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Title:
CAMERA FOCUS CORRECTION METHOD AND SYSTEM
Document Type and Number:
WIPO Patent Application WO/2010/149763
Kind Code:
A1
Abstract:
The present invention relates to a camera comprising a housing including an optical lens and an autofocus system for determining a first focus setting of the optical lens. A sensor is adapted to detect a change in spatial orientation of the housing after determination of the first focus setting and generating a signal in accordance with the change. The camera comprises a processor with an input connected to the sensor for reception of the signal from the sensor and an output for controlling the autofocus system. The processor being further configured to determine a focus correction parameter based on the signal from the sensor and controlling the autofocus system to refocus from the first focus setting to a second focus setting in accordance with the determined focus correction parameter. The autofocus correction parameter includes correction for lens field curvature.

Inventors:
POULSEN ANDERS (DK)
Application Number:
PCT/EP2010/059056
Publication Date:
December 29, 2010
Filing Date:
June 25, 2010
Export Citation:
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Assignee:
HASSELBLAD AS (DK)
POULSEN ANDERS (DK)
International Classes:
G02B7/08; G02B7/04; G02B7/28; G03B13/36; H04N5/232
Foreign References:
US20080317453A12008-12-25
US20070122129A12007-05-31
US20070098380A12007-05-03
US5227832A1993-07-13
US20080317453A12008-12-25
US20070098380A12007-05-03
Attorney, Agent or Firm:
Henrik Bagger-Olsen (Hans Bekkevolds Allé 7, Hellerup, DK)
Download PDF:
Claims:
CLAIMS

1. A camera comprising: a housing including an optical lens, an autofocus system for determining a first focus setting of the optical lens, a sensor for detecting change in spatial orientation of the housing after determination of the first focus setting and generating a signal in accordance with the change, a processor with

- an input connected to the sensor for reception of the signal from the sensor,

- an output for controlling the autofocus system, the processor further being configured to determine a focus correction parameter based on the signal from the sensor and controlling the autofocus system to refocus from the first focus setting to a second focus setting in accordance with the determined focus correction parameter, characterized in that the autofocus correction parameter includes correction for lens field curvature. 2. The camera according to claim 1 , wherein the optical lens is detachably mounted to the housing.

3. The camera according to claim 1 or 2, wherein the sensor is configured to measure tilt and/or swing of the housing.

4. The camera according to any of the claims 1-3, wherein the sensor comprises at least one of {a gyro, a rate gyro, an accelerometer} or a combination thereof.

5. The camera according to any of the claims 1-4, wherein the processor is configured to determine the focus correction parameter only after the autofocus system has established a focus setting.

6. The camera according to any of the claims 1-5, wherein the camera further comprises a device for measuring distance from the camera to an object viewable from the lens.

7. The camera according to any of the claims 1-6, wherein the camera comprises a focus locking device for locking the first and second focus settings.

8. The camera according to any of the claims 1-7, wherein the camera is a digital camera or a camera having a digital back.

9. A method for correcting focus settings in a camera, the camera comprising an autofocus system and a sensor for detecting change in spatial orientation of the housing, the sensor configured to transmit a signal to a processor configured to determine, on the basis of the signal, a correction parameter for the autofocus system, the method comprising:

- establishing a first focus setting by the autofocus system,,

- the sensor transmitting a signal to the processor in response to a change in spatial orientation of the housing,

- the processor determining an autofocus correction parameter for a second focus setting based on the signal from the sensor, characterized in that

- the autofocus correction parameter includes correction for iens field curvature.

10 The method according to claim 9, wherein the autofocus system locks the second focus setting based on the autofocus correction parameter

11 The method according to claim 9 or 10, wherein a first focus plane at the first focus setting is at a distance d from the camera,

- shifting the first focus plane by δ given by the equation δ = d x (1- cos θ) to obtain the second focus plane at a second focus setting when the camera is moved an angle of θ degrees.

12 The method according to claim 11 , further comprising the step of adjusting the first focus plane by an amount corresponding to δ to obtain the second focus plane

13 The method according to any of the claims 9-12, further including determining a distance from the camera to an object to be photographed.

Description:
CAMERA FOCUS CORRECTION METHOD AND SYSTEM

The present invention relates to a camera comprising an autofocus system for determining a first focus setting of an optical lens. A sensor is adapted to detect a change in spatial orientation of the housing after determination of the first focus setting and generating a signal in accordance with the change. The camera comprises a processor configured to determine a focus correction parameter based on a signal from the sensor and controlling the autofocus system to refocus from the first focus setting to a second focus setting in accordance with the determined focus correction parameter. The autofocus correction parameter includes correction for lens field curvature.

BACKGROUND OF THE INVENTION

When using a camera equipped with an auto focus system with a centrally positioned focus measurement area the camera cannot measure correct focus position if the main object is not in a center of the image. A problem arises if the photographer wishes to compose the image in a manner where the object that the photographer wishes to be in focus is not at the center of the image.

Today at least two different solutions to this problem are generally used: 1. Multiple focus measurement areas over the image area. 2. Means to lock the focus position where the main subject is first centered in the image and focus position is locked. Next the camera is moved to position the main subject in the image as desired. Examples of movement include tilt and swing.

At least the following problems or shortcomings of the solutions above are:

1. In order to handle arbitrary objects, the camera must have many focus measurement areas and this makes the focus measurement system complicated and expensive. Also user selection of desired measurement area is cumbersome.

2. The locked focus position will shift the focus in relation to the subject when the camera orientation is changed, e.g. a combination of camera tilt and swing.

US 2008/317453 describes a digital camera with an autofocus system that can move a focus lens between two different focal positions. A moving distance acquiring unit is configured to acquire a moving distance of the camera body and a focus correction unit corrects a position of the focus lens based on the acquired moving distance. US 2007/098380 discloses methods and systems for improved autofocus in digital imaging systems. In response to locking of lens focus on a subject image, the autofocus system may determine an initial subject focus distance and an initial attitude, and in response to a request for an exposure, determining a finai attitude. An initial subject distance may be determined based on the initial subject focus distance and a focus correction distance may be determined where the focus correction distance is based on the difference between the initial attitude and the final attitude.

SUMMARY OF INVENTION

A first aspect of the present invention relates to a camera comprising a housing including an optical lens and an autofocus system for determining a first focus setting of the optical lens. A sensor is adapted to detect a change in spatial orientation of the housing after determination of the first focus setting and generating a signal in accordance with the change. The camera comprises a processor with an input connected to the sensor for reception of the signal from the sensor and an output for controiling the autofocus system. The processor being further configured to determine a focus correction parameter based on the signal from the sensor and controlling the autofocus system to refocus from the first focus setting to a second focus setting in accordance with the determined focus correction parameter. The autofocus correction parameter includes correction for lens field curvature. The above-mentioned autofocus system is contemplated to allow a photographer to establish a first autofocus setting and then re-orientate the camera to achieve a desired composition of the image at a second focus setting. Since correction of lens field curvature is included in the focus correction parameter utilized to determine the second focus setting, imperfections in lens geometry are also compensated in an efficient manner. This is particularly useful in combination with the re-focusing process between the first and second focus settings because the object to be photographed will as a result of the photographer's re-composition of the image end up at a position offset from the center of optical lens.

Ideally, an optical lens has a perfectly flat field of sharpness. A picture of a flat object will show perfect sharpness all over the entire image area in such an optical lens. However, most practical optical lenses have a sharpness field that is slightly curved and will not produce images with full sharpness over the entire image area when photographing a flat object. This lens characteristic is called "lens field curvature" and effects thereof have been illustrated in further detail below in connection with the description of Figs. 7 and 8, With detailed knowledge about the optical lens design, image position and aperture used, it is possible to calculate a distance deviation between a plane of optimum or best sharpness deviates and a theoretical flat sharpness plane. Performing such calculations on a number of optical lenses offered by the present assignee/applicant shows that the distance deviation caused by lens field curvature correspond to an off-set, measured on the image sensor (e.g. CCD chip), between 0.1 and 0.3 mm for a 1 meter object distance, d, and typical lens geometries. Based on such calculations, it is possible to compensate the focus correction parameter by a distance deviation which corresponds to the off-set determined on the image sensor resulting in a higher precision of the final image. Furthermore, focus errors introduced by lens field curvature and re-focusing between the first and second focus settings are of comparable magnitude under normal image recording/shooting conditions as described in further detail in connection with Figs. 7 and 8 below. This makes the above-described compensation for determined lens field curvature particularly useful and significant to achieve optimum image sharpness at the second focus plane where the picture is recorded.

The present invention is most useful for, but not limited to, handheld photographing or handheld photography.

In the optical lens is preferably detachably mounted to the housing. Exchangeable optical lenses allow a photographer to use a number of different optical lenses, e.g. lenses having different focal length or other characteristics, for different purposes.

The camera may further include a detection device for detecting the type of lens attached to the camera housing. The processor may include information on the lens type when calculating autofocus correction parameter and/or lens field curvature so as to specifically adjust the focus correction parameter to optical characteristics of the optical lens in question and improve focus correction.

The sensor may comprise at least one of {a gyro, a rate gyro, an accelerometer} or a combination thereof. The type of sensor or sensors chosen for a specific embodiment may depend on factors such as space in the camera housing, power consumption of the sensor etc. The sensor or sensors may be adapted to measure tilt and/or swing or pan of the camera housing. The sensor or sensors are preferably additionally adapted to measure translatory movement of the camera housing so as to allow all types of re-orientation of the spatial orientation of the camera housing between the first and second focus settings to be accurately measured and compensated. The adjustment of focus setting may be performed continuously while the camera is reorientated or be performed when the camera is again substantially at rest. In one embodiment the processor may be configured to determine the focus correction parameter only after the autofocus system has established a focus setting. The camera may comprise a focus locking device for locking the first and/or second focus setting(s).

In an embodiment the camera may be a digital camera or a camera having a digital back. By digital camera is meant a camera having a light sensitive sensor for recording a digital image, such as a CCD sensor or the like. A digital back may be attached to a camera not originally designed for digital photography. The camera may be a camera recording images on film.

The camera may comprise a device for measuring distance from the camera to an object viewable from the optical iens.

According to a second aspect of the invention, there is provided a method for correcting focus settings in a camera. The camera comprises an autofocus system and a sensor for detecting change in spatial orientation of the housing and the sensor is configured to transmit a signal to a processor. The processor is configured to determine, on the basis of the signal, a correction parameter for the autofocus system. The method comprising: - establishing a first focus setting by the autofocus system,

- the sensor transmitting a signal to the processor in response to a change in spatial orientation of the housing,

- the processor determining an autofocus correction parameter for a second focus setting based on the signal from the sensor, characterized in that the autofocus correction parameter includes correction for iens field curvature.

This is contemplated to allow a photographer to use the autofocus system by pointing the center of the optical [ens towards the object to be photographed, indicate that this is the focus setting that he/she desires and then recompose the image by displacing and/or tilting and/or panning before recording the image. The recomposed image will then have the same areas in focus as the initial composition with the object at the center of the lens.

The autofocus system may be configured to adjust or lock the second focus setting based on the autofocus correction parameter. The locking or adjustment is preferably automatically performed by the autofocus system. in another embodiment, a first focus plane at the first focus setting is at a distance d from the camera. The method comprises shifting the first focus plane by δ given by the equation: δ = d x (1- cos θ) to obtain the second focus plane at a second focus setting when the camera is moved an angle of θ degrees.

Tilting the camera housing around more than one axis of rotation may be incorporated in the above-mentioned equation. Further, displacement of the camera housing may also be incorporated in the equation. The first focus plane may advantageously be adjusted by an amount corresponding to δ to obtain the second focus plane. Thereby, the δ value calculated above may form the basis for a calculation of the required adjustment of the focus of the optical iens.

The method preferably further includes determining a distance from the camera to an object to be photographed. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed in more detail with reference to the drawings in which:

Fig. 1 is a schematic illustration of a view in a viewfinder with one central focus point according to prior art autofocus systems, Fig. 2 is a schematic illustration of a view in a viewfinder with several focus points prior art autofocus systems,

Fig. 3 is a schematic illustration of a setup where a person is in the center of the image when setting the autofocus and the consequence of tilting the camera subsequently,

Fig. 4 is a schematic illustration of a setup where a person is in the center of the image when setting the autofocus and the consequence of swinging or turning the camera subsequently,

Fig. 5 is another schematic illustration of a setup where a person is in the center of the image when setting the autofocus and the consequence of tilting the camera subsequently, Fig. 6 is a schematic illustration of geometrical relations between two orientations of the camera,

Fig. 7 is an exemplary schematic illustration of the effect of lens field curvature, Fig. 8 is a schematic illustration of the effect of lens field curvature and a correction parameter or factor; and

Fig. 9 is a schematic illustration of a camera in accordance with a preferred embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 schematically illustrates the view 10 in a viewfinder showing a single focus point 12 or focus area at the center marked with a cross in accordance with prior art autofocus systems. Fig. 2 schematically illustrates the view 14 in a viewfinder showing a multitude of focus points or focus areas, each marked with a cross, in accordance with prior art autofocus systems. One problem with multiple focus areas is that due to the nature of lenses the areas farthest from the center is subjected to less light that the area at the center. This may cause difficulty for the autofocus system to achieve a sufficiently focused image if the desired object is located in these areas.

A central focus area, as shown in Fig. 1, is contemplated to provide the best outset for the autofocus system as this area is most likely to have the best light condition.

In Fig. 3 is illustrated a setup where a photographer uses a camera 16 having a central focus area such as the one illustrated in Fig. 1. The photographer wishes to focus on the face of the person 18. As illustrated on the left side of the figure, the photographer initially orients the camera 16 so that the face of the person 18 is at the center of the image. The autofocus system accordingly focuses on the face and the photographer holds or locks a first autofocus setting. The locking may be achieved by pressing the trigger halfway down or by operating a separate autofocus button on the camera. The photographer subsequently tilts the camera 16 as illustrated in the right-hand side of the figure so that the image is composed as the photographer desires. As illustrated, the photographer wishes to compose the image so that the entire body of the person 18 is visible in the image 20. As illustrated by the grey area in the left-hand side of Fig. 3, the focus plane is parallel with the face of the person when the first autofocus setting was performed. After the photographer has recomposed the image, this focus plane is now moved i.e. tilted relative to the face of the person. The resulting image will accordingly not be focused where the photographer intended.

A similar situation where the photographer pans or swings the camera 16 is illustrated in Fig. 4. A more detailed illustration is given in Fig. 5 where in the left-hand side the autofocus has focused on the eyes of the person in a first focus setting. It is illustrated that the photographer holds the camera 16 at a horizontal orientation, but the starting point may of course be that the camera 16 is tilted and/or swung relative to the orientation that the photographer intends when recording the desired image. When the photographer tilts the camera 16 as illustrated in Fig. 5, the focus plane 22 is tilted and shifted so as to position the ears of the person 18 in the focus plane instead of the eyes as in the the first focus setting.

Fig. 6 illustrates some geometrical relations associated with the shift of focus plane as illustrated above. When the photographer has performed the autofocus, left-hand situation to focus at the person's eyes at a first focus plane 22, and subsequently tilts the camera 16, right-hand situation, by an angle θ, and assuming that the person is at a distance, d, from the camera, the first focus plane 22 is shifted by δ to be at 24. δ is given by the equation: δ = d x ( 1- cos θ )

On the basis of the parameter δ the amount of adjustment for the lens may be determined so that the desired focus plane, at the person's eyes, may be maintained in the second focus setting. The camera comprises a sensor that detects the movement of the camera. The sensor may detect the acceleration and/or tilt and pan of the camera. The angular movement of the camera may be determined on the basis of a signal from the sensor. Sensor types may be any suitable sensor and may be any one of gyroscope, accelerometer, piezo-fiim or piezoelectric sensor, strain-gauge, optical, gravity based measurement devices, or any combinations thereof. In an embodiment the sensor senses change in one direction and in an embodiment the sensor senses change in two directions, and in an embodiment the sensor senses change in three directions. The sensor may be composed of several individual sensors such as accelerometer(s) and gyroscope(s).

The camera may include memory for storing orientation information so that change in orientation may be determined. The sensor may provide information regarding the present orientation of the camera or changes of orientation of the camera. In one embodiment, the autofocus system integrates respective signals from rate change gyros to detect relative changes in spatial orientation of the camera housing, such as tilts and pans, of the camera between the first focus setting and the second focus setting. This embodiment may additionally comprise accelerometers to detect translational movement or displacement of the camera housing.

In an embodiment the sensor may be constituted by image recognition software or devices, where the view from the optical lens is analyzed to determine change in orientation or movement of the camera.

Ideally, a lens has a perfectly flat field of sharpness. This means that taking a picture of a flat object will show perfect sharpness all over the entire image area. However, most lenses have a sharpness field that is slightly curved and will not produce images with full sharpness over the entire image area when photographing a flat object. This lens characteristic is called "lens field curvature". With detailed knowledge about the lens design, image position and the aperture used, it is possible to calculate how much the plane of best sharpness deviates from the theoretical flat sharpness plane.

With this knowledge, it is possible to further "correct" or compensate the focus correction described earlier between the first and second focus settings by the deviation caused by the lens field curvature, resulting in a higher precision.

Figs. 7 and 8 schematically illustrate an optical lens where the plane of best sharpness 28 bends toward the optical lens. Different optical lens types may have a behavior where the plane 28 bends away from the optica! lens. Fig. 8 illustrates how the focus correction parameter 5 computed above, in connection with refocusing from the first to the second focus setting, is further compensated or corrected with the illustrated distance or value "x". The distance "x" represents a calculated lens field curvature error at this image height which may be computed on basis of a measured camera tilt angle and a measured subject distance. As illustrated by Fig 8, the focus plane 26 is shifted to be at plane 30 by the above-described shift of focus plane in accordance with the computed value of the autofocus correction parameter δ if no account is given to lens field curvature compensation. However, the true focus piane at the first focus setting lies at the curved plane 28, rather than at plane 26, due to the lens field curvature. The curved plane 28 is subsequently shifted to be located at curved plane 32 in connection with refocusing from the first to the second focus setting as described above. As illustrated in Fig. 8, the correction by δ without taking account of lens field curvature causes the curved plane 32 to lie outside the intended focus area (at the person's eyes). By correcting the position of the curved plane 32 with the computed distance, x, the second focus piane, where the image is recorded, is shifted from the curved plane 32 to the desired focus plane 30. To asses the magnitude of errors introduced by lens field curvature, optical lens characteristics of Hasselblad camera model H4D-60 was used as an exemplary illustration of the benefits of the present invention. Computations made by the present inventor revealed a lens field curvature error of about 0.28mm at the image edge, measured on the image sensor (e.g. CCD chip), for a 1 m object distance using an HC100@f/5,β objective lens. The errors introduced by re-focusing from the first to the second focus plane were subsequently computed to lie between 0.05 mm and 0.30 mm, also measured on the image sensor (e.g. CCD chip), under the same conditions as outlined above. Consequently, focus errors introduced by lens field curvature and re-focusing are of comparable magnitude under normal image recording/shooting conditions underlining the considerable benefits achieved by present invention where the focus correction parameter δ includes the above-described correction for lens field curvature, distance "x". Fig. 9 is a schematic illustration of a camera 90. The camera 90 comprises a processor 1 arranged inside a camera housing 91. The processor 1 is electrically connected to three sensors 2, 3 and 4. The sensors may provide information regarding the cameras orientation and/or position and/or motion. The processor may determine the cameras motion or relative motion based on signals from one or more sensors 1, 2 and 3. Based on the sensor signal or signals the processor determines a focus correction parameter for the autofocus system so as to compensate or correct the change in focus caused by a change of spatial orientation of the camera housing 91 as described above. A device 5 such as a motor or linear actuator is configured to move a lens system 6 so as to maintain a desired focus setting at a desired focus plane.




 
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