BetterPhoto Q&A
Category: New Answers

Shutter Speed and Aperture

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September 03, 2001

First, about lens aperture, shutter speed and exposure:
The combination of lens aperture and shutter speed set an exposure. The exposure required for film cameras is determined by subject brightness level and film speed. The CCD sensor in a digital has the equivalent of a single film speed. Thus, the exposure required for the CCD to record the image is determined by subject brightness. Think of exposure for recording an image like filling a water glass. The object is to exactly fill the glass. Subject brightness is like the water pressure, lens aperture is the diameter of the water pipe, and shutter speed is how long you leave the faucet open. Low lighting is like low water pressure. Bright lighting is like high water pressure. A larger diameter pipe allows more water to flow and a smaller one restricts the flow. The lens aperture works much like the pupil in your eye which gets larger in low light and smaller in bright light. The shutter speed is how long light is allowed to strike the CCD to record the image.

For a given exposure required, you can trade aperture and shutter speed settings. In other words, if you double the area cross-section of the pipe (increase diameter by about 1.4X), you can leave the faucet open half as long and still get the same amount of water (and vice versa).

Shutter priority means you set the shutter speed and the camera will set the lens aperture needed. Aperture priority means you set the lens aperture and the camera will set the shutter speed. There are only so many combinations of shutter speed and aperture that will work for a given subject brightness. Your camera has limits on how wide or narrow an aperture it can set, and on how fast or slow a shutter speed it can use. This means if you try to set a very fast shutter speed in low light conditions using shutter priority, it is possible the camera cannot open the lens up enough for a proper exposure. The same applies if you pick an aperture in aperture priority mode that requires a shutter speed outside the range the camara can set.

The most common reason for wanting a fast shutter speed is to stop action. 1/125th second will freeze nearly all movement by people. 1/1000th second will stop nearly all motion. A 90 MPH fastball travels about 1.5 inches in 1/1000th second.

Lens aperture also affects how much in front and behind the lens focus distance will appear to be in focus in the image. This is called Depth of Field. The most common reason for wanting a narrow aperture (high f-number) is to have a very deep depth of field in which everything (or very nearly so) appears in focus. A very deep depth of field is almost always desired for landscape photographs. A wide aperture (low f-number) will create a shallower depth of field. The wider the aperture, the shallower the depth of field. One of the common uses for wide apertures is in portraiture, to isolate the subject from a distracting (possibly cluttered) background.

As you gain experience with your camera, think about which one you want most and experiment some with it to get a feel for how fast a shutter speed will stop certain types of action, and how wide an aperture will blur a distant background (without blurring part of the subject).

-- John

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September 04, 2001

I am not trying to take away anything from this site. It is indeed an excellent place to get wonderful information.

However, I think the best answer to your question is found at another site called webslr.
It offers a live working model through a JAVA-enabled web browser and it also provides a training course in aperture settings, shutter settings, and exposure settings. I think anyone starting off in photography should take advantage of such a model.

The only draw back is that the course costs approx. $30 for a 1-year membership. It is well worth the expense, as I've taken the course myself and can attest to how wonderful it is.

Cheers,
Glenn

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September 09, 2001

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March 28, 2002