Photography 101 - Part 2 - One Exposure, Many Looks

In the last part we learned how shutter speed, aperture, and ISO work together to form an exposure. As promised, this part we will be discussing how changing one or more and compensating with the others can offer the same exposure but a very different photo.

First, let's talk about aperture. The range of values you can set it to depends on your lens. “Fast” lenses let you set the aperture really wide. Wide apertures have small numbers like 1.4. This is the end that varies the most from lens to lens and faster lenses are usually big, heavy, and expensive. At the other end is how small you can make it, usually around 22. (If you’re wondering why wide apertures have small numbers, the short answer is that they’re actually fractions.) Changing the aperture affects how much of your image is sharp (known as depth of field (DOF)).

In some cases, you may want the entire image to be sharp (large DOF). Landscapes are a good example of this. You generally want everything from right in front of the lens all the way to the distant horizon to be as sharp as possible. For this, you'll want a small aperture (large number). To help you remember, think of squinting your eyes to read something. When you stop down the lens to make the aperture small, not as much light can get in. To compensate, set a longer shutter speed. [Note: if you use the AV mode on your camera, it will automatically set the shutter speed for the aperture you choose.]

Many times, though, you want only part of the image sharp. By having just your subject, say a person, or a bird, sharp and the rest out of focus, your subject really seems to pop out at you. This is partly achieved by having a large aperture. (It's also affected by the distance between you and your subject and your subject and the background). Opening the lens up with a wide aperture gives you a smaller DOF and helps with that blurred background (known as bokeh). Besides blurring the background, a wide aperture means you need to set a faster shutter speed so you can work with less light. The downside is that, because sometimes a very small area is sharp, you need to be more careful with your focusing to be sure you focus on the right bit.

The third part of exposure, ISO, is generally best kept as low as you can get away with for the shooting conditions. Being able to turn it up gives you more options with the aperture and shutter speed but also adds noise. Unless you consider noise an artistic look, don't turn the ISO up unless you need to.