Focus distance information in EXIF data

[Kim Pilegaard]

From the EXIF data it is possible to extract information on Focus Distance. Does anyone know how this information is derived and how precise it is? 

[Birna Rørslett]

There is a mechanically based read-out in the lens. Probably not very exact, as far as I can evaluate the numbers seen in my EXIF headers.

It would be easy to set up a controlled experiment to learn the accuracy, however.

[Kim Pilegaard]

Thanks for your answer, Birna.

Yes, I also thought to make a test with different known distances, but maybe someone already did that.

It seems from the results, I get, that it is not a continuum of distances, but only a specific set of distances (probably lens specific) is registered.

When for instance you photograph birds, it is nice to know the actual distance. It can also be worked out form a lens formula if you know the size of the bird.

[Birna Rørslett]

The contraption inside the lens is a contact "finger" that slides across a print with intermittent gaps. So yes, the distances are "zoned" and probably specific to a given lens.