Andrew S Gibson is the author of Understanding EOS: A Beginner’s Guide to Canon EOS cameras.
Regardless of what camera you own, there are two settings that dramatically affect the look of the photo:
1. The focal length of the lens.
2. The aperture
Focal length is a topic for another article. Today I’m going to look at the effect that aperture has on the look of your photos.
Three ways to use aperture
There are three ways to use aperture. Even if you’re not aware of it, you are selecting one of these approaches every time you take a photo:
1. Use a narrow aperture to maximise depth-of-field
This is the sort of approach you may take if you are shooting a landscape where you want everything to be in focus from the front of the image to the back. I used an aperture of f11 to ensure everything was in focus in the above seascape.
Typically, this means setting an aperture of f11 or f16 and using a wide-angle lens. It’s okay to use longer focal lengths too, but depending on the framing it may not be possible to get everything in focus, even at f22.
One thing to be aware of is the effect of diffraction. This is an optical effect caused by light passing through a small hole (in this case the aperture iris of your lens). If the hole is small enough, it causes the light to spread out, softening the image.
Because of diffraction, images taken at f22 are softer than images taken at f8 or f11, despite the extra depth-of-field. The amount of diffraction depends on the lens and camera in use as well as the aperture. It’s worth testing your lenses to see at what aperture settings the softening caused by diffraction becomes visible.
Are narrow apertures useful in any other way? Yes, they are. A narrow aperture restricts the amount of light entering the lens. Combine a narrow aperture with a low ISO and that means that you need a slow shutter speed to obtain the correct exposure.
If you’re shooting in low light (ie dusk or before sunrise) you may need shutter speeds of 30 seconds or more. Some photographers take advantage of this to take long exposure photos that blur anything within the frame that is in motion, such as water. The long exposure photo above was taken with a shutter speed of 15 seconds at f16 and ISO 50.
2. Use a middle of the road aperture
A ‘middle of the road’ aperture is anything from about f5.6 to f11. The usual result is that the subject is in focus and the background is either in focus or a little out of focus. This is fine if it’s what you want, but it’s not always as exciting as the next option:
3. Use a wide aperture to take photos with a sharp subject and blurred background
This is an exciting technique to use because it lets you take some really dramatic images. Blurred backgrounds work for three reasons:
- The out of focus background doesn’t distract from the subject.
- The out of focus background is mysterious, especially if it is darker than the subject. The viewer can’t see the background clearly and fills in the gaps with their imagination. Like the tease of a burlesque dancer, this can be much more effective than revealing all in sharp detail.
- You will often use a wide aperture in low light, the quality of which is usually beautiful.
There are several subjects where out of focus backgrounds are very effective:
Perhaps the most common of all. An out of focus background concentrates attention on your subject and adds a sense of mystery. I created the above portrait by using an aperture of f2.8 with an 85mm lens.
Flowers also look good with out of focus backgrounds. Try to find flowers that are some distance from what is behind them. That will help you defocus the background. I used an aperture of f1.8 and my 85mm lens to create the above photo.
You can try this with animals too. I took this photo of a parrot by focusing on its eye and letting the background fall out of focus. Zoos are a good place to try this out, as long as you can get close enough. 85mm lens, f1.8.
Travel and street photography
You can use a wide aperture to concentrate attention on your subject and throw distracting backgrounds out of focus. This works well if you are using a telephoto lens, like I did in this photo taken in Potosi, Bolivia. Focal length 150mm, aperture f2.8.
Primes lenses versus zoom lenses
Prime lenses have wider apertures than zooms. This helps you create image with out of focus backgrounds. I own both a 50mm and 85mm prime lens, with maximum apertures of f1.4 and f1.8 respectively.
The depth-of-field at these apertures is amazingly narrow. Care is required to make sure you focus on the correct point. I often use these lenses at f2.8. The depth-of-field is still very small, but not narrow enough to give me major focusing problems.
If you own a zoom lens you can still join in the fun. Even if the only lens you own is a kit lens (with a typical maximum aperture of f5.6 at the longest focal length), you can still take photos with a narrow depth-of-field once you understand the following principles:
1. The closer you are to your subject, the less depth-of-field you will get.
2. The further your subject is from the background, the more out of focus the background will be.
3. The longer the focal length of the lens, the less depth-of-field there is.
These apply no matter which aperture you select, but it’s also worth mentioning again that the wider the aperture, the less depth-of-field there is.
Aperture Priority mode
The easiest way to use your camera when you want to select a specific aperture is to set it in Aperture Priority mode. This is marked by the letters Av (aperture value) on the Mode Dial of my EOS cameras (check your camera’s instruction manual if you’re not sure how to set Aperture Priority on your camera). Once in Aperture Priority, set the aperture and ISO values you want to use. The camera sets the shutter speed automatically according to the light reading from its internal meter. Some cameras offer an Auto ISO setting, but my personal preference is to avoid that and set the ISO that I want.