BetterPhoto Q&A
Category: New Answers

slr macro work

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March 29, 2003

If you are planning lighting and film speed beforehand . . . or are using a hand-held meter or an estimating method that is not measuring through the camera lens:

The exposure correction factor required can be calculated for the lens' focus setting in the infinity position if the length of the extenstion tube or bellows is known:
Extension Factor = [(f + x) / f]^2
f = focal length
x = extension (or bellows) length
"Extension Factor" is similar to filter correction factors. It is only a factor for multiplying by the shutter speed.

To adjust an aperture setting, divide the aperture f-number by the square root of Extension Factor for the new aperture f-number. To find the compensation in EV (stops), use the following:
Extension Compensation = 2*log[(f + x) / f] / log(2)
f = focal length
x = extension (or bellows) length
For the logarithms you can use base 10 logarithms (a.k.a. "common") or natural logarithms (a.k.a. base "e" or Naperian) . . . it doesn't matter.

-- John

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March 30, 2003