As we continue our discussion of aperture keep in mind that in addition to allowing more or less light in, the aperture also has a creative application: Depth of field.
Depth of field is the distance in front of and behind your subject that appears in focus. Shallow depth of field has a very narrow plane of sharp focus, the rest of the scene is blurred out. Conversely, deep depth of field has a very wide plane of focus, from foreground to the distant background.
A “Wide Open” aperture, represented by a low number has the narrowest plane of focus. As you “stop down” your aperture, making it a physically smaller opening (higher f/number) the depth of field increases. A good way to remember this is to think of what happens when you squint, more things become clear.
But of course that can’t be all there is to the subject of depth of field. In fact there are four more factors to be aware of, aperture being only the first among them. Here they are in no particular order:
Size of imaging sensor
As your sensor size increases your natural depth of field decreases. So f/4 on a full frame camera might look suspiciously like f/2.8 on a crop sensor. This is also why we rarely discuss aperture regarding point and shoot cameras. They have small imaging sensors and so changing your aperture does not have a significant impact on depth of field.
Focal length is our next factor. It’s simple to understand; the longer our focal length, the shallower our depth of field. To help you visualize the effect, here is a photograph of a bird.
f/8, but 300mm focal length
Our final factor is the physical proximity to our subject. The closer we are to our subject the shallower our natural depth of field. Again, a visual aid to help the discussion.
f/11, but close proximity
So go out and use your aperture priority mode – and remember you can use exposure compensation too! Just adjust your aperture to adapt to changing lighting conditions or to achieve a particular depth of field.