At 09:25 PM 7/31/03, you wrote:
I figured someone would "best" me somehow. :)
At first I thought this would be a very minor correction. Then I thought
of the 50mm f/3.5 macro. When that helical is racked all
the way out for close focusing the lens extension (LE) would be well worth
correcting for with slide film.
I guess if you have internal focussing, like the AF camp usually does, LE
is essentially zero?
Lama
No. Some have an "effective" extension as if it were physically extended;
others exhibit focal length variation with focusing; and yet others exhibit
both. Using a zoom lenses that employ internal focusing with extension
tubes is much more difficult than with primes, and not just with exposure
compensation (if it's required). Sometimes its easier to calculate
compensation based on magnification. With that method, exact isn't
necessary. A reasonable estimate works. Estimating magnification is
relatively simple if subject size is easily measured or estimated in
English units, using a film frame of 1 X 1.5 inches, and estimating how
much of the film is filled horizontally or vertically with the
subject. Equations for this are:
Exposure Factor = ( 1 + M )^2,
where M = magnification on film.
Use this to increase shutter speed.
EV Compensation = 2 * log( 1 + M ) / log(2)
This represents compensation required in f-stops.
As with the other equations, further discussion is on my web site:
http://johnlind.tripod.com/science/scienceexposure.html#Macrophotography
I most often use the magnification method when working with monolights and
simply dial in the EV compensation needed on the Gossen Luna Pro F dial.
-- John
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