OK, this seems like a good place to talk about of depth of field in more depth (pardon the pun) It’s quite a hard topic to pin down in a technical course, as it’s affected by several factors, so could easily fit in a number of areas. Best I cover it now, as I’ve been promising more info for ages.
Now, one proviso – what I’m about to say isn’t actually, strictly, totally 100% scientifically accurate. However, this is one of those instances where knowing the real science makes no difference, and what you need to know is the practical tools to make something work. So don’t get too concerned if further down the line in your photographic career you discover that some of these facts aren’t quite right – this is what works in the real world. The reason I’m explaining things this way (and forever couching what I say in caveats) is that the actual scientific explanation can be quite confusing, and all that matters is what you do practically!
So, as we’ve already talked about, depth of field (d.o.f.) is the area either side of your chosen point of focus that’s also in focus. Here’s an image with a small (shallow) depth of field:
And here’s one with a large depth of field:
Factors affecting depth of field
Now, you may want a different amount of depth of field, depending on what effect you want in your shot. A common use for lots of d.o.f. is a landscape shot, where you want to get everything from the foreground to the background in the distance in sharp focus. Shallow depth of field is often used in portraits or sports images to concentrate attention on the subjects eyes, or isolate them against the background. But how do you achieve each effect?
Depth of field is affected by (remember – in practice):
The focal length of your lens
The aperture you shoot at
The distance between the camera and the subject (the point you’re focusing on)
Image sensor size
And the variables are:
The shorter the focal length of the lens, the greater the d.o.f.
The smaller the aperture, the greater the d.o.f.
The further your subject from the camera, the greater the d.o.f.
The smaller the sensor, the greater the d.o.f.
And the opposite is true for all these variables. So, to take an image with a huge depth of field, use a short focal length lens (a wide angle), a small aperture (say, f16), make sure your subject is a decent distance from the camera, and shoot with a camera with a small sensor – something like a camera phone tends to have a huge depth of field, and struggles to give you anything else. Flip these around for a shallow depth of field – longer lens, wider aperture, closer subject distance, and larger sensor.
As with most things, depth of field exists on a curve – there’s no on or off button for it. When you bear in mind that you probably can’t change the size of the sensor you’re shooting with, you’ve only really got the other 3 variables to alter if you want to change it. With the variables at your disposal, you’ll only have so much range of depth of field to play with, before you end up changing your shot so dramatically that it’s not what you intended to shoot. You may only have one lens with you, or the location you’re shooting in may only be big enough to allow you get a short distance from your subject. As with so many of the “rules” of photography, you don’t have to play along, but as long as you understand the principles at work you can either make informed choices in order to create the shot you want, or decide that with the kit you’ve got with you, in the situation you’re in, you’re going to struggle to get the look you’re after.
How can you tell how much depth of field you’re going to get in the final image? Well, if your camera has one, then you can use a little button called a depth of field preview. During normal operation, the aperture on your camera is held open at it’s maximum, in order to let lots of light in so you can compose and focus easily. At the moment you press the shutter, the aperture will then stop down to the required setting (if it’s less than the maximum of course) so that your picture is correctly exposed. By pressing the depth of field preview button you stop the aperture down to whatever setting you’ve picked, and providing you can still see through the viewfinder (it can get quite dark!) you’ll now be able to see how much of the image is in focus. Exactly where this button is will vary from camera to camera, and increasingly these days you’re able to customise which button you want to use. It’s not a tool you’ll use every day, but it can be very handy.