How do I get a properly blurred background in a photo?

Shallow depth of field gets talked about as though it were bought with a lens. It is not — it is a geometric consequence of four variables, and two of the four are free. The diagram shows how the zone of sharpness changes as each one moves.

Step by step

1. Understand what depth of field actually is.The lens focuses on one exact distance. Everything nearer and further is technically out of focus; depth of field is simply the band around that distance where the blur is small enough that the eye accepts it as sharp. Anything that makes the blur grow faster with distance shrinks that band.
2. Open the aperture.A wider opening — a smaller f-number — makes each out-of-focus point spread into a larger circle. Going from f/8 to f/2.8 is a dramatic change. This is the control everyone knows, and it is the one most limited by what lens you own.
3. Get closer to the subject.Depth of field shrinks rapidly as focus distance decreases. Halving your distance to the subject reduces the depth of field to roughly a quarter. This costs nothing and it is the most powerful of the four for most people.
4. Move the subject away from the background.This does not change the depth of field at all — it changes how far outside it the background sits, which is what determines how blurred the background actually looks. A person a metre from a hedge cannot have a soft background at any aperture; the same person ten metres forward can.
5. Use a longer focal length.At the same framing, a longer lens gives a more blurred background, because it magnifies the background blur along with the background. Standing further back with a 135mm produces far more separation than standing close with a 24mm, even at the same f-number.
6. Frame the same shot two ways to see it.Photograph a person filling the frame at 35mm from a metre away, then at 135mm from four metres. The subject is the same size and the backgrounds are unrecognisably different. This one exercise teaches the whole idea.
7. Watch how little is sharp when you push it.At f/1.8 close up, the sharp zone can be two or three centimetres deep. Focus on the near eye. Focusing on the nose or on the ear leaves the face soft, and it is the commonest reason a wide-aperture portrait looks slightly wrong.
8. Know when you want the opposite.Group photos, landscapes, product shots and anything where several planes matter need depth, not blur. Stop down to f/8 or f/11, step back, and focus roughly a third into the depth you need sharp.
9. Accept the limits of a small sensor.A phone or compact needs to be much closer, at the same framing, to get the same background blur, which is precisely why phones simulate it. A larger sensor is not magic — it lets you use a longer lens for the same framing, and that is where the effect comes from.

Tips

• Distance to the background is the free variable everybody forgets. Ask the subject to take five steps forward before you change anything on the camera.
• Very wide apertures on cheap lenses are often soft. If your f/1.8 shots look mushy rather than blurred, try f/2.5 — most lenses sharpen up noticeably one stop down.
• Background blur quality depends on what is in the background. Small bright highlights — leaves with sun behind them, distant lights — render as pleasing discs. A plain wall just goes soft.

Common mistakes

• Buying a fast lens and standing in the same place — Aperture is one of four factors and the most expensive one to change. A cheaper lens used close, at the long end, with the subject moved forward, beats an expensive lens used badly.
• Using the widest aperture for a group — At f/1.8 the front row is sharp and the back row is not. Group photos need f/5.6 to f/8, with everyone arranged on as few depth planes as possible.

If it doesn't work

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