Method and apparatus for measuring the focus performance of a camera and lens combination

In one embodiment, the present invention is a method and apparatus for measuring the focus performance of a camera and lens combination. One embodiment of a test target for measuring a focus performance of a camera and lens combination includes a target body, the target body displaying a pattern including: a primary pattern covering a portion of the target body and a secondary pattern superimposed over a portion of the primary pattern, the secondary pattern being aligned along an edge of the target body. The test target also includes a ruler positioned adjacent to the edge of the target body, a ruler positioned adjacent to the edge of the target body, such that the secondary pattern directly abuts a zero line of the ruler.

FIELD OF THE INVENTION

The present invention generally relates to the field of photography, and more specifically relates to the calibration of image capturing devices.

BACKGROUND

An autofocus optical system uses a sensor, a control system, and a motor to focus fully automatically or manually on a selected point or area. Cameras and other image capturing devices have used built-in autofocus systems for many years, and yet many photographers still struggle with the accuracy and repeatability of these systems.

Autofocus is a great convenience, especially in fast shooting situations; however, autofocus error still causes many images to be improperly focused. Photographers tend to blame the improper focus on the camera choosing the wrong object (or the wrong point on the object) as the focal point. However, another possibility is that the autofocus feature is simply not functioning properly. In such cases, the photographer may have to return the lens and/or camera body back to the manufacturer for recalibration or replacement. This option can be both time consuming and expensive.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is a method and apparatus for measuring the focus performance of a camera and lens combination. One embodiment of a test target for measuring a focus performance of a camera and lens combination includes a target body, the target body displaying a pattern including: a primary pattern covering a portion of the target body and a secondary pattern superimposed over a portion of the primary pattern, the secondary pattern being aligned along an edge of the target body. The test target also includes a ruler positioned adjacent to the edge of the target body, a ruler positioned adjacent to the edge of the target body, such that the secondary pattern directly abuts a zero line of the ruler.

DETAILED DESCRIPTION

In one embodiment, the present invention is a method and apparatus for measuring the focus performance of a camera and lens combination. Embodiments of the invention aid a photographer in correcting the autofocus feature of his camera for use with a particular camera and lens combination. This correction data may be stored in the camera for later use with the same camera and lens combination. Thus, the present invention may be used to easily calibrate the interchangeable lenses that may be used with modern cameras such as modern digital single-lens reflex (DSLR) cameras.

In particular, the present invention provides a portable test target that allows the photographer to check the camera and lens calibration on-site.FIG. 1illustrates one example of a test target100, according to the present invention.FIG. 2illustrates the test target100ofFIG. 1in its compact form. With reference toFIGS. 1 and 2, the test target100generally comprises a base102, a target body104, and a ruler106.

The target body104is mounted to the base102. In one embodiment, the target body104is mounted to the base102via one or more hinges108. The hinges108allow the target body104to be folded down flush with the base102as illustrated inFIG. 2(e.g., for transport of the test target100) or to be rotated upward so that the target body104is positioned in a substantially perpendicular orientation relative to the base102as illustrated inFIG. 1(e.g., for use). In one embodiment, the target body104is formed of a sturdy plastic resin.

Printed on the face of the target body104is a high contrast two-toned (e.g., black and white) pattern. For example, the pattern may comprise white markings printed on a black target body104. In one embodiment, the pattern is substantially rectangular in shape. For instance, in one embodiment, a primary pattern110covers a majority of the target body104and comprises a square divided into four substantially equally sized quadrants. Two of the quadrants are black, and two of the quadrants are white. The black quadrants are positioned diagonally from each other, and the white quadrants are also positioned diagonally from each other. A perimeter around the square is also black and white. The portions of the perimeter that contact the black quadrants are white, while the portions of the perimeter that contact that white quadrants are black.

In addition, a smaller secondary pattern112having substantially the same configuration as the primary pattern110is superimposed over a portion of the primary pattern110and is aligned along an edge of the target body104In particular, the secondary pattern112is superimposed over a portion of the perimeter and over portions of two quadrants (i.e., the two quadrants closest to the ruler106) of the primary pattern110. In one embodiment, the secondary pattern112includes an additional second perimeter surrounding a first perimeter. The second perimeter is also black and white, where the black portions of the second perimeter contact the white portions of the first perimeter, and the white portions of the second perimeter contact the black portions of the first perimeter. The secondary pattern112is a high resolution pattern.

In one embodiment, the secondary pattern112is superimposed over a portion of the perimeter (and two corresponding quadrants) of the primary pattern110that abuts the zero marking on the ruler106(when the test target100is in its unfolded position as illustrated inFIG. 1). In one embodiment, the primary pattern110and the secondary pattern112share a horizontal center line124(i.e., a line that separates the upper two quadrants from the lower two quadrants in the primary pattern110and the secondary pattern112). This horizontal center line124is also the zero marking on the ruler106when the test target100is in its unfolded position; thus, the secondary pattern112is precisely lined up with the ruler's zero marking. Moreover, the secondary pattern112directly abuts the ruler's zero marking (i.e., there is no perimeter or other optical pattern or surface between the edge of the secondary pattern112and the zero marking). Thus, the small, highly resolved secondary pattern112is positioned right at the focal point of the camera when the text target100is in its unfolded position. This allows the depth-of-field and the resolution of the lens to come from the focal center of the lens. In one embodiment, the secondary pattern112is less than or equal to approximately one fourth the size of one of the quadrants of the primary pattern110. The position and smaller size of the secondary pattern112relative to the primary pattern110makes for minimal visual “clutter” on the target body104.

An extension122is coupled to the target body104for structural purposes (i.e., the extension122is not a part of the target). The extension122includes a flange116that defines a small shelf when the target body104is rotated upward.

The ruler106is positioned adjacent to the target body104and is also mounted to the base102. In one embodiment, the ruler106is mounted to the base102via one or more hinges114. The hinges114allow the ruler106to be folded down flush with the base102as illustrated inFIG. 2(e.g., for transport of the test target100) or to be rotated upward so that the ruler106forms an acute angle relative to the base102as illustrated inFIG. 1(e.g., for use).

In one embodiment, the ruler106is formed of a sturdy plastic resin. In one embodiment, the ruler106comprises a white inlay (formed, e.g., of plastic) having black markings. In one embodiment, the back of the ruler106(i.e., the side without the markings) includes a lip approximately at a midpoint (e.g., the zero line) along the length of the ruler106.FIG. 6, for example, illustrates a side view of the ruler106, allowing the lip128to be viewed. The lip128allows the ruler106to be locked into place against the shelf defined by the flange116that rotates upward with the extension122(e.g., as illustrated inFIG. 1). The flange116may also be folded flush with the base102(e.g., as illustrated inFIG. 2).

When the ruler106is rotated upward and locked into place, the zero line126of the ruler106is substantially even with the horizontal center line124of the primary pattern110and the secondary pattern112. The zero line106represents the midpoint of the ruler106and is the focal point of a camera in three dimensions.

In one embodiment, the test target100further comprises a tripod mount120formed in the base102. The tripod mount120is a threaded hole that allows the test target100to be mounted to a tripod. The base102may also include a level118(i.e., a device used to measure true horizontal or relative heights). For example, the level118may be a bubble level.

FIG. 3is a flow diagram illustrating one embodiment of a method300for using the test target100illustrated inFIGS. 1-2. Specifically, the method300illustrates how to use the test target to calibrate the autofocus feature for a given camera and lens combination.

The method300starts in step302. In step304, the user selects a location with appropriate lighting for the autofocus calibration. In one embodiment, the lighting must not be too dark or contain too much uncontrolled light directly behind the camera. Light from other angles in front of the target location is generally acceptable.

In step306, the user sets up the test target100. As discussed above, the test target100is opened from its flat locking position (e.g., as illustrated inFIG. 2), and the ruler106is positioned in the correct orientation with the target body104(e.g., as illustrated inFIG. 1). The test target100is then placed on a substantially flat and level surface. In one embodiment, the test target100is mounted on a tripod, via the tripod mount120. The level118may be used to determine the most effective alignment of the tripod with the camera.

In step308, the user selects the camera and lens combination. The user may elect to calibrate several lenses in combination with a given camera in order to determine the range of autofocus settings. In one embodiment, the camera is a DLSR camera or other camera capable of storing lens autofocus corrections, and the lens is an auto-focus lens capable of being driven by the camera.

In step310, the user sets the camera and lens combination to autofocus. In one embodiment, the camera is also set to the most open aperture setting available (e.g., 4.0 or 2.8 on many cameras). This produces the clearest example of the focal plane. The camera may need to be in aperture priority mode or manual mode in order to correctly set the aperture.

In step312, the camera is used to capture at least one image of the target body104(i.e., specifically the test pattern). In one embodiment, the camera is aimed at the center of the primary pattern110to be sure that the camera autofocuses on the correct plane. In another embodiment, the central focusing point of the camera is aimed at the edge of the secondary pattern112, at the exact zero point of the ruler106, for improved calibration accuracy. However, because this practice may in some cases disrupt the camera's auto-focus performance (due to the discontinuity of the surface at the attended point), another embodiment approximates this focus by aiming the camera at the center of the secondary pattern112. This allows the neighborhood of a single point to be assessed simultaneously in three dimensions. This also minimizes any error caused by not having the test target100orientated exactly perpendicular to the axis of the lens. In fact, even when the test target100is orientated exactly perpendicular to the axis of the lens, there will still be some focal distance error when focus off the lens axis is considered (e.g., as when the initial autofocus focuses on the middle of the primary pattern110).

As discussed above, there may be situations in which the ideal location to aim the camera is not at the edge of the secondary pattern112. For instance, cameras that have large focusing points (e.g., digital-single lens reflex cameras using wide angle lenses) will tend to include both the secondary pattern112and the ruler106inside the focusing point area, causing inconsistency and error in focusing performance.

If the camera highlights the chosen focus point (e.g., the center of the primary pattern110or the center of the secondary pattern112), then this confirms that the target plane has been selected as the intended focus plane.FIG. 4illustrates a configuration of camera and test target100in which the camera is positioned to capture an image of the target body104.

In step314, the user determines whether the autofocus settings are properly calibrated for the camera and lens combination being tested. In one embodiment, this involves selecting an image captured by the camera and zooming the image in on the portion of the ruler106that is positioned adjacent to the target body104.FIG. 5, for example, illustrates an image that has been zoomed in on a portion of the ruler106. At the camera's maximum zoom, the numbered markings on the ruler106are examined to determine which marking is sharpest. The sharpest mark represents the center of focus, as defined by the camera and lens combination using the autofocus feature. If the autofocus settings are properly calibrated for the camera and lens combination, then the sharpest mark should be the mark numbered zero.

If the autofocus settings are not properly calibrated for the camera and lens combination being tested (i.e., the sharpest mark is a mark other than zero), it may be necessary to adjust the autofocus settings in step316. It is necessary to identify whether the autofocus feature is front focusing or back focusing, and by how many markings on the ruler106. The autofocus settings may then be adjusted, for example, by moving the autofocus adjustment setting the appropriate number of points forward or backward.

In step318, the adjustments made to the autofocus settings are optionally stored in the camera. Specifically, the adjustments are stored with information for the camera and lens combination being tested. This allows the adjustments to be retrieved and duplicated the next time this camera and lens combination is used.

The method300terminates in step320. It may be necessary to repeat at least steps312-318to determine whether an adjustment made in step318was correct and to further adjust, if necessary. The method300may also be repeated for multiple different camera and lens combinations, or repeated for the same camera and lens combinations on a periodic basis.