Focus, and what it is for

A lens forms a sharp image at exactly one distance. Focusing is choosing that distance.

Everything nearer or further is technically not in focus. depth of field is the band either side where it is close enough that nobody notices — but there is still only one plane that is genuinely sharp.

The part the camera cannot do

Modern autofocus is fast and accurate. What it cannot do is know which part of the scene you care about.

Left to itself, a camera tends to pick whatever is nearest, or highest in contrast, or in the middle of the frame. For a portrait against a busy street that is often a lamppost.

The fix is to choose the focus point yourself rather than letting the camera choose. On most cameras this means switching from an automatic area mode to single-point, and then moving that point onto the thing that matters.

Single and continuous

Two modes, and the choice is about whether the distance is changing.

• Single focuses once and locks. Right for anything still: a landscape, a building, a person sitting down.
• Continuous keeps adjusting for as long as you half-press. Right for anything moving towards or away from you, because by the time a single lock has happened the distance has changed.

Sideways movement matters much less than movement towards you. Someone walking across your view stays roughly the same distance away; someone walking towards you does not.

Focus and sharpness are different things

A photograph can be perfectly focused and still not sharp — blurred by camera shake, or by the subject moving. Conversely it can be pin-sharp and focused on the wrong thing.

Worth diagnosing separately, because the cures are unrelated.

Key takeaways

• Only one distance is truly sharp; depth of field is the band that looks sharp enough.
• Autofocus is accurate but does not know what you meant — choose the point yourself.
• Single autofocus for still subjects, continuous for anything moving towards you.
• For portraits, focus on the nearest eye.