A passport photo is one of the few pictures in life that is graded by a machine against a written specification, and the specifications disagree with each other. The United States wants a 51 × 51 mm square with the head at 50–69% of the height; the United Kingdom wants 35 × 45 mm with the face at 29–34 mm and a cream or light-grey background; Canada wants a tall 50 × 70 mm frame in which the same face must shrink to 44–51%. A photo that passes one country’s check is, measurably, the wrong photo for another — which is why this section is organised by document rather than by a one-size crop.
Each guide states its numbers the way the issuing authority publishes them, links to the official page, and is honest about the edges: where an authority publishes no numeric head band (India), the guide says the framing is guidance; where a rule cannot be satisfied by software at all (Canada’s photographer stamp, Australia’s ban on altered photos and glossy-print requirement), the guide says that too, in plain sight, rather than implying a download solves everything. The maker then does the parts a browser genuinely can do — exact dimensions, guided composition, permitted background colours, and print layouts at a true 300 DPI — without ever taking your photo off your device.