because i always lose this piece of paper
Easy bellows extension exposure compensation. All you need is [a] a list and [b] two numbers.
a) The list: F-stops in 1/3 increments. Keep this close.
3.5
4
4.5
5
5.6
6.3
7.1
8
9
10
11
13
14
16
18
20
22
25
28
32
b) First number: know the focal length of your lens in inches; divide mm by 25.4 to arrive at the customary unit. Some common conversions: 90mm=3.5″ 150mm=6″ 210mm=8.26″ 240mm=9.4″ 300mm=11.8″ Locate this number (as close as possible) on the F-stop list.
3) Second number: measure the bellows extension in inches from the center of the lens plane to the center of the film plane. Locate this number (again, as close as possible) on the F-stop list.
Method: total the number of 1/3 stop differences on the list between the focal length and the bellows extension and that will be the approximate increase in exposure required.
Example: 150mm/6″ lens is about 6.3 on the F-stop list and with a bellows extension of 9″ would be 1 full stop of added exposure. Another: 210mm/8.26″ lens is 8 on the F-stop list with a bellows extension of 10″ would equal a 2/3 stop increase in exposure.