GoofyNewfie wrote:
F/stops are fractions , so f/4 (1/4) is bigger than f/16 (1/16)
True. But actually it might be more accurate or understandable to think of f-stops as a "ratio"... F = focal length.... So an F-stop is "focal length divided by X". (In fact, many lenses are actually marked with a ratio such as 1:3.5, instead of f/3.5. But when discussing f-stops, we tend to write the latter.)
For example:
A 100mm lens at f/4 will have an opening that's 25mm in diameter (100/4 or 100 ÷ 4). At f/16 the same lens aperture is closed down to give an opening that's 6.25mm in diameter (100/16 or 100 ÷ 16, which will allow less light to pass through).
On a 200mm lens f/4 would make for a 50mm aperture, while f/16 would provide one that's 12.5mm in diameter.
On a 50mm lens f/4 is 12.5mm and f/16 is 3.125mm.
When you think of it as illustrated in the examples above, it might make more sense why the higher the number, the smaller the aperture opening.
F-stops regulate the amount of light that can pass through a lens. Each "full" f-stop smaller reduces the amount of light being allowed to pass through by one half. Full stops are f1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc. (Note that every other smaller f-stop the divisor "doubles".... 1, 2, 4, 8, etc. and 1.4, 2.8, 5.6, 11, etc. This can help when trying to remember the sequence of stops.)
F-stops ALSO control depth of field (which is also effected by focal length & distances). Basically, a larger aperture makes for shallower depth of field (the "plane of apparent sharp focus"), while a smaller aperture increases DoF.
Hope this makes sense... it only took me about 10 years to finally "get it"!