Blurred backgrounds, and what they cost.

A camera focuses on a plane, not on you.

The first thing to unlearn is the idea that a camera focuses on a person. It focuses at a distance, and that distance describes a flat plane standing upright in front of the lens like a pane of glass. Only what sits on that pane is truly sharp.

Nothing else in the picture is sharp, strictly speaking. What happens instead is that things close to the plane are only slightly unsharp, and at some point that slight unsharpness stops being visible. The zone where it is still invisible is what gets called depth of field, and its edges are a judgment call rather than a hard line.

This is why a face is a more demanding subject than it looks. A head is not flat. Sitting square to the camera, the tip of your nose, your eyes and your ears occupy several inches of depth. Turn your head and that spread grows, because the far eye moves further away from the lens than the near one. The band has to be deep enough to cover all of it, or something you would rather have sharp will not be.

That word and a few others get used constantly around a shoot without ever being defined, which is what the photography terms glossary is for.

Three things make the band narrower.

There are only three, and once you know them the whole subject stops being mysterious.

• The size of the opening in the lens. This is the f-number, and it runs backwards: a small number such as f/1.8 is a wide opening and a shallow band, while a larger number such as f/8 is a narrow opening and a deeper band.
• How close the camera is to you. Distance matters enormously. The same lens at the same setting gives a razor thin band up close and a generous one from across a room.
• The focal length of the lens. Longer lenses render a shallower band at the same distance and the same opening, which is part of why portrait lenses behave the way they do. The rest of that story is in the guide to focal length and your face.

There is a fourth thing that changes how the picture looks without changing depth of field at all, and it is the one people mix up most: how far the background sits behind you. Move the background further away and it becomes more blurred, because it is further outside the sharp band. Depth of field itself has not moved an inch. That single distinction explains most arguments about blurry backgrounds.

Why softness behind you does any work at all.

Attention follows detail and contrast. Given a frame with sharp edges in two places, an eye will wander between them. Remove the detail from everything that is not you and there is only one place left to look.

That is the entire mechanism, and it is worth understanding because it means blur is not the only tool for the job. Separation can equally be made by tone, choosing a ground that sits well away from the brightness of your skin, or by keeping less light on the background than on you. A backdrop does it with tone rather than with blur, which is the subject of the guide to white, gray or black backdrops.

Separation of some kind is not optional, though. A subject with no separation reads as though it were printed onto the background, and that flatness is one of the first things a viewer reads as amateur without being able to name it. It sits near the top of the list of what makes a photo look expensive, which is not really about money at all.

Enough, and too much.

Very shallow depth of field is the most overused effect in modern portraiture, and it is overused because it is easy and it looks impressive on a phone screen. On a real face it starts costing you things.

Here is the checklist of what goes first as the band narrows:

• The far eye, when your head is turned. Two eyes at different distances cannot both be sharp in a very thin band, and a soft far eye reads as a mistake rather than as style.
• The near ear and the hairline beside it, which turn to mush and take the sense of a solid head with them.
• The frames of your glasses, which sit forward of your eyes and go soft while the eyes behind them stay sharp, an effect that looks like a fault in the picture.
• The shoulder nearest the camera, which is often several inches ahead of the face and starts to smear.
• The tip of the nose, in the most extreme cases, which is when the whole thing collapses.

A working portrait wants a band that comfortably holds the eyes, the nose and the near ear, with the background clearly softer but not liquid. In practice that lands most head and shoulders work somewhere in the region of f/4 to f/8 rather than wide open, and the exact figure depends on the distance and the lens. The blur that remains should look like a consequence of the picture rather than the point of it.

Why blur added afterwards gives itself away.

Optical blur is produced by physics: light from a point outside the sharp band arrives at the sensor as a small disc rather than a point, and that happens smoothly, everywhere, in exact proportion to how far out of the band each thing is.

Blur applied after the fact works completely differently. Software has to decide where you end and the background begins, draw an outline, and then blur one side of it. Everything that goes wrong follows from that outline being a guess.

• Hair is the hardest edge in the world to trace, so strands get cut off, or a halo of sharp background survives inside your outline.
• The arms of glasses, a raised hand, a lifted collar and anything else that leaves your silhouette get treated as background and blurred away.
• Shoulders turned at an angle should get progressively softer toward the back, and instead the whole body is treated as one flat cut-out at one distance.
• The blur is often uniform, when real optical blur increases gradually with distance, so a background that should recede stays flat.

None of this is an argument about any particular device. It is a limit of guessing at an outline instead of recording depth, and it is one of the specific gaps behind why professional work beats a phone photo. You can see it for yourself in about five seconds: zoom into the hair and the glasses on any picture made this way, and the edges will tell you.

In a controlled room the problem changes shape.

On a plain backdrop there is no clutter behind you that needs hiding, so blur has almost nothing to do. Separation comes from tone and from light instead: the background set brighter or darker than you, and often an edge of light along the hair or the shoulder to draw a clean boundary.

That frees the depth of field decision to be about your face alone, which is how it should be. The band gets chosen so that everything from the near ear to the far eye is sharp, and the softness behind you is a gentle byproduct rather than an effect.

It is also why the way the background is lit matters more in a studio than the aperture does. A slightly out of focus gray field and a perfectly sharp one look nearly identical, whereas a background that is a shade too bright will visibly attach itself to your shoulders. Which source is making that light is its own question, covered in natural light compared with studio light, and it decides more about the final picture than the lens setting does. That is the same reason my sessions happen in a studio rather than in front of whatever happens to be behind us.

How to read it in a portrait you are shown.

Run these four checks on any portrait, the ones on my headshots page included. They take seconds and they tell you a great deal.

• Are both eyes sharp when the head is turned. If the far one is soft, the band was too thin for the pose.
• Look at the ear on the camera side. It should be slightly softer than the eyes and still clearly an ear.
• Follow the outline through the hair. Real blur passes smoothly through flyaway strands. Painted blur cuts them off.
• Look for a halo, a thin bright or sharp line hugging the shoulders and head. That is an outline that did not fit.

Do this on your current profile picture before you replace it, because it usually explains the vague feeling that something is off about a photograph you cannot fault on any other grounds. The rest of what a good one gets right is set out in the guide to professional headshots.