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Aperture/Shutter Speeds

Now I am ready to learn more about this. Could someone please explain aperture and shutter speeds? Examples?

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November 05, 2000

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November 05, 2000

The following are some very rudimentary basics:

Exposure is controlled by two things, lens aperture and shutter speed.

The average "luminance" of your subject is how much light you have to work with in setting the exposure. Obviously there is much more luminance outdoors in direct sunlight than there is in open shade on the shady side of a building, or deep shade under the trees in a thick forest, and these have more luminance than in a dimly lit night club. The subject luminance and your film speed determine how much you need to expose the film in the middle of its "latitude" so that you capture detail in both strong highlights and deep shadow.

If you look at the shutter speeds available under manual control, they will run from something like 1 second to 1/1000th or 1/2000th second. Each setting, or "stop" is approximately half or twice as long as the one next to it (1/60th, 1/125th, 1/,250 etc.). Often only the denominator number is found to save space. Thus, moving the shutter speed by a "stop" either allows in twice as much light or half as much light.

Now look at your lens aperture settings. You should see "f-stop" numbers like 1.4, 2, 2.8, 4, 5.6, 8, 11, and 16. The lens aperture is approximately the lens focal length divided by the aperture diameter. f/16 is a very small diameter allowing very little light through the lens, and f/1.4 is a very large diameter allowing a lot of light through the lens. By doing it this way, an "f-number" means the same amount of light regardless of focal length. However, the amount of light is directly proportional to the aperture area, not its diameter. The area is related to the square of the radius (half the diameter). This is why adjacent f-stop numbers are approximately the f-stop number divided or multiplied by the square root of 2 (1.414). In other words, f/2 lets in twice as much light as f/2.8, and four times as much light as f/4 does.

Now you should realize that you can trade one shutter speed stop for one f-stop. That is, if you increase the shutter speed by one stop and open up the lens aperture by one f-stop, you get the same exposure. Therefore, for a proper exposure there will be a set of shutter speed and aperture combinations you can choose.

What comes after this is complex, because it involves decisions about how fast a shutter speed you need to stop action (or not quite stop action for a feeling of motion), while maintaining a the depth of field you desire (the range of distances around the focus distance that will appear to be in focus). This is a trade-off. If you increase shutter speed to stop action, you have to open up the lens aperture to keep the exposure the same. However, a wider open lens has less depth of field (DOF), and if you open it up too much the DOF will be too shallow to capture the depth of your subject in focus. "eglm" is right in his suggestion to visit the library. This is where you need to find book(s) that discuss the finer points of these decisions in making photographs. There should also be some diagrams and photographs that show you exactly what is going on, and what the effects will be when you adjust each one.

Hope this helps get you started.
-- John

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November 07, 2000