DPI, Paper and Why a Correct File Prints Wrong

A file can have the right dimensions in pixels and still print at the wrong physical size, or print at the right size and look soft. Both come down to DPI, which is the least understood number in the whole process.

What DPI actually is

Dots per inch is a ratio between pixels and physical size. It is not a property of the image on its own. The same 600 x 600 pixel file is a 2 inch square at 300 DPI, a 1 inch square at 600 DPI, and a 6 inch square at 100 DPI.

Changing the DPI number without resampling changes nothing about the pixels. It changes the instruction to the printer about how large to make them.

This is why "my photo is 300 DPI" is an incomplete statement. The useful question is how many pixels the file has, and what size it is being printed at.

The numbers you need

Take more pixels than you need and crop down. A modern phone photo has ten times the resolution required, so there is no reason to be near the minimum.

Paper

This is where home printing most often fails, and the failure is visible before anyone measures anything.

Plain paper is refused. It absorbs ink, which shifts colours toward the dull and soft, and the surface texture is obvious. A counter clerk will hand it back without measuring.

Matte or semi-gloss photo paper is the safe choice. It holds colour, resists fingerprints, and does not produce the reflections that make a photo hard to scan.

High gloss works but is riskier. It reflects overhead lighting at a counter and at some scanners, and a few authorities specifically prefer matte for this reason.

Weight matters less than surface, but very thin photo paper curls and is hard to cut cleanly. Something around 200 gsm or heavier behaves better.

Printing more than one

Passport photos are almost always required in pairs, and printing a single 2 inch square on a sheet of 6 x 4 photo paper wastes most of the sheet. Laying out several on one sheet and cutting them apart is both cheaper and more accurate, since you cut once along a straight edge rather than trimming each separately.

Cut with a guillotine or a craft knife against a steel rule. Scissors wander, and a photo trimmed to the wrong size by a millimetre is a rejection for a reason that had nothing to do with the photograph.

When printing at a shop is better

If you do not have photo paper and a printer you trust, a print shop or pharmacy will produce a better result for very little. What they will not do is fix a photo that is wrong — they print what you bring. Getting the crop and head height right first is the part that matters, and it is covered in printing a passport photo.

Colour, briefly

Printers interpret colour differently, and a photo that looks correct on screen can print with a colour cast — usually warm from an inkjet, or slightly green. The background is what shows it, since it is meant to be plain white and any cast is obvious against that.

If the print comes out tinted, the fix is the printer's colour settings rather than editing the photo, because the file was correct. Printing from a phone directly to a photo printer removes several places a cast can be introduced.