T3D Re: Med Formatn needs more light? (TECH-3D digest 379)

[bobh@xxxxxxxxxxxxxxxxxx]

RE: Tom is right about square law distance..BUT he forgets that is 
why we have the f/stop system of goofy numbers. BY
 MAKING that number a ratio of focal length and aperture diameter, 
all f/stops give the right exposure for any lens AT INFINITY where 
they are calibrated.  So f/8 on a 1-inch lens on a movie camera is a 
1/8 inch aperture 1 inch from the film. It gives exactly the same light 
on its film as an 8-inch lens on a medimu format camera at f/8 
which is a  1-inch aperture eight inches from its film. The one inch 
aperture and the 1/8 inch one are equalled by the square law fall off 
at the focal length respectively.
NOW when you do close up focusing that apreciably extends the 
lens from the infinity point, the calibrated f/stop number no longer 
holds and you need the new distance factored in..it is common 
knowledge that  double extension (of normal focal length at inf.) 
gives life size image and requires two stops or 4x the exposure as at 
inf.  
So it is NOT true that medium format needs more exposure if you 
use f.stops and correct of extenstion..i.e. f.stops take care of the 
format differences and is why they are used. 
BobH
BOB HOWARD (bobh@xxxxxxxxxxxxx)
DO NOT SEND PICTURES OR PROGRAM FILES TO LIGHTSIDE!
Send those to:  robhoward@xxxxxxxxxxx or wa6dli@xxxxxxxxxxxxxx
or bobh626@xxxxxxxxxxx


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