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Distance to subject from subject size in frame.

dankeny

Roland, US
1555 posts

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dankeny Gold Member Nikonian since 29th May 2006
Tue 23-Jul-13 06:43 PM

Assume you know the height of the subject. How to you get the distance. My logic (?)

tan t = a/b where t = the angle theta between sides a and b, a is height and b is the distance we are solving for.

if the subject is the exact size of the diagonal (Max angle of view) then the distance is given by

b = a / tan t

Lets say the lens is 400mm on the 7100. theta = 4 degrees, tan theta = .07
Assume the subject fills the frame diagonally and is 14 feet (or meters or cubits, come to it)

b = 14 / .07 = 200 feet (or meters or whatever)

if the subject is a fraction of the the frame say 720 px, then the distance increased by the inverse of the fraction. for simplicity the diagonal of the D7100 is about 7200 px. So the distance is:

b = 200 feet * 7200 / 720 = 2000 feet.

If you followed me so far, you may be saying, so what? The what is that it's wrong. And I don't know why.

David

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