A human preparing a document photo usually has a simple goal: one clean file that survives the next screen in the application. The advice below is organised around that goal.
Keep the units straight
Millimetres and inches describe physical output; pixels describe digital dimensions; DPI connects pixels to physical size during printing. If an authority already states the required pixel dimensions, use them directly. If you are building a print file from millimetres, the basic relationship is pixels = millimetres × DPI ÷ 25.4.
Where calculations stop being enough
A mathematically correct file can still fail because the face is the wrong size inside the frame, the background is unsuitable, the compression introduces visible artefacts, or the application expects a different file type. Finish every conversion with a visual and application-specific check.
When in doubt, retake
If the source is badly blurred, distorted or poorly lit, another photograph is usually better than a stack of corrective filters. Identity images benefit from a clean capture more than from heavy editing.
Make the requirement explicit
Write down the destination, frame, file rules and editing limits before exporting. Once those variables are explicit, most of the actual photo preparation becomes straightforward.
The part worth saving
If another person will handle the upload, send the final file together with its intended destination. This avoids the common mistake of submitting the correct portrait in the wrong format.
Visa Photo Pixel Size Guide
FastIDPhoto is useful when you need to turn one good portrait into a measured file or print sheet for a particular application.