 BetterPhoto Member |
what does equivelent exposure mean What does the phrase equivelent exposure mean?
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Jason G. Gainey |
Equivalent exposure is a camera setting that is equivalent to the meter reading. Example. You point your camera at a scene and obtain a meter reading. i.e. f/5.6 @ 1/125 sec You can then change the aperture or shutter speed to enhance whatever effect you are trying to obtain. If you are taking portraits you may want a soft background to enhance your subject. In which case you would decrease the number for the aperture. However you will have to compensate for the extra light entering the lens so you would want to increase the number for your shutter speed. If you only changed it by one stop you new setting would be f/4.0 @ 1/250 sec. Conversely if you wanted to have some blur motion you could decrease your shutter speed say, 2 stops, but you would have to increase the number for your aperture. Example. f/11.0 @ 1/30 sec.
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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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