Understanding aperture

The shutter and aperture are the main players in the game of exposure. Last time we covered what the shutter does and its effect over exposure and the image. This time we’re tackling aperture.
Aperture is the one thing that seems to cause the most confusion. There’s that sequence of funny numbers that seem to work backwards and that little matter of depth of field. Both of these combine to trip up a lot of people early on so if you can get a grasp on them now, the rest will come much easier.
The aperture is the opening in the back of the lens that regulates the amount of light entering the camera. It’s adjustable by a ring on the lens or by a dial on the camera, sometimes both. Many modern lenses lack an aperture ring, so the camera must control aperture. By turning the aperture ring or making the adjustment on the camera (see your manual), you make the opening bigger or smaller, letting in more or less light.
Here’s the sticky part: the F-stop numbers. What you see on the aperture ring is a series of numbers like this: 2.8, 4, 5.6, 8, 11, 16, 22, 32. Some lenses have more, some less. You’ll see the same sort of series on your camera’s LCD readouts. So what in the world do these numbers mean? They’re actually ratios of the lens focal length to the size of the aperture opening. You probably don’t really care about that at this point. The important thing about that fact is this: Any given aperture will let in the same amount of light, no matter the focal length of the lens. That means that f/8 will let in the same amount of light whether you’re using a 50mm lens or a 500mm lens.

Like shutter speed, think of the f-stop numbers as fractions. It helps that they’re usually written that way. Smaller f-stop numbers are actually larger openings (larger openings let in more light). f/2.8 is a larger opening than f/8 and a much larger opening than f/22. So, out of the series above, f/2.8 lets in the most light and f/32 the least.
And like shutter speeds, each f-stop in the series above lets in twice as much or half as much light as the numbers next to it. f/5.6 lets in half the light that f/4 does and twice the light as f/8 does. Does this sound familiar? Remember the doubling and halving of light associated with shutter speeds? This is important and something we’ll get to in the next lesson. For now though, just understand the concept that as the f-stop number gets bigger, the aperture opening gets small. Bigger number, less light.

Like the shutter speeds, there are also some in between numbers in the sequence above. Depending on your camera, you may have either one or two additional numbers in between each of those listed. But realize, if you do have those in between numbers, they DO NOT represent a doubling or halving of the light. They are intermediate steps that allow greater control over your exposure.
I don’t want you to get confused at this point, (I’ll save that for later) but I do want you to realize that, depending on your particular camera, each f-stop number may or may not represent a doubling or halving of the light. Again, check your manual (you do have it at your side, don’t you?)

Here are a few terms you’ll hear or read: “Opening up” means you’re making the aperture opening larger, letting in more light. “Closing down” or “Stopping down” is just the opposite. “Wide open” means, as you’ve probably guessed, setting the aperture at its widest setting (smallest f-number).

To review, the aperture's part in controlling exposure is by opening up or closing down, letting in more or less light. That should make sense.

Depth of Field

What may not make sense is how the aperture actually affects a picture. Besides having a hand in controlling exposure, the aperture controls something called Depth of Field (DOF). Depth of Field is a zone of apparent focus in a picture. I say “apparent” because a lens only focuses at one point and has a plane of focus parallel to the plane of the film (the back of your camera). Depth of Field means that it looks like there’s more in focus than there really is. This area of “focus” extends in front and behind the true point of focus.
The size of the aperture determines the depth of field, the smaller the opening (larger f-stop number) the more depth of field. Therefore, f/22 has a lot more depth of field than f/2.8. Think of it this way: Bigger f-stop number, bigger depth of field.
With lots of depth of field, you get lots of detail. By using a smaller f-stop number (a wider opening), you’ll get a shallower depth of field and less detail overall, besides what you’re actually focused on.

What are some reasons you may want to use one f-stop over another? If you’re making landscape photos and you want everything sharp from foreground to background, you need lots of depth of field, which means small aperture openings like f/16 or f/22.
Portraits, whether of people, animals, or flowers, generally use wider openings. A wider opening gives you less depth of field—fewer things appear to be in focus. This helps in reducing any distractions in the background that take attention away from the main subject. So if you want to isolate your subject, use a wide aperture like f/2.8 or f/4.
Another reason to use one aperture over another has to do with shutter speed. If you need a faster shutter speed, you can open your lens more to let in more light. You then compensate by setting a faster shutter speed. We’ll cover this in more detail when we discuss using aperture and shutter speed together.

I know this is a lot to take in, but it’s important. For now, just make sure you understand these two things:
As the f-stop number increases, light entering the camera decreases.
As the f-stop number increases, depth of field increases.

Get out your camera and put a lens on it. Have your camera manual handy and go through all the available f-stops on the lens. You may need to be in the manual exposure mode to do this. Do this for all the lenses you own.

NOTE: For those of you mathematically inclined, something that helped me to grasp these numbers was realizing that the numbers in the main series (2, 2.8, 4, 5.6….) have one thing in common: the square root of two. You can multiply or divide any number by the square root of two to get to the next number in the series.

Next, A stop of light...