i would like to know (both rough & approximate) how to determine magnification(i mean object size to size of image recorded on film ratio)from data like focal length and focusing distance. thank for answering

-- legnum (legnum212@email.com), September 18, 2001

Answers

First primciples:
1/F = 1/v + 1/u ; where F is the focal length of the lens; v is the distance from the rear node of the lens to the focal plane; and u is the distance from the subject to the front node of the lens. For most practical purposes you can take the front and rear nodes to be co-incident at the physical centre of the lens.
Then the magnification is given by m = v/u.
These two equations are about all you'll ever need. They can be mixed and matched to derive everything from exposure compensation to depth-of-field.

For example: To derive the magnification with a lens focal length of 100mm and a subject distance of 1m (1,000mm)
1/v = 1/F - 1/u = 1/100 - 1/1000 = 0.009. So v is the reciprocal of 0.009, which (according to the Windows calculator) is 111.111.mm. The magnification therefore is 111.11/1000, or 0.1111, or 1/9th in plain English.

-- Pete Andrews (p.l.andrews@bham.ac.uk), September 20, 2001.

BTW, 'primciples' are like principles, only more inhibited.
Jeez, I wish I could spot these typos BEFORE I hint tha Smubit bottom.

-- Pete Andrews (p.l.andrews@bham.ac.uk), September 20, 2001.

Pete's answer is excellent; but since you asked for "rough & approximate", I can get rougher and approximater. The magnification ratio is always the distance from the lens to the film divided by the distance from the lens to the subject. At "normal" focusing distances, the lens-to-film distance is approximately the focal length, so at a distance of 500mm, you have 50/500 = 1:10. It's actually more like 1:9 due to the slight lens extension, but i promised to be rough so we'll ignore that. As you get closer, you have to factor in the lens extension. If the extension is 1/2 the focal length, magnification will be 1:2. When extension equals focal length, magnification is 1:1. At extension if double the focal length, magnification is 2:1, and so on.

rick :)=

rick_oleson.tripod.com

-- Rick Oleson (rick_oleson@yahoo.com), September 21, 2001.