Why this standard exists
ISO/IEC 19794-5 was published to give biometric systems a single, vendor-neutral way to encode and judge a face image. Before it existed, every passport agency and every face-recognition vendor had its own definition of an acceptable portrait. The standard fixes that by describing the image itself - geometry, illumination, expression, and a set of measurable quality attributes - independent of any particular country's policy.
Full-frontal versus token image
The specification defines several face image types. The two that matter for document photos are the full-frontal image and the token image. Full-frontal is the broader format used for stored enrolment images and shoulder-up portraits. The token image is a tightly cropped, normalized derivative used for matching: it fixes the inter-eye distance, the eye-line position within the frame, and the overall aspect ratio. Most modern e-passport chips store a token-style image so that border gates do not have to re-crop before matching.
Geometry and pixel budgets
ISO 19794-5 anchors the portrait on the eyes. Inter-eye distance drives everything else. For machine matching to be reliable, the standard expects at least 90 pixels between the eye centres in the stored image, with 120 pixels recommended for higher accuracy. Head height, defined from chin to crown, sits in a defined ratio relative to the image height. Roll, pitch, and yaw - the three head-rotation axes - each have tolerances measured in degrees rather than vague phrases like 'face the camera'.
Quality attributes
The standard enumerates a long list of quality attributes that an automated checker can score: sharpness, contrast, exposure, lighting uniformity, background uniformity, presence of red-eye, presence of shadows on the face or background, eye openness, mouth closure, glasses reflections, and so on. Each attribute has a numeric threshold or a pass/fail rule. This is what lets a photo tool produce a deterministic compliance score rather than a subjective opinion.
Relationship to ICAO 9303
ICAO 9303 is the policy document a passport agency cites. ISO/IEC 19794-5 is the technical document that ICAO 9303 leans on when it talks about portrait pixels and quality. If you read both, you will see ICAO 9303 deferring to the ISO standard whenever the question is 'how do we actually measure this'. National rules - the US 2 by 2 inch print, the Schengen 35 by 45 millimetre print, the UK no-glasses rule - sit on top of both.
What changed in 19794-5:2011 and beyond
The 2011 revision of ISO/IEC 19794-5 tightened expression rules, clarified the token image geometry, and added explicit quality scores for non-uniform backgrounds and shadows. Newer profiles continue to add scoring for digital artefacts such as JPEG compression banding and beauty-filter smoothing. If you are building a checker today, target the most recent profile your agencies cite, but expect them to lag the ISO release by several years.