Most guidance about professional portraits is about how to look. What decides whether an image works is three measurements, and they are dull enough that almost nobody checks them.
The display size
Not the file size — the size it is actually seen at. In a list, roughly forty-eight pixels. On a conference badge, about fifteen millimetres. On a business card, less.
At those sizes skin texture, eyebrows and fine gradients are gone, leaving the outline of the head and two or three masses of light and dark.
The crop ratio
Everything below the collarbone is wasted area. The most common fixable problem in the whole subject, and it needs no reshoot.
The contrast between hair and background
Invisible until the image is small, then total. The fix is separation from your hair colour, which is a different colour for different people — which is why blanket advice to use white fails for about half of people.
Then the conventions
The general treatment covers those, and it is worth reading against the three numbers rather than instead of them. Advice about expression and wardrobe is real but second order; geometry decides first.
Going deeper by subject
The material splits into genuinely different problems and it is worth landing on the right one. The index lists them.
The tools section is about choosing software, where the criterion that matters is not sample quality but whether the output still looks like the specific person.
The industry section covers profession-specific conventions, which genuinely contradict each other — medicine wants warmth, law wants restraint, and no single photograph satisfies both.
The professional portraits section collects the rest.
The thing you can give someone else
A gift card rather than a finished image, because a portrait is a statement about how someone wants to be seen and that cannot be made on their behalf. Assigning someone a face lands as an obligation; giving them the ability does not.
The test
Scale your photograph to the size it is displayed at and set it beside the original. If you cannot tell where your head ends, that is the version almost everyone sees, and no resolution changes it.
