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Wesley R. Elsberry

Posts: 4928
Joined: May 2002

 Quote (khan @ April 18 2012,19:15) Can someone explain what that f.. stuff means?

f-stops are a measure of light-gathering or light transmission capability.

Start with a pinhole. A small hole will serve to produce an image. The closer it is to the image plane, the brighter the resulting image is. So for a fixed aperture, or size of hole, the distance from image plane to the pinhole can change the amount of light arriving at the image plane. For a pinhole, the distance from the image plane can be treated as the focal length. A smaller pinhole that is closer to the image plane and a larger pinhole that is further from the image plane will produce the same amount of light impinging on the image plane. You can, in fact, derive a formula for this equivalent light-passing capability:

f-stop = {focal length} / {diameter of aperture}

Small f-stop numbers indicate aperture diameters closer to the focal length. Larger f-stop numbers mean the aperture diameter is small relative to the focal length.

Now, pinholes have to have a high f-stop in order for the images produced to look anywhere near sharp. When we start thinking about lenses, these can be considered ways to make our pinholes larger without losing so much sharpness. The lens elements focus light paths diverging from a subject that touch all across their surface to (just about) one point. (If the lens is well-adjusted, etc.)

An f-stop is easily calculated and works for just about anything you put in front of a piece of film or a sensor. Handheld light meters for photography simply present shutter speed and aperture combinations to tell the measured light level, which is pretty impressive given how many different lenses have been produced over the history of man's use of optics.

But f-stops are not the whole story. Passing light through a complex lens with many elements will tend to lose a small amount of light to reflection at each surface encountered on the way in. Modern lenses have coatings on the surfaces to reduce this light loss. The cinematic folks tend to use lenses that have T-stop ratings. These are on the same scale as f-stops, but take into account the particular light loss that each lens has.

There are tradeoffs. Getting lower f-stop numbers and thus greater light-gathering capability requires larger pieces of glass and more esoteric means of correcting optical errors that refraction introduces into images. That means that "faster" glass is more expensive to make. A common "fast" f-stop is f/2.8, or a light-gathering element with an effective diameter just a bit larger than one-third the focal length. Once you get past f/5.6, certain features like autofocus tend not to work. Those need a certain amount of light to do their work. Lou's new lens is an f/1.2, an exceptionally fast lens. It will permit almost three stops or eight times as much light through it wide-open as will an f/2.8 lens.

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"You can't teach an old dogma new tricks." - Dorothy Parker

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