@rpt

Yes, an 'impossible' task, so I'm simply going to use the focal length as an approximation.

After all, the distance between the sensor and the principal plane of the lens, for non-macro work, is insignificant compared to the principal plane of the lens to the focus distance; especially for wide angle work.

Also, the depth of field equations are already an approximation, i.e. assuming a simple, thin lens and ignoring the pupil magnification.

Cheers

Garry