About: HOWTO: Calibrating the distance of the A620

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Richard Lemieux, 2007-05-24 The purpose of distance calibration is to get correct distance numbers from the camera software. The intent is to be able to compute distance related numbers such as the depth of field and the optimal stereo separation distance. The process of calibration returns parameter values that are used in a program that displays distance related numbers such as program rf_3.c [2]. In the following, I describe the procedure I used to calibrate my A620. Expect to be taking many measurements if you do this procedure.

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rdfs:label	HOWTO: Calibrating the distance of the A620
rdfs:comment	Richard Lemieux, 2007-05-24 The purpose of distance calibration is to get correct distance numbers from the camera software. The intent is to be able to compute distance related numbers such as the depth of field and the optimal stereo separation distance. The process of calibration returns parameter values that are used in a program that displays distance related numbers such as program rf_3.c [2]. In the following, I describe the procedure I used to calibrate my A620. Expect to be taking many measurements if you do this procedure.
dcterms:subject	CHDK
abstract	Richard Lemieux, 2007-05-24 The purpose of distance calibration is to get correct distance numbers from the camera software. The intent is to be able to compute distance related numbers such as the depth of field and the optimal stereo separation distance. The process of calibration returns parameter values that are used in a program that displays distance related numbers such as program rf_3.c [2]. The A620 already provides a distance number. The distance number provided by Canon is used in the 'distance scale' shown in manual focus mode and while doing 'autofocus lock' (shutter half-press and down arrow). The 'Canon distance number' can also be found in the EXIF data (Look for FocusDistanceUpper if using Phil Harvey's Exif tool). The CHDK will display that number in the OSD/Misc window. The 'Canon distance number' returned by the A620 is an integer between 0 and 65535 (a 16-bit number) and is interpreted as a distance in millimeters. Both the CHDK and the EXIF viewer may scale that number and show the distance either in meters or centimeters. The 'Canon distance number' appears to be pretty close to the real distance if the distance is measured from the back of the camera and is small (roughly less than half the hyperfocal distance). At larger distances the number returned by Canon is way off; the errors are more apparent at smaller focal lengths but are still there at the maximum zoom position. Focusing is performed by moving a lens group within the lens. This is different from the movement of the lens occurring when using the zoom control. The movement of the lens group associated with focusing is done by a stepping motor and associated gears and is very precise. It is so precise that the camera can memorize the focus position and return to it days later if using the Custom mode. Calibration is made possible by the use of a model relating the "Canon distance number" and the measured distance. Such a model is developed in [1]. The resulting model appears to be good enough for practical use. The model returns a formula that translates a 'Canon distance number' into an actual distance. The formula is dependent on three parameters. The purpose of calibration is to find out figures for those three parameters. An open question is whether the parameter figures obtained by calibrating one A620 can be used for all other A620's. I conjecture that two of the parameters at least ('d0' and 'a1') are pretty much fixed, while the third parameter 'si' could be obtained by simply looking at the EXIF data of pictures of far away subjects. In the following, I describe the procedure I used to calibrate my A620. Expect to be taking many measurements if you do this procedure.

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