The safest starting point is the application itself: identify the document, the country, and whether the photo is being uploaded, printed or both.
The measurement first
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 good stopping point
Once the image meets the published requirement, stop editing. Extra sharpening, smoothing or retouching can create new problems even when the original task has already been solved.
Why this detail matters
A small technical mismatch can stop an otherwise complete application. File type, dimensions, framing and image quality are separate checks, so treat them separately when you prepare the output.
A note for this exact search
A practical habit is to preserve the source image and note why the final crop was made. Future versions can then be created from the original instead of from an already-resized JPEG.
How to Convert 35×45 mm to Pixels
For online workflows, verify the final dimensions and file size on disk before uploading.