Film Speed, Shutter Speed and Aperture – Digital Photography Tip of the Week

One of the keys to a good photographic image is proper exposure. Proper exposure is made up a couple of elements, all working together; shutter speed, aperture and film speed. However, there is never one proper exposure, but many that will provide a good print. I will explain why.

Film, whether traditional or digital, requires a certain amount of light to fall on it’s surface to be properly exposed. The amount of light necessary is determined by the film or sensor speed. The faster the film/sensor speed, the less light needed to produce a proper exposure. I discussed this last week. How much light reaching the film is determined by a combination of shutter speed and aperture. Shutter speed is how long the light reaches the film, while aperture is the size of the lens opening letting light into the camera.

All three aspects deal in increments of 2. Film speed of 200 is two times as fast as a film speed of 100. A shutter speed of 1/30 second is two times as long shutter speed of 1/30 of a second. An aperture of f8 is two times as small as a shutter speed of f5.6 (this one is a little confusing, the larger the number, the smaller the lens opening, the less light falling on the sensor.) To maintain a proper exposure, as you increase one factor, another must be decreased. The best analogy I have to explain this interaction is the process of filling a glass of water. The size of the glass relates to the film/sensor speed. A larger glass requires more water to fill, or more light to properly expose the sensor (a slower film speed). The amount of water coming out of the tap is your aperture. And the length of time the tap is open, shutter speed. So, to fill a glass of water, you can crack the tap open to a drip and let it run a long time, or open it full for just a second. The result is the same, a full glass of water.

Aperture vs. Shutter Speed

As aperture increases, the size of the lens opening decreases and more time is necessary to provide a properly exposed image. Or, as shutter speed increases, a larger aperture (smaller lens opening) is needed to provide a properly exposed image. In the above example, each combination will produce a properly exposed image. Changing film speed would allow one of these figures to slide, giving an exposure of f8-1/125 if we had gone from 100 to 200 ISO film. So, how can this be applied to your photography?

If your camera allows you to operate in a manual mode, after taking a shot with your digital camera, look at the image details for exposure information. Many cameras will give you the settings it used to take the image. If it was shot at f5.6 for 1/125 second, you can obtain the same exposure by shooting a f4 for 1/250 second (double the amount of light but for 1/2 the time) or f8 for 1/60 second (1/2 the amount of light, but for twice as long). So long as you apply the same adjustment to the shutter speed as you do to the aperture, you will still get the same exposure for your image. Any of the above combinations would work. As I mentioned last week, shorter shutter speeds tend to stop motion, while longer shutter speeds will help show motion. If your exposure doesn’t match what I have above, simply line up the two corresponding values from your camera. It might be f4-1/500 which is the same as f5.6-1/250 second.

Next week I will discuss Aperture Priority and Shutter Priority camera modes.

 

The digital photography tip of the week is written by the PCIN Assistant Editor, Chris Empey. Chris is a long time photographer and is currently the vice-president of the Niagara Falls Camera Club. You can see more of his photography at his Photo of the Day website.
If you have a tip to send Chris, or a question about digital photography he can address in the newsletter, send it to chris@pcin.net.

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