Sunday, March 29, 2009

Photo 101: The Basics of Exposure

ISO, Shutter Speed, and F-Stop
By Tim Farmer

I've seen many photographers buy expensive cameras and leave them set to "Auto" because they never take the time to learn the proper techniques for manual exposures. But with the investment of a few minutes of time to learn the basics (okay, maybe a few hours...), you can be more creative and take better photographs.

Three camera settings can be easily manipulated while shooting to adjust an image's exposure: ISO, shutter speed, and aperture (f-stop).

ISO refers to the image's sensitivity to light. A lower ISO is usually more suitable for brightly lit situations where lack of light is not a problem. On the other hand, using a higher ISO setting is often used to take photos in low-light situations, where greater sensitivity to available light is needed.

Back in the days of film, you were pretty much stuck with whatever ISO you chose when you selected your film, unless you could specially process your film to squeeze out another f-top or two. With today's digital cameras, each image can have its own individual ISO setting, so you can change your ISO setting to suit the lighting conditions and the effect you hope to achieve.
Like most everything in life, there's a tradeoff between ISO settings and camera noise in your image. Most cameras have an optimal ISO setting, which may or may not be its lowest setting. But in general, a higher ISO setting usually results in more noise being introduced into the image.

Shutter Speed
Shutter speed is exactly what it sounds like, the speed at which
the camera's shutter takes the picture. A faster shutter speed will stop fast-moving action, freezing the beating wings of a bee, for example. A slower shutter speed can be used to create the feeling of motion through blur, but may also show unintended camera shake or subject movement.

Aperture, or f-stop, refers to the size of the opening of the lens and determines how much light can come through the lens in a given amount of time.
ISO, f-stops and shutter speeds work together in an inverse relationship; in other words, when one goes up, the other goes down to achieve an equal exposure.

Think of filling a bucket of water from a faucet (bear with me on this). Let's say you want to fill the bucket to a preset mark (your perfect exposure). Two things work together to fill the bucket to the mark: the rate at which the water comes out of the faucet (the size of the lens opening, i.e. your aperture setting), and how long you leave the faucet running (your shutter speed).

If you turn the flow of the water half-way on, it will take twice as long to fill the bucket as if you were to turn the water on full flow. Similarly, if a proper exposure in a given lighting situation calls for 1/500th of a second shutter speed at f5.6, the same exposure will be achieved if you set the shutter for 1/1000th of a second (half as long) and set the aperture to f4.0 (twice the "flow," or light allowed in the lens).

Or, the same would be achieved if you set the shutter to 1/250th of a second (twice as long) and the aperture to f8 (letting in 1/2 as much light).
When one goes up, the other goes down.

Do it in equal, inverse steps and the exposure is the same. It's called an inverse relationship.

Similarly, changing the ISO results in the same inverse relationship. ISO 200 is twice as sensitive to light as ISO 100, and half as sensitive as ISO 400. A higher ISO will allow a faster shutter speed or smaller aperture opening; likewise, a lower ISO will permit a slower shutter speed or a larger aperture opening.

By manually manipulating your ISO, aperture and shutter speed, you can achieve creative results such as silhouettes or overcome difficult lighting situations. You can use a long shutter speed to blur movement and create the illusion of motion, or you can use a fast shutter speed to stop the action of a kayaker running a waterfall.

If you take your camera off "Auto," the decision is up to you, not your camera.
Copyright 2009 Tim Farmer