How to measure distance to subject in Depth of Field calculator?

Question

I used Depth of field calculator here.

I should specify subject distance.

How to measure distance to subject?

In theory this is a distance to front principal plane or point. What is in practice?

I tried to calculate expected image size and then made shot with camera. In calculation I measured the subject distance from front lens of photo lens. This result that "theory image" about 1.5 times larger then "practical image".

Asked by sergdev

Answer

According to the FAQ for the particular calculator you're using, the calculations are performed for a thin lens, and hence the front nodal point of the lens should be used. The author recommends using the front surface of the lens, on the assumption that the front nodal point is somewhere inside the lens, and this assumption will yield a conservative estimate of the DoF.

By the way, note that in lens specifications, focusing distances are specified from the film/sensor plane. This location is often marked with a special symbol on the camera body. So these measurements are not exactly comparable to what the DoF calculator uses.

Answered by coneslayer

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