For example, if the bellows extension is 18 inches and the focal length is 12 inches then:
that is: 18 x 18 = 324, 12 x 12 = 144, 324 divided by 144 = 2.25, or to keep the numbers
smaller: 18 divided by 12 = 1.5. 1.5 x 1.5 = 2.25. Your bellows extension is 2.25.
(2) The second method. Divide the image size by the original size, add one, and then square
the result as shown by the following formula:
For example, if the image on your glass is 8 inches high and the original is 12 inches
high, then the BEF equals:
Your bellows extension factor is 2.8.
(3) If you are working with millimeters (mm) the equation is the same. Remember that 1
inch equals 25.4mm. For practical purposes, we round that off to 1 inch equals 25mm. This is close
enough for normal copy work. If you are doing very critical scientific copy work then the 25.4 figure
should be used.
(a) Using the same lens and bellow extension as in (1) above we find that 18 inches
equals 450mm and 12 inches equals 300mm. The equation then reads:
(b) Your bellows extension factor is 2.25. It makes no difference if you use inches or
millimeters, just make sure all the numbers are the same, all inches or all millimeters.
(4) After completing one of the above steps, you now multiply your basic exposure by the
bellows extension factor. If your BEF is 2.25 and your basic exposure was 8 seconds at f/16, then your
new exposure would be 18 seconds at f/16:
8 x 2.25 = 18, or 18 seconds at f/16