# Recovering physical distance from perceived distance in telephoto lens

I am trying to find a way to estimate the DOF from the perceived distance through a telephoto lens. The problem is, most DOF calculators use the physical distance from the subject to the lens to calculate the DOF. But for the telephoto range, it is much easier and practical to estimate the perceived distance of the object as it appears in the EVF rather than the actual physical distance due to how far the subject is from the camera. So suppose the focal length is `f` and the subject seems to be `x` meters away from me through the lens, is there a way to recover the physical distance `s` of the subject?

Also, due to perspective distortion, the scene will look compressed through a telephoto lens. Suppose, looking through the lens, I estimate the distance between two objects to be `y` meters (along the depth axis), and I want both objects to look sharp. Should the physical depth of field be `y`, or should I adjust that as well?

What is perceived distance? If you know the physical size of the object you can determine its' physical distance by how much of the FOV it occupies.

The math is: (sensor dimension ÷ focal length) = (size ÷ distance)

Or, there's an app for that

Secondly, the DOF always has to be equivalent to the actual physical spacing of the objects ("y").

And thirdly, there is no such thing as perspective distortion nor compression. There is only perspective (relative distance) and it is the same for every lens at the same distance (the different lens FOV's only crop the scene before you).