The conventions differ enough that the wrong set hurts

This index is long because professional portraiture is not one subject. It is several, and they contradict each other.

The clearest contradiction

Medicine wants competence and approachability at once. Law wants restraint, with warmth optional and past a point counterproductive.

Put both briefs side by side and no single headshot satisfies them. That settles the idea of one correct professional photograph faster than any argument.

The inversions

Acting portraits show range rather than reliability, which means less retouching, not more. Makeup artistry cannot be retouched at all without erasing the evidence. Interior design is the one case where the background may legitimately be a set.

What is genuinely universal

Contrast between hair and background, a crop matched to the display size, frontal rather than side light.

Everything else is convention — worth knowing precisely so you can tell when a guide has mistaken its own field's habits for physics.