Mike,
Effective aperture is a function of subject magnification on film. That is
determined by total lens extension from infinity focus and the lens' focal
length. The equation for this is sometimes referred to as the "bellows
factor":
f = lens focal length in millimeters
x = bellows or extension tube length in millimeters
y = lens extension from lens focusing helical,
if focused closer than infinity
A = aperture
For exposure compensation by adjusting shutter speed:
Extension Exposure Time Factor = [(f + x + y) / f]^2
This is similar to compensation for filters.
Multiply metered shutter speed by this factor.
For exposure compensation by adjusting lens aperture:
Extension Exposure Aperture Factor = f / (f + x + y)
Multiply metered aperture by this factor.
For exposure compensation in EV:
Extension EV Compensation = 2*log10[(f + x + y) / f] / log10(2)
Effective aperture using aperture markings on lens:
(Effective A) = A * [(f + x + y) / f]^2
I have more discussion of exposure compensation when using lens extensions
on my web site here:
http://johnlind.tripod.com/science/scienceexposure.html#Macrophotography
IMPORTANT:
Remember that compensation is *only* required if metering using a hand-held
ambient light or flash meter. If you meter TTL, or control a flash TTL,
the camera body's metering will automagically compensate as it's measuring
the light through the lens. I didn't get into using these compensation
equations routinely until I started using studio strobes, the output of
which cannot be controlled by the OM-4 or OM-2S with their TTL/OTF
metering. They can only trigger the strobes. If they're operated in
TTL/OTF mode with a flash such as the T-32, on or off camera (using a TTL
cord), manual compensation is not required.
-- John
At 22:04 9/24/02, Mike wrote:
What happens to the effective maximum aperture of the macro lenses (80,
135) when they are mounted on the tele auto tube or bellows for which
they are obviously designed to be primarily used. Are they rated at f4,
f4.5 with the tube? And I suppose the total amount of extention matters?
How would I compare the 80/f4 for example with the 50/f2 + 25mm? Or say
the 50/2 + 2xa and the 135/4.5? I'm totally overwhelmed by the
possibilities. Can someone recommend a book that would help sort this
out?
mike
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