On page 222 in View Camera Technique, Leslie Stroebel, 7th Edition, Focal Press, the author recommends focusing, and then increasing the lens-to-film distance by 1/400th to correct the focus for infrared film.
This might be difficult to implement on some cameras. It can be done more easily for finite subject distances by working with the subject distance instead of the image distance.
s = lens-to-subject distance
f = focal length of lens
Then the corrected infrared subject distance is
s’ = 401sf/(s + 400f)
Example: s = 3 meters = 3000mm and f =75mm
s’ = 401*3000mm*75mm/(3000mm + 400*75mm) = 2734mm
For several distances we get the following combinations:
s = 1.5 meters, f = 75mm, s’ = 1432mm
s = 1.75 meters, f = 75mm, s’ = 1658mm
s = 2 meters, f = 75mm, s’ = 1880mm
s = 2.5 meters, f = 75mm, s’ = 2313mm
s = 3 meters, f = 75mm, s’ = 2734mm
s = 3.5 meters, f = 75mm, s’ = 3142mm
With no infrared film to test this, I cannot confirm its usefulness. The above formula is equivalent to Mr. Stroebel’s infrared focus-correction recipe.
Last edited by Ian C; 01-27-2013 at 09:52 AM. Click to view previous post history.