Originally posted by L&D Hi Lowell,
thanks for the reply. No i am not considering macro.
Just full body shots of people, so i'd say image height is slightly greater than subject height by few inches. I know subject distance can be easily obtained through viewfinder, by getting closer or farther from the subject (with prime lens), keeping the difference between subject and image height to minimum. But i was thinking perhaps there is a formula to get the numeric value of subject distance, so that it can be properly used for DOF calculations. I'm sure Angle of View of the lens has also something to do with the subject distance.
OK if this is what you are looking for, then start with switching this formula around,
Distance = subject height * focal length / Image height
The image height is easy, that is 16mm high and 24mm wide (i.e. the format of your sensor)
then, go on line and take any DOF calculator and consider the DOF vs aperture for the lens you pick. BUT, I did an exercise a while back where I looked at the change in DOF vs shooting distance and found that what it all boiled down to was that in a first order, the Depth of field in actual units of measure did not change as you changed focal length and shooting distance to maintain the subject image the same.
What this means is if you shoot from 5 feet at F2.8 with a 50mm lens, shooting from 10 feet at F2.8 with a 100mm lens will have basically the same depth of field.