 BetterPhoto Member |
F/2.6 What does the F# stand for on my camera lens?
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BetterPhotoJim.com - Jim Miotke  Contact Jim Miotke Jim Miotke's Gallery |
Good question! The f # (or f-stop) is a way photographers have developed for keeping track of how much light gets to the film. The number comes from dividing the focal length of the lens by the diameter of the hole that the light is coming in through. So a 55mm "normal" lens with an aperture of 5mm would mean an f-stop of 11. Interestingly, this terminology dates back to when photographers controlled aperture by attaching various plates - each with a different sized hole drilled into it - to the camera to "stop" varying degrees of light from reaching the film plane. (Be grateful we don't have to do that anymore!) Used in conjunction with the shutter speed - how long the hole is opened - your selection of an f-stop controls how much the film gets exposed to light. One final note: in addition to the ring of f numbers you can choose from to control aperture, you will often see a single f number or range of f numbers written on the lens. This number, such as f2-f3.5, represents the minimum f-stop the lens can achieve. This is useful to many sports, action, and news photographers because it tells them how much light they will be getting when they shoot "wide open" - at the largest aperture. While these photographers generally need as much light getting to the film as possible - to stop action - nature and landscape photographers like Ansel Adams want the exact opposite. They like to use as high as possible of an f number to get the most depth of field. That's why he and others started the f64 club. This f number on the lens is a range (rather than a fixed number) when the lens can zoom from one focal length to another. More about using f-stop and shutter speed to control exposure
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John A. Lind |
Shawn, Some additional information about the f-stop numbers on a lens. You will notice the numbers 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and sometimes 22 commonly used. The amount of light admitted is directly proportional to the *area* of the aperture, not its diameter. If you double its diameter, you quadruple (4 times) the amount of light. Similarly, if you halve the diameter, you only admit 1/4th the amount of light. This is because the area of a circle is pi*(1/2*d)^2 (proportional to the diameter squared). A one "stop" change in exposure is either twice or half the previous exposure setting. You will notice with your shutter speeds that they go up by half the previous one, cutting the light in half with each "stop" in shutter speed. Pull out your calculator now and find the square root of two. It is approximately 1.414 which should look vaguely familiar. The f-stop numbers are powers of the square root of two, rounded off to one or two digits, which allows changing the aperture a "stop" at a time by doubling or halving the previous *area* (not the diameter) which doubles or halves the amount of light. To double the area of a circle, you increase its diameter by multiplying it times the square root of two. Thus the mystery of why the specific f-stop numbers are what they are is revealed: 1.4 ~= sqrt(2) Close is good enough for an aperture diameter. In the late 1800's during days of Waterhouse Stops (the plates with circles in them betterphoto.com mentioned) the aperture diameter was measured and the f-stop of the hole being used calculated using its diameter and the focal length of the lens into which it was inserted. Without a calculator, approximations were heavily used and this carries through to the numbers lenses are marked with today.
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