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Square crop in a Polaroid frame — Effective Ppi Before Printing

Guide

Effective PPI before printing

A print file has two resolutions: the one it is tagged with, and the one its photograph actually delivers once it has been fitted to the paper. Only the second decides how sharp the print looks.

Tag and reality

A file made for a 4 × 6 inch sheet at 300 PPI is 1200 × 1800 pixels. Every such file is “300 PPI”. What differs is how many pixels of the original photograph were behind those 1800: a 6000-pixel original was reduced, and has detail to spare; an 800-pixel one was enlarged, and has the detail of 800 pixels spread over six inches.

Effective PPI is that second number: source pixels divided by printed inches, on whichever axis limits the fit. It is the figure the print-size page reports, with a warning under 200 and a stronger one under 150.

Worked figures

A landscape photo on a 6 × 4 in sheet, 5 mm margins, 300 PPI
PhotoPrinted widthEffective PPI
4000 × 30004.81 in832
1600 × 12004.81 in333
1200 × 9004.81 in250
800 × 6004.81 in166

The image area is 1682 × 1082 pixels; a 4:3 photo is limited by the height and drawn 1443 × 1082 pixels, 4.81 × 3.61 inches. Divide the photo’s own width by 4.81 and the effective PPI follows.

Questions

My file says 300 DPI — is it print quality?

Not necessarily. The tag says how the pixels should be spread over paper; it says nothing about how many there are. A 600 × 400 file tagged 300 is a 2 × 1.33 inch print. Stretched to 6 × 4 inches, it prints at 100 effective PPI.

What effective PPI is good enough?

300 is the usual target for photo prints viewed close up; 200 is acceptable for most; under 150 looks soft at normal viewing distance. Large prints viewed from across a room tolerate less.

Can I raise effective PPI?

Only with more source pixels: a less cropped original, or a smaller print. Enlarging in software adds pixels, not detail.

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