Digital SLR photography — Why does a telephoto lens blur the background?
Published April 3, 2011
In the first page of this beginners’ guide to SLR photography I talked about the close relationships between depth of field, aperture and shutter speed. Aperture becomes a brilliant tool for controlling depth of field, especially in those instances where you want to blur the background away. But using a telephoto lens can soften the background too.
A combination of aperture setting and focal length allowed me to reduce the cluttered background in this shot to just a blur. The bird, by the way, is a Willy Wagtail.
Switching between a 50mm lens and a 400mm lens makes a huge difference to the background, despite the fact that the aperture is the same.
A lot of people get confused about depth of field. To put it simply, depth of field is the distance between the closest and most distant areas in focus. You can use a small depth of field to blur a background or you can use a telephoto lens to blur the background. Or you can use a combination of both. So, if depth of field is controlled by aperture, then what is that telephoto lens doing? Does that also involve depth of field?
Take a look at the two photos at right. Both shots were taken with the same camera using the same aperture. The only difference was that I changed lenses. You’ll see right away that the background looks much softer in the 400mm shot.
Is the depth of field the same?
I would say it is! In both shots, the very same bits are in focus (the whole flower and none of the background).
So why is the background more out of focus in the 400mm shot?
Actually it’s not! What it is, is more ‘stretched’.
To understand what’s going on, have a look at the diagram above. We’re starting with the 50mm lens. A 50mm lens takes in a sweeping, wide expanse of background behind your subject. That background might be out of focus but there will be a lot of it in the frame.
Now look at the next diagram (above). This shows what the 400mm lens ‘sees’. Looking through the 400 is like looking through through a long pipe where you lose all your peripheral vision. The background is still out of focus but you’re only seeing a small patch of it, and that small patch fills the entire frame behind your subject.
Can you see now, what I meant when I said the 400mm (telephoto) lens ‘stretches’ a small patch of background in a way that makes it look softer?
It’s that stretching (or not stretching) which makes the backgrounds look so different. When you combine that effect with the depth of field controlled by your aperture, then suddenly you have a lot of power over how your shots turn out!
So if you want to make your subject really stand out — by getting it in sharp focus against a very, very soft backgrounnd — use your longest lens opened up to a nice big aperture (small f-stop number) and better still, try to have the background a long way behind your subject.
It’s not rocket science. What it is, is another cool trick you can use when you’re out taking photos.