purposes of illustration and description. It is not intended
to be exhaustive or to limit the invention to the precise form disclosed,
and other modifications and variations may be possible in light
of the above teachings. The embodiments were chosen and described
in order to best explain the principles of the invention and its
practical application to thereby enable others skilled in the art
to best utilize the invention in various embodiments and various
modifications as are suited to the particular use contemplated.
It is intended that the appended claims be construed to include
other alternative embodiments of the invention except insofar as
limited by the prior art. |