 Daniel Deschner |
Understanding f-stop ratios. I want to understand how f-stop ratios work. I would like to know what does '1/3' and '2/3' or '2-2/3' f-stops mean? Thank you,
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Jon Close |
Each full stop change in exposure is a doubling (or halving) of the amount of light falling on the film/sensor. For shutter speeds and ISO rating, 1 stop changes are by a factor of 2x (with some rounding for simplicity): 1 sec., 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000... ISO 25, 50, 100, 200, 400, 800, 1600, 3200,... 1 stop changes in aperture are by a factor of the square root of 2 (appox. 1.4142 or 1.4), again with rounding for simplicity: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. This is because the area of the diaphram opening changes with the square of the diameter, so a x1.4142 change in diameter gives a x2 change in area. Exposure changes can also be made in smaller 1/2 or 1/3 stop increments. For shutter speeds and ISO, 1/2 stop increments are by a factor of 1.414x (square root of 2), so the change from 1/90 to 1/125 is 1/2 stop. 1/3 stop change is 1.2599x (3rd root of 2), so the difference between 1/100 and 1/125 is about 1/3 stop. ISO is most commonly adjustable in 1/3 stops only, the progression being 25, 32, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800,... For apertures, 1/2 stop changes are a factor of 1.1892x (4th root of 2) so f/5.6 to f/6.7 is 1/2 stop; and 1/3 stop changes are a factor of 1.1225x (6th root of 2) so f/5.6 to f/6.3 is 1/3 stop.
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