We're probably all familiar with the notion of aperture controlling the depth-of-field in our photos. By using a faster aperture, you create a shallower depth-of-field. To keep more of your image in focus, you need to use a smaller aperture. But there's a whole lot more to depth-of-field than adjusting your aperture to get more or less of the scene in focus.
Let's start with setting out what we mean by depth-of-field. It's the range of distance in a photo that is considered to be 'acceptably sharp', or what we would regard as 'in focus'. Only the actual point of focus in a photograph is definitively sharp and 'in focus'; depth-of-field describes the zone of acceptable sharpness either side of it. A wider band of 'acceptable sharpness' running through an image equates to a greater depth-of-field. To introduce more blur into your photos you would want a shallower depth-of-field with a narrower band that's 'acceptably sharp'.
It's worth remembering that there's no sudden transtition from 'sharp' to 'unsharp': focus falls off gently on either side of the plane of focus, regardless of the aperture you use. It is fair to say, though, that larger apertures have a more rapid transition from in- to out-of-focus than larger apertures.
Controlling the depth-of-field in an image is achieved primarily by adjusting your aperture—a smaller aperture for a greater depth-of-field; a larger aperture for a shallower depth-of-field—however, there are other factors that affect it, too.
You'll often hear people say that telephoto lenses have a shallower depth-of-field than wider angled lenses. This isn’t strictly true. It’s more accurate to say that because telephoto lenses are mostly used to magnify subjects, and the subject will then fill more of the frame relative to the background, the depth-of-field appears to be shallower.
All the same, it's worth capitalising on the magnifying effect from telephoto lenses to pick out your subjects and surround them with blurred foregounds and backgrounds.
If you've ever practised macro photography, you'll appreciate how getting closer to your subject makes it harder to get it all sharp. The closer that you position your subject to your lens, the shallower your depth-of-field will be. Choose a subject further into the background and you'll find that the depth-of-field surrounding it is larger.
Distribution of acceptable sharpness
Depending on the focal length you use, you will find that the depth-of-field isn't divided equally in front of and behind the plane of focus. Instead, the area of acceptable sharpness behind the point of focus is generally larger than that extending in front of the focal plane. As focal length increases, so too does the distribution of the depth-of-field in front of the subject.
When you shoot with a focal length of 15mm about two thirds of the depth-of-field will be behind the subject and one third in front of it. When you get to 400mm it's closer to a fifty-fifty divide.
Depth-of-field: not just about aperture.