The Aperture Effect

Of all the ingredients that make up a photograph-shutter speed, ISO, and white balance, to name a few-the last one I understood was aperture. I simply could not wrap my mind around the difference in the numbers. It was when studying math with my daughter for the second time in my life that my eyes finally opened.

In my example (above), you have two circles, one distinctly larger than the other. The amount of light entering the lens in any given space of time is directly dependent on the size of those circles. This is known as aperture.

Think of it as a piece of pie. One-half (1/2) of a pie is a greater portion than one-sixteenth (1/16). Therefore, one-half of the pie will hold more apples in it than the one-sixteenth portion. Applying this to photography, the greater the slice of pie, the greater the amount of light.

"Okay," you say, "I get that, but why does it matter?"

It matters because of the effect it has on your final photograph. There are a number of reasons to use Aperture Priority (AP on a Nikon DSLR) and all of them involve depth of field. Now, before you bash your head against the wall, remember depth of field is the amount of the scene before your lens that is in focus.

Depth of field is affected by both your choice of aperture and your particular lens. For instance, in macro photography, depth of field is very shallow. This means less of the scene will be in focus, and the closer you get to that object, the more blurry it will become. In contrast, landscape photography has a greater depth of field-more of the scene is in focus (trees, buildings, mountains, etc.)

There is no right or wrong depth of field. But there are some depths that work better for certain scenes, and your choice aperture is the key to changing them. In this first photograph of a butterfly, I was dealing with an insect whose wings (laid flat) are the size of a dime. It is also a quick moving species, so finding it still at all was a relative miracle. I wanted to see as much of the butterfly as possible, so I chose a smaller aperture (F20) to have greater depth of field. At F20, I was dealing with a trickle of light. Therefore, I encountered an additional problem-time.

Again, think of the slice of pie. But this time think of it as time, not aperture. In the numerical system twenty is a larger number than two, so eating a "twenty" slice of pie will take much longer than eating a "two" slice. In photography, more time means more light. If you are using a smaller aperture and thus restricting the flow of light, then your camera will require more time to eat the pie.

Now ... there are formulas for this which match the size of aperture to specific shutter speeds for the optimum exposure, and I have read them and re-read them multiple times by many authors. Yet since I do not have a math brain like some, they tend to fly right past me. I simply cannot take a photograph and do the math first. I do, however, love dessert so the pie reference always works for me.

When I realize I want the effect of a smaller aperture, I ask myself how much pie this gives me. If the pie is small, I will need more time to eat it, and conversely, if the pie is large I need less time. (So, okay, the analogy seems weird here because typically larger pie requires more time, so instead change it to the size of the opening you are inserting the pie into, aka your mouth.)

Ever watch one of those eating contests where the skinny dude wins and the overweight guy doesn't? How did he do it? He crammed as much in his pie-hole as was possible at one time. This is your large aperture.

Large apertures give the effect of less depth of field. There are times when you want this. Less depth of field isolates a subject from its surroundings. It creates bokeh, which can make for magical specular elements. In the next example, I again was photographing a butterfly, but I didn't want to see the texture of the grass in the background. By using a longer lens (300mm) and a large aperture, I saw the butterfly and not the grass.

However, there is a trade off because when you change your aperture, you change your shutter speed-always. You also always change your depth of field. This means I'd have less background in the second butterfly picture but more chances of having a fuzzy image. I compensated for this by waiting for the butterfly to be square (flat) to my lens. This makes every part of its wings the same distance away and thus the same amount of focus.

I know. At this point, you need to stop and digest all this pie I've been feeding you, and that's perfectly all right because the best way to learn aperture is to keep chewing. Take a series of images of the same object using different apertures and then view the effect afterward.

Take another series at the same aperture but at different times of day or in different amounts of light. (Be prepared to use a tripod on the longer exposures.) Eventually, the whole pie will digest and you'll have an important tool to taking the photograph you desire. Knowledge, after all, is the ultimate power.

Now I think I'll go find some pie.