Photo specifications mix two unrelated systems of measurement, and the mixing is where most of the confusion lives. Millimetres describe a physical print. Pixels describe a digital file. DPI is the exchange rate between them.
The three quantities
Print size is what the photograph measures when it comes out of a printer - 35 by 45 mm, 2 by 2 inches, 50 by 70 mm. It is the only one that matters for a paper application.
Pixel size is how many samples the image file contains - 600 by 750, 600 by 600, 413 by 531. It is the only one that matters for an upload form.
DPI (dots per inch, often written PPI for screens) is a number stored in the file that says how many pixels should occupy an inch of paper. It changes nothing about the image itself. It is an instruction to the printer.
The arithmetic
Pixels = millimetres ÷ 25.4 × DPI. At the 300 DPI most authorities expect:
| Print size | At 300 DPI | At 600 DPI |
|---|---|---|
| 35 × 45 mm | 413 × 531 px | 827 × 1063 px |
| 2 × 2 in (51 × 51 mm) | 600 × 600 px | 1200 × 1200 px |
| 50 × 70 mm | 591 × 827 px | 1181 × 1654 px |
| 33 × 48 mm | 390 × 567 px | 780 × 1134 px |
This is why a US digital passport photo is specified as 600 by 600 pixels: it is a 2 inch square at 300 DPI. The two numbers are the same requirement written twice.
Where it goes wrong
Changing DPI without resampling. Setting a 413 by 531 pixel image from 300 to 600 DPI does not add detail. It tells the printer to pack the same pixels into half the area, so the print comes out 17 by 22 mm. The file is unchanged and the print is wrong.
Resampling to hit a pixel count. Enlarging a small image to reach 600 by 600 produces a file with the right dimensions and no more real detail than it started with. Sharpness checks notice, and so does anyone looking at the print.
Assuming DPI is enforced. Some upload portals read the DPI tag and some ignore it entirely, working purely in pixels. Tagging it correctly costs nothing and occasionally matters, so tag it.
Printing with scaling on. The most common failure of all. A file can be perfect in every respect and still print at 96 percent because a print dialog had "fit to page" selected. See printing passport photos at home.
How much resolution do you actually need?
Enough that the print at 300 DPI does not have to be invented. For a 35 by 45 mm photo that is 413 by 531 pixels in the final crop - not in the original image. Since the crop keeps only the head and shoulders, the original needs to be considerably larger.
Any recent phone camera produces far more than enough, provided the face fills a reasonable part of the frame to begin with. The problem case is always a crop from a group photo taken at distance.
Colour profile
The quiet companion to all of this is colour space. Portals and printers expect sRGB. A file tagged with a wide-gamut profile can come back with shifted colours, which matters when the background has to be a specific grey.
IDShot exports at the exact pixel size the document asks for, tags DPI and sRGB, and generates print sheets at true size so the millimetre and pixel specifications agree with each other by construction.