Re: [Leica] Re:50mm f1.5 Nokton question

["Dan Post" <dpost@triad.rr.com>]

Point to ponder:

When we talk about the degrees in between an 'f stop', normally, older
camera books talked about 1/3 increments.
This is because the increase of exposure is logrithmic in most sensitometric
situations.
If you have a given amount of light entering a lens/camera, doubling it
gives a logrithmic increase of exposure of .3; If you double it again, the
logrithmic value of the overall increase is .6, and doubling it again, a
value of .9.
Since common logrithms are based on 10, an exposure increase of .9 means an
overall increase of 8 time the original exposure ( 2X2=4, 2X4=8) if you go
to a log increase of 1.0 from .9, then the exposure is ten times the
original-( 10 to the first power is 1, 10 to the second power is 10!)

By determining the log increase of light with each increment of aperture
diameter, it is relatively easy to caluclate the aperture needed to give
exposure differences of  log .1

Since there is a lot of 'slop' in the process of processing film- it is hard
to be exact all the time, I would venture to say- and this is from my
experience, that any increment smaller than 1/3 or even 1/2 of a stop is not
going to be detectable unless you are in a lab or darkroom equipt with
sensitometric instruments.

I am sure Erwin might be able to enlighten us in that area. In testing
lenses, I am sure that what we call 'falloff' in the corners, and edges of
lenses is the cosine error introduced by a lens and if it is measurable,
then I am sure that the log differences in the various 'f stops' could be
measured, and tested against actual exposure on the film.

Dan