Image size calculation

vdonovan · 07-23-2008

I want to shoot a portrait in which the face fills the frame. I want to use a telephoto lens to get some foreshortening effect. I used the following calculation to see if a lens would give me what I want. The lens is a 250mm which can focus to 9 feet. Is this correct?

F = lens focal length = 250mm = 9.84 inches
G = object size = 12 inches (size of a human head, more or less)
A = distance from lens to object = 108 inches (9 feet)

I = FG/A

I = 9.84 * 12 / 108

i = 1.09 inches

So the resulting image will be 1 inch high, which would fill a 35mm frame but not a 120 frame.

Right? Not right?

ferider · 07-23-2008

Correct.

To fill a 6x6 you have to be 12 * 9.84 / 2.25 inches = 52 inches away.

Fred, there are plenty of 250s for medium format.

Roland.

vdonovan · 07-23-2008

Thanks, gents. I am indeed shooting medium format. I'm on a quest for the perfect close-up portrait lens for my Bronica so I wanted to make sure I was using the formula right.

Thanks for the compliment Fred. How 'bout I shoot your portrait for free next time I'm in NY? I'm out there about twice a year.

thanks again and best regards,