Aperture is one of the three settings that make up "the exposure triangle." Along with shutter speed, your aperture essentially controls how much light reaches your image sensor. Your image sensor, in turn, is responsible for forming the image, which is then saved to your memory card.
Because aperture is one of the three settings that you can use to control exposure, it may not be immediately clear why it might make a difference whether you choose a large aperture or a small one, just so long as you're getting the correct exposure. But while getting good exposure should be one of your primary goals as a photographer, it doesn't address things that you can do creatively to change your results. So with that in mind, here are a few situations where you might need a large aperture—both from a practical standpoint and from a creative one.
How aperture works
Before we start talking about the ways that you can creatively use your camera's aperture, let's talk a little bit about what aperture is and how it contributes to making a photograph.
Aperture simply refers to the size of the diaphragm, which is between your lens and your camera's image sensor. When you select a large aperture (or smaller f-number), the diaphragm becomes very wide, and when you select a narrow aperture (or larger f-number) the diaphragm becomes very small. I'm sure I don't have to say that the wider aperture lets in more light while the narrower aperture of lets in less light, but that's really the essential difference.
Aperture is different from shutter speed because it has to do with the amount of light that's allowed to reach your camera's sensor rather than with the amount of time that your sensor is exposed to that light. When you select a very wide aperture, you can take a photograph in low light even with a faster shutter speed—the shutter doesn't have to be open as long because there is already so much light coming through that wider opening. But the flipside to this is that with a wide aperture, you will get less depth of field in your image. The term "depth of field" refers to how much distance there is between the sharp parts of the scene and the blurry parts. An image with broad depth of field will be sharp from foreground to background, while an image with shallow depth of field will have only a very small area that is sharp. The reasons why are complicated, but have to do with the direction of individual light rays. Light rays don't come from a single direction, they come from all over—so when you shoot with a very wide aperture you're getting angled rays of light as well as focused rays of light. When you shoot with a narrow aperture, on the other hand, you're only capturing those focused rays of light, which means that your foreground will be as sharp as your background.
One of the best reasons for using a wide aperture is so you can get a well-exposed photo in low light. Of course you always have the option to pop up your flash if it looks like there's not going to be enough light, but it's generally a better idea to simply use that wider aperture (your popup flash can wash out your subject and cause red-eye). A wider aperture lets in more light, which means that that you can potentially take a photo in low light without having to resort to a high ISO or slow shutter speed. Now, if you're using a kit lens, which is the typical mid-range zoom lens that comes with your camera, you won't be able to select a very wide aperture (most kit lenses can only go as wide as f/5.6 or f/4). So if you are finding that you still can't get clear photos in low light with your kit lens, consider investing in a 50mm prime lens (a “prime” lens is a fixed-focal length lens, or a lens that doesn’t zoom). Most prime lenses have an maximum aperture of up to f/1.4, and even budget versions will let you go as wide as f/1.8. A prime lens is a great investment for anyone who does a lot of shooting in low light situations such indoors or in the early morning or late afternoon.
20110604_roots_picnic_429 by Flickr user jaredpolin
Any time you shoot a portrait of a person or an animal, you're going to want to use a wider aperture in order to blur out any additional distractions that may happen to be in the frame. For example, let's say you are taking a picture of your girlfriend in an outdoor café on the sidewalk of the big city. There's quite likely going to be a lot of distractions in that scene. You're going to have cars driving back and forth, you're going to have pedestrians, you might have some shop signs, billboards, or maybe even neon. If you shoot this photograph with a narrow aperture, you're going to end up with a lot very sharp, visible distractions in the scene. Your viewer isn't going to know whether she should look at all that stuff in the background, or whether she should look at your girlfriend. The easy way to resolve this problem is to simply shoot the scene with a wider aperture. That wider aperture is going to get rid of all those background distractions, and then there will be no doubt who the subject of the photograph is.
Isolating a small part of a macro subject
When you shoot macro, you get a very limited depth of field even if you are using a very narrow aperture. That's because the closer you get to your subject, the less depth of field you're going to have overall—sometimes as little as a few millimeters. So while most of the time you probably do want to select the narrowest aperture you can in order to achieve as much sharpness as possible from one part of your subject to another, there are certain situations where you may want to make the artistic choice to do exactly the opposite. For example, let's say you are shooting a very close up photograph of the stamen of a flower. You can create a surrealistic and almost magical effect by shooting the scene with a very wide aperture. When you have only a single element in the frame that is in tack-sharp focus and the rest is falling off into blur, you are giving your viewer a subject that's unlike anything he's going to see with his own eyes. This can make for a very compelling and beautiful photograph, and it's a technique that anybody who shoots macro should try if only because of its great creative potential.
Flower by Flickr user Salvo.do
If you want to shoot fast action, you need to have fast shutter speed. And to achieve a fast shutter speed you also need to use a wide aperture. That's because fast shutter speeds don't allow very much light to reach your camera's image sensor, and you generally need to compensate for that by opening your aperture to let more light in. And your aperture will increase as the speed of your subject increases, and also as the light decreases—for example, you may not need a wide aperture to photograph a runner during a bright part of the day, but you may need a very wide aperture in order to capture a race car on an overcast day.
Now ISO, of course, comes into play here as well. This isn’t an article about ISO in particular, but some discussion is needed simply because you can’t effectively use aperture if you don’t understand ISO. ISO is the setting that changes how light-sensitive your camera is—if you’re shooting that racecar on an overcast day, you may find that your camera simply won’t allow you to select a fast shutter speed even at the widest aperture your lens is capable of. For that scenario, you need to make your camera more sensitive to the light, and you do that by turning up the ISO.
Malibu Bird by Flickr user Danny Perez Photography
Grass, water and light by Flickr user Sorin Mutu
Another great use for a wide aperture is to capture bokeh, which is that lovely blurred effect you get in the background of some images. We typically associate the word “bokeh” with those pretty orbs of light as in the above example—but it really can be used to describe any strong background blur. To achieve those orbs of light, you need to make sure that your background contains bright points of light—Christmas lights work beautifully for this effect. Position your subject some distance in front of those points of light, and use a wide aperture to capture the scene.
Wide apertures are really fun to experiment with, so if you haven’t really done much outside of auto mode, try switching over to aperture priority mode and playing around with those wider apertures (remember that a wide aperture corresponds to a small f-number—f/1.8 is a wide aperture, while f/22 is a narrow aperture). Try some shots in all of the categories above and compare them to your auto-mode photos. You may never go back to auto-mode again.
- How aperture works
- When to use a wide aperture
- Low light
- Isolating a small subject
- Action photos
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