Originally Posted by
Mark Sawyer
If measuring the aperture is difficult in your usual situation, just make a chart of the aperture sizes at the standard f/stops. Say you have a 210mm lens; at f/5.6, divide 210mm by 5.6, your actual aperture size is 37.5mm. At f/8, 210mm divided by 8 is 26.25mm, etc. Now just measure your bellows extension and divide by your aperture size and you'll have your actual, real, working f/stop value.
Or you can divide the square of this by the square of that and get another number to multiply your exposure by...
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