Although you only focus your DSLR camera on one point of a scene, you’ll have noticed that some pictures appear to be sharp all the way through from front to back, while others are only sharply focused at one point. The depth of this zone of sharp focus is caused by the size of the hole in the lens, so the aperture setting isn’t just for ensuring correct exposures – it’s also used for controlling which areas of a picture are sharp, and which are defocused.
Though small apertures (large f-numbers) require longer shutter speeds to get enough light onto the sensor, they also give the greatest depth-of-field, so if you want a picture that’s sharp from front to back, you’ll need f/16 or f/22 to make it happen. Unless it’s extremely bright, you’ll probably also need a tripod to keep the DSLR camera steady because the shutter speed will be much longer than at f/2.8.
There are many times when photographers want to isolate a particular part of a scene and throw the rest out of focus, though, and for these occasions you need a large aperture, like f/2.8 or f/4. This will ensure that only a narrow zone around the point you focus on will remain sharp. This means that focus has to be as accurate as possible.
Many DSLR cameras are equipped with a depth-of-field previews button, which helps take the guesswork out of what will be soft and what will be sharp in your picture. Depress this button when you’re framed-up, and it’ll reveal how the shot will be in the viewfinder.
The only downside of depth-of-field previews is they make the scene very dark with small apertures (because less light is entering the lens) and this can make it tricky to see what’s happening – especially in low-light conditions. If you’re struggling to see what’s going on, just take the shot and check the screen image to see what’s sharp and what’s not.
Depth-of-field & digital compacts
Although creative compact cameras have an Aperture Priority mode which generally allows all apertures from around f/2 to around f/8, they cannot offer the same depth-of-field control as DSLR cameras because of the smaller sensor size and very short focal lengths on the lens units.
For shots like landscapes where you’ll often want a large depth-of-field, this is not a problem as huge depth-of-field comes built-in, but when you want a narrower plane of sharp focus, it’s a real issue. Even at large apertures of f/2 or f/2.8, compacts can’t produce the same kind of defocused backgrounds as DSLR cameras, so you’ll need to resort to Photoshop’s Gaussian Blur or Lens Blur filters to generate these kind of effects.