Aperture and Depth of Field

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As we covered in Chapter 1, your camera’s lens covers the aperture: a hole that lets light into the camera. The aperture’s width is controlled by an adjustable diaphragm. Widening the aperture lets in more light; narrowing it lets in less. On cheaper compact cameras you might not have direct control over the aperture, though its settings may be affected by your choice of shooting mode. Even if you’re unable to control it directly, its effect on your image is important.

Aperture measurements may seem a little odd the first time you see them. They’re expressed in f- stops (as opposed to the plain old stops we’ve already covered), usually written either as f and a

an absolute measure of the aperture’s size, but is the related focal length of the lens; therefore, an aperture of any given f-stop will let in the same amount of light regardless of what lens you use.

The first slightly tricky point to remember about apertures is that the smaller the f-stop, the larger the aperture, and the more light that enters the camera. To increase exposure, set your aperture to a smaller number. To decrease exposure, make the f-stop larger. It might help to think of the number as a measure of how much the hole is closed.

Another aspect of aperture that takes a bit of getting used to is the numbering. Standard lens apertures are usually a subset of the sequence f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, and so on. Each number is roughly 1.4 times the previous one. This is no huge joke played by math nerds on those of us who are more numerically challenged: 1.4 is the square root of 2, rounded off to fit on a camera’s display. If you multiply the width of the circular aperture by 1.4, you double the aperture’s area and let in twice as much light. You can see this demonstrated in Figure 2.5.

Figure 2.5. Each f-stop is half the area of the previous one

When you change the aperture from f/1.4 to f/2, you halve the amount of light the camera lets in.

Go from f/8 to f/5.6 and you double it. You don’t need to understand the math, but it’s a very good idea to learn the sequence of f-stop numbers. It might seem counterintuitive that f/8 means half as much light as f/5.6, but knowing it well enough so that it’s second nature to you might one day save you missing a shot. Most cameras allow you to adjust apertures in half-stop steps; some allow steps of a third of a stop, but you’ll find some part of that sequence of f-stop numbers in your camera’s settings if you have manual control over the aperture.

The aperture’s primary effect is to control how much light passing through the lens reaches the sensor. As a secondary effect, the aperture controls depth of field.

As we saw in Chapter 1, depth of field is the amount of scene that’s in focus in front of and behind the point you’ve focused on. A small depth of field means that only a part of your image will be in

focus and the rest will be blurred, as illustrated by Figure 2.6. A large depth of field gives you a greater distance in front of and behind your subject in focus. A wide aperture gives you a shallow depth of field; a narrow aperture gives you a deep one.

Figure 2.6. The size of the depth of field

A bit of blur can be a good thing. If you want to shoot one flower in a garden full of flowers, or a portrait in front of a busy background, limiting your depth of field so that only your subject is in focus will separate the subject from the background. Background details, which would be distracting if in focus, are blurred, leaving your subject standing against a more abstract background. In the following flower pictures, at f/22 (Figure 2.9) the trees in the background clash with the flower stems in the foreground, while at f/1.8 (Figure 2.7) it’s hard to tell that there are any trees. An aperture in between (Figure 2.8) shows all the flowers on the one plant yet still blurs the background.

(Automatic portrait modes use depth of field in this way to isolate a subject.)

When shooting a landscape or a large group of people, you might wish to keep a number of objects in focus that are all at different distances from the camera. You’ll need a narrower aperture to do this, so if it’s necessary to increase the exposure, you’ll have to add more light (difficult for a land­

scape photograph) or adjust one of the other corners of the exposure triangle.

There are other factors involved in depth of field besides aperture. For example, the closer you are to the subject, the shallower the field at any given aperture. The previous flower examples were all shot from within a meter of the plant. Had the focus been on the trees in the background instead, the depth of field would likely have extended to infinity at f/22.

Figure 2.7. Flowers on a bush at f/1.8

Figure 2.8. The same flowers at f/8

Figure 2.9. And again at f/22

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