You need only carry a pocket inch tape measure.
When doing close-up work and your subject is closer than 8 times the focal length from the lens you will need to compensate for the extension. Take the focal length of the lens in inches and the length of the bellows in inches. Make the measurements unit the f-stop and judge what is the difference.
I.e. 210mm lens = 8"(approximately}
With a 16" extension of the bellows the extension factor would be 2 stops.
210mm=8"=F8 16"extension =F16
The difference between F8 and F16 is 2 stops.
You should compensate as much as possible by shutter speed and deal with the fraction of a stop with the aperture.
Hope this helps.
Rod
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