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.
A useful record to keep
Note the date taken, destination document, dimensions and final format. This creates a simple trail that helps when the same photo must be prepared again for another portal.
A passport photo is an application-specific deliverable
The same face can legitimately need several outputs: one physical print, one square digital image, or a country-specific frame. The portrait is reusable; the final file is not necessarily interchangeable.
Keep the original portrait separate so each version can be prepared without degrading the master.
What to remember from this page
When you finish, record the authority or form beside the output. That turns a generic image file into a traceable application asset and reduces mix-ups between similar document photos.
How Compression Affects Passport Photos
Create your document-specific version from the original master and keep the final file clearly labeled for the application.