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Candace L. Carrillo
member since: 6/12/2008
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F-stops for Lenses?
What does the f/stop mean on lenses? For example, if you have a 85mm-200mm f4 lens, does the f4 mean that f4 is the widest aperture you will be able to use with that lens? Thanks!
1/7/2009 4:29:45 PM
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Pete H
member since: 8/9/2005
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Candace, It is odd there are not TWO numbers on your zoom. If there is only the one f/stop number; it probably means at 200mm your widest F/stop is f/4. Usually zooms have 2 numbers. For instance, the 18-200mm Nikkor is rated at 3.5/5.6 which means the widest aperture at 18mm is 3.5 and 5.6@200mm.
These are best case numbers.
Pete
1/7/2009 6:39:32 PM
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John G. Clifford Jr
member since: 8/18/2005
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The f-stop, or aperture, is a number that indicates the relative size of the opening which light will pass through. F-stops are relative, not absolute, because they depend on the focal length of the lens. The f-stop is really a ratio. For instance a 50mm lens with a maximum aperture of f/2 has a 25mm maximum opening for light to go through (50mm divided by 2 = 25mm). At f/4, the opening would be 12.5mm, at f/5.6 it would be 50/5.6, or 8.9mm. Equally obvious, a 200mm f/4 lens has a max opening of 50mm (200 divided by 4). You can see that a 300/2.8 lens is going to be a lot larger around than a 300/4, and that a 70-200/3.5 lens will be larger in diameter than a 70-200/4.5 lens.
1/7/2009 7:50:45 PM
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John G. Clifford Jr
member since: 8/18/2005
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Yes, the f-stop number given on the lens is the maximum (largest) aperture that the lens provides. For instance, a 70-200/4 has a maximum aperture of f/4, and a 70-200/3.5-4.5 has a maximum aperture at 70mm of f/3.5, decreasing to a maximum aperture of f/4.5 at 200mm.
1/8/2009 1:34:36 AM
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Alan N. Marcus
member since: 3/4/2006
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Hi Candace
The camera lens acts like a slide projector only in reverse. Its job is to project an image of the outside world onto a screen at the back of the camera. This screen is the surface of the digital chip or film. How much light will be admitted is a function of the lens’s aperture.
This is a circular opening is like the pupil at in the center of our eye. The area surrounding the human pupil is called the Iris because it is colored (blue – green – brown etc.) named for the Greek goddess of the rainbow. The camera’s aperture is also adjustable as to diameter. The mechanism is a series of overlapping thin metal leaves. This device is called an iris diaphragm. The large the diameter of the aperture the more light admitted. In addition to the size of the aperture, the focal length of the lens also has a direct effect on how much light is admitted. Long focal lengths project a larger narrow angled image however they admit reduced amounts of light. Short focal lengths project a tiny wide-angle image however they admit oodles of light. Both the aperture and the focal length must be taken into account when assessing how much light will be admitted. Knowing this amount is critical to setting the camera for correct exposure. Thus aperture and focal length are intertwined. Only math can take the chaos away. The problem: with a world filled with lenses of all different diameters and focal lengths, how can we come up with a universal system that makes sense? Because the volume of light admitted in proportional to the working aperture divided into the focal length, a ratio best fills this bill. In English speaking countries we write this focal ratio as f/2, f/2.8, etc. The beautiful thing about the f/number system is; any lens functioning at the same f/number as any other lens, regardless of size, admits the same quantity of light thus the chaos is remived. By convention the number engraved on the lens barrel is the one that admits the most light. Since a zoom lens changes focal length a mechanical linkage must adjust the size of the aperture as the lens is zoomed. This linkage keeps the f/number constant through the various changes in focal length. Most zooms have more zoom than aperture meaning as the zoom approaches maximum the iris runs out of ability to enlarge. Thus two f/number values are engraved on the barrel. One is the f/number at maximum zoom, the other the effective f/number for reduced zoom focal lengths. Nobody said this stuff is easy! Alan Marcus (marginal technical gobbledygook)
1/8/2009 8:09:30 AM
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