Skip,
While that may work if you can provide TTL/OTF exposure control, or are
using available light and use the TTL metering, everything suddenly changes
when using flash that isn't under TTL/OTF control. Exposure compensation
is required when subject distance is closer than about 8X lens focal
length. I had to become saavy about how to calculate the amount of
compensation when I started making macros using studio strobes on stands
and firing them with a PC cord connected to the camera. Although my Mamiya
M645 has TTL metering which works for available light, it has no AE mode
and cannot control any flash TTL/OTF, on or off camera.
Although the strobes have an adjustable output, incident light level must
be measured using a flash meter. The reading from the flash meter must be
adjusted for exposure compensation depending on magnification (this is
dependent on total lens extension from infinity and the focal length of the
lens).
One can do the math using the equations I presented, or there's perhaps
another method I had nearly forgotten about. Kodak's Master Photoguide
(aka Pocket Photoguide) and Kodak's Professional Photoguide both have a
nifty wheel that can be used to find exposure compensation for macros. It
has markings for lens extension and for magnification, depending on what is
easiest to measure.
I have also worked the problem using subject magnification by simply puting
a small ruler into the subject material being photographed temporarily at
critical focus distance once the image to be made is composed and
focused. By measuring the width or the height of the subject from edge to
edge as seen through the viewfinder the magnification can be
estimated. The 35mm film frame is about 1 inch tall and 1.5 inches
wide. Divide the appropriate one (width or height) by what is seen on the
ruler and Bob's your uncle, one has a sufficiently accurate estimate of
subject magnification for estimating exposure compensation.
It's not that difficult after doing it a few times . . . and it works well,
even with finicky transparency films (including Kodachrome).
-- John
At 08:48 9/25/02, you wrote:
Mike,
what "real" difference does it make? The AE OM's TTL exposure will handle
the light falloff issues, right? If you're doing macro with a manual body,
you're more of a man than me. Or at least one with more patience.
Translation: I don't know the answer and I'm too lazy to research it,
preferring to throw hardware at the problem instead of figuring it out.
Skip
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