Importance of Aperture

Categories: Photography
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Published on: 2011.01.29

(All images in this blog entry are the intellectual property of Wil C. Fry and are Copyright © 2008 by Wil C. Fry.)

As always, light is the most important element in photography. Nothing happens without light. Controlling light is the number one absolute best way to improve your photography.

But given a specific type of lighting, your camera’s settings can help you work with the light. The “exposure triangle” consists of three camera settings: exposure speed (more on exposure), ISO sensitivity, and aperture (f-stop). This article is about aperture.

Without getting into the technical details, physics or math of how lenses work, aperture is simple. It’s a hole inside your lens that lets in light. The brilliant part of lens design is that you can change the size of your aperture to let in more or less light. This has the adverse effect of narrowing or widening your depth-of-focus. (Scroll down for example images.)

Aperture is commonly measured by “f-stops.” The larger the number, the smaller the hole (this is because of the aforementioned math, which I’m still not getting into). Examples: f/1.8 is a relatively large aperture, whereas f/16 is a relatively small aperture.

Why is aperture important?

As I’ll discuss in other places, changing your shutter speed or ISO can have adverse affects. Too slow of an exposure, and your image will be blurry. Too fast of an exposure and it’ll be dark. High ISO sensitivity can cause grain or “noise” to appear on your images. Aperture is just as important in determining what your image will look like.

The following is very simple, yet many beginning photographers haven’t learned it yet:

1. If you need more light, you can widen your aperture (choose a lower f-number). This will narrow the depth-of-focus.

2. If you need less light, you can shrink your aperture (choose a higher f-number). This will deepen the depth-of-focus.

See the following example images, taken from the same spot, with the same lens, in the same evening light. (Lens used was the Canon EF 85mm f/1.8 USM, attached to a Canon EOS Rebel XTi.)

Fence Post, f/16.0
f/16.0, 1/50 sec., ISO1600

I chose f/16 for this example, focusing on the fencepost in the center of the frame. The narrow aperture allows more of the surrounding objects to be in focus, such as the other fence posts , plants, concrete blocks, and even the brick house in the background. Only the center fence post is sharp, but the other objects are recognizable.

Using this aperture caused two problems with this image. One, I didn’t really have enough light for such an aperture. By choosing f/16, the image would have been too dark, so I had to use a much higher ISO (1600 in this case). It was still too dark, so I was forced to use a slower exposure: 1/50 sec. Handheld, I was lucky this shot didn’t have more camera-shake blur. Two, the image looks junky; too much is visible because of the deep depth-of-focus.

Let’s solve the light problem first.

Fence Post, f/5.6
f/5.6, 1/400 sec., ISO1600

By changing the aperture to f/5.6 for the second image, I was able to increase the exposure time to 1/400 sec., which is plenty fast for a handheld shot, even if there had been action involved. You can also see that less of the surrounding objects are in focus.

However, I’m still at ISO1600, which produced unwanted noise in the full size image. And the frame is still too junky for my tastes.

Fence Post, f/2.8
f/2.8, 1/400 sec., ISO400

Now we’re getting somewhere. I selected f/2.8 for this third image. This let me reduce the ISO to 400, which produces barely noticeable noise/grain. Additionally, the background and foreground are even more out of focus, though still recognizable.

Let’s try one more.

Fence Post, f/1.8
f/1.8, 1/1000 sec., ISO400

I’ve now maxed out the lens at f/1.8, as wide as it will go. The ISO is still at 400, so the image would have been too bright. This forced me to increase the exposure speed to 1/1000 sec. (Alternately, I could have changed the ISO to 200, and left the exposure time unchanged.)

Now we have a fence post that’s completely cut off from the background and foreground by the narrow depth-of-field. We have a blazing-fast shutter speed that can freeze motion (sports, kids playing, dogs running, whatever), and a low enough ISO so we don’t have to worry about noise.

Most consumer (affordable) lenses aren’t capable of such wide apertures. Each lens maker usually only has one or two lenses in the lower price ranges that can achieve f/1.8 or wider. The “kit lens” that came with your camera probably maxes out at f/3.5 (f/5.6 if you zoom). This is why anyone who’s been shooting for a while will almost always recommend a wide aperture (“fast”) lens.

Not everyone likes the extremely narrow depth-of-field you see in my fourth example. Also, if I’d missed my focus by a few inches, then nothing in the last example would be in focus. But I have the option, and that’s why it’s often worth it to get at least one fast prime. If there had been less light available, and if any subjects had been moving, going wider (f/1.8) might have been my only option for a usable image.

Regardless of your lens, it’s important to understand what aperture does, and how it affects your other settings.

To recap: “aperture” means “hole.” Widening the hole lets in more light, yet narrows the depth-of-focus. Narrowing the hole lets in less light and provides a deeper depth-of-focus.

Here are a few more examples. Click the thumbnails to see larger versions, including all relevant exposure information
f/1.8

Tree, f/1.8

f/3.5

Tree, f/3.5

f/5.6

Tree, f/5.6

f/16.0

Tree, f/16.0

(This entry is part of my growing Photography FAQ. The wording above may change at any time.)

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