Why we ask for 4000 pixels on the long edge
When contributors write to us, the first question is often about size. Why 4000 pixels, when most phones now save more than that? Because 4000 is the number at which most of the layouts our customers make stop breaking.
Start from the print size
A magazine page prints at about 300 pixels per inch. A half-page picture of 20 centimetres across is 7.9 inches, so it needs roughly 2,400 pixels. A full A4 page is 8.3 inches wide and needs about 2,500. A poster at 50 by 70 centimetres is looked at from further away, so 150 pixels per inch is enough, which gives you 2,950 pixels. A 4000 pixel long edge covers all three, with room left to crop.
Cropping is the real reason
Designers rarely use the picture as it was taken. They crop to a square for a feed, to a tall strip for a banner, and to a wide band for a header. Cropping to a quarter of the frame throws away three quarters of the pixels. A 4000 pixel image becomes a 2000 pixel image, still fine for the web, and still usable at A5.
What does not matter
Megapixels on their own tell you little. A sharp 16 megapixel file beats a soft 45 megapixel one, and we reject files that are upscaled or heavily sharpened. We check at 100 percent zoom in the corners, where lenses are weakest, and we look at the sky and any smooth surface for banding and noise.
If your camera is older and gives you 3,500 pixels, write to us anyway. We make exceptions for pictures that are unusually good. We just cannot make them for pictures that are merely adequate.