The easy way is to program a pocket calculator and take it with you. If you have trouble programming it there are many high school or college students whoíd do it for you in a minute. You likely know at least one such person whoíd do it for you.

Once thatís done you carry a metric roll-up tape measure or a dressmakerís tape (they have metric versions very cheap at any fabric shop). Hereís all you do.

1. Compose and focus

2. Measure the distance E from film plane to diaphragm in millimeters (this isnít fussy)

3. Enter f, E, and press the execution key to display the answer in f stops

What could be simpler?

The equation is

x = 2*ln(E/f)/ln2

where x = number of additional f stops for the bellows extension, E = bellows extension measured from the film plane to the diaphragm, and f = focal length of your lens.

It isnít rocket science and you really donít need to know any math to use it.