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[OM] ?simple? problem driving me nuts... calculating magnification from

Subject: [OM] ?simple? problem driving me nuts... calculating magnification from focal length and focusing distance
From: jking@xxxxxxxxxxxxxx
Date: Wed, 2 Nov 2005 10:00:37 -0000 (GMT)
ok, I may be about to open myself up to calls of idiot but here goes
anyway....

I would like to calculate how much light loss there is depending upon the
lens focusing distance and lens focal length. This is bcause I mainly use
an incident light meter and want to be able to compensate for the light
loss at close focusing distances e.g. macro but also taking photos of
statues at close range etc.

I found a nice equation that can calculate the light loss based upon the
magnification ratio and the tamron 90mm macro lens has course
magnification marks on it. Fine. I converted the light loss ratio into
stops of light loss, ploted and plotted a graph. I found that until you
get to about 1:8 the light loss is less than 1/3 of a stop. By 1:1 its two
stops and very significant. Great what about my other lenses that only
have focusing distance and not magnification ratios marked on them...

Intuition says to me it must be possible to get the magnification ratio
from the focal length and the focusing distance but everything I tried did
not work. Part of the problem was that I had little additional information
about my other lenses other than minimum focusing distance and for some of
them the magnification ratio at that distance so I was playing with three
variables, focal length, minimum distance and mag ratio. I got out my copy
of ansel adams the camera and used his equations for working out image to
nodal point vs magnification and subject to nodal point vs magnification
and I could find no match between them and the markings on the tamron!
HELP
am I crazy? (probably) can it be done... I guess so since macro lenses
have magnification ratios on them as well as focusing distances...


James



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