Question

f/2.8 f 11 f/16 f/5.6 f/22

from wiki it is saying

The f-number N or f# is given by:

$N={\frac {f}{D}}\$

where is the focal length, and is the diameter of the entrance pupil (effective aperture). I feel really confused about what is this D mean? for example, in the first circle, D is 2, but second D is 2.8. How could that possible the size of mid area(D = 2) is larger than size of (D = 2.8)?

Answer 1

the relation is right. but

Aperture openings are measured as fractions of the focal length of a lens. That is what the 'f' stands for in the aperture rating, 'focal length'. Assuming we have the epitome of lenses, the 50mm, with an aperture of f/2.8, we can determine the actual diameter of the aperture opening like so:

50mm / 2.8 = 17.85mm

If we open the aperture up to its maximum of, say, 1.4, we can measure that as well:

50mm / 1.4 = 35.71mm

The difference between an aperture of f/2.8 and an aperture of f/1.4 is a difference of four times as much light...or two stops. We know this because the area of the aperture opening itself is four times as large at f/1.4 (1001.54mm^2) as it is at f/2.8 (250.25mm^2). A stop in photography nomenclature means a difference of one exposure value, which is the doubling, or halving, of the amount of light reaching the sensor. There are a few standard "full stops" that f-numbers are rated in

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64

These aperture settings all differ by one full exposure value, or one full "stop", and create the full f-stop scale. When you close down your 50mm f/1.4 lens from its maximum aperture of f/1.4 to an aperture of f/2.8, you are "stopping down" by two full stops.