 BetterPhoto Member |
F/stop - What is That? What is the meaning of the term "F/stop"?
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Hermann Graf |
With f-stop, the size of the aperture (= lens opening, abbr. f) is meant. With most lenses, there are stops at the aperture ring when reaching full sizes, such as f = 1.4, 2.0, 2.8, 4.0, etc. With the aperture ring, one can alter the size continuously in manual operation. Increasing values mean decreasing openings, i.e., less incident light. Exactly spoken, the numbers are the ratios of the focal length and the effective diameter of the opening of the lens.
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John P. Sandstedt |
The f-number is the ration of the focal length of the lens to the widest opening of the shutter. For a 50 mm lens, about 2 inches, the shuter is about 1 inch. So, 2/1 = 2 [f/2]. This is a measure of the speed of the lens. There actually are f/1 lens [Canon made one] and, of course, many manufacturers make f/1.4 lenses. Your lens probably has a number of stopping point [indents] at f-numbers such as f/2, f/3.5, f/4 f/5.6, f/6.3, f/8, f/11, f/16 and f/22. As you shift from one to another, you are "stopping down the lens. In fact, moving one stop, halves the amount of light allowed to reach the film plane. You also adjust your speed starting at, let's say 1/2000th sec. Each "normal adjustment" downward [slower] allows more light to pass to the film plane. Got the trend here? Set up a chart - starting at 1/1000 and going to 1 sec across the page. Then, on the next line put f/2 under 1/1000 and continue with the f-number sequence across the page. What you have created is one line of the Exposure Value Table. Any combination of f-number and speed will let in the same amount of light as any other combination on your chart. The EV table is the basis of your camera's automatic metering system. Hope this helps. John
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