Why the camera focus distance is not accurate (Technical)

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Richard Turton Regular Member • Posts: 321
Why the camera focus distance is not accurate (Technical)
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This shows the function the camera uses to convert lens focus position (EXIF tag Focus Step Count) to subject distance (EXIF tag Focus Distance).

This particular curve is for the 60mm f/2.8 macro lens, but the curves for other lenses will have a similar shape but different numerical values.  This lens was used because I was investigating focus stacking.  The function transitions abruptly from almost zero to almost infinity, making accurate distance estimation extremely difficult.

This shows the sharply curved part of the function below 5m, compared to the actual measured distance.  It can be seen that the camera approximates the actual smooth distance curve by a series of straight line segments.

The actual distance curve is very close to an inverse square function for distances less than 1.5m, but this function may be too time consuming to implement in real time, especially if the processor does not have a hardware divide instruction.

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