Two habits that stop you paying for a reprint

The arithmetic behind print resolution is one line long and almost nobody gets it wrong. Reprints happen anyway, and when you look at how, the cause is nearly always the same: the right calculation, performed at the wrong moment.

Crop first, then check the density

This is the one that costs money. A 24-megapixel file is comfortably enough for a 16 × 20 inch print — until you straighten the horizon, trim to a 4:5 aspect, and tighten in on the subject. Each of those throws pixels away, and none of them announces it.

Crop a 6000 × 4000 file to 3800 × 4750 and you still have 18 megapixels, which sounds fine. At 16 × 20 inches that is 237 PPI, not the 300 you were planning on. It will probably look acceptable. It will not look like the proof you approved.

The habit is simply an ordering rule: finish the crop, then run the numbers on what is left. Check the density on the file you are actually sending, never on the file you started with. The print size calculator takes the final pixel dimensions and reports the size they support; the DPI checker reads them straight out of the file if you have lost track.

Match the density to the distance, not to a rule

The second habit is a decision rather than a check. “300 DPI” is repeated as though it were a law of physics; it is a sensible default for one specific viewing distance and wasteful outside it.

Acuity is angular, not absolute — what the eye can separate depends on how far away it is — so the density you need falls off with distance:

• Held in the hand, 25–40 cm — 300 PPI. Below about 240, fine detail visibly softens.
• Wall art at a metre — 150 to 200 PPI is indistinguishable from 300 for most subjects.
• Posters at two or three metres — 100 to 150 PPI.
• Banners and billboards — 30 to 72 PPI is normal, and files are often built at quarter scale and scaled up on the RIP.

The practical effect is large. Take a poster at 24 × 36 inches: insisting on 300 puts it beyond 75 megapixels, which almost no camera delivers, while 150 lands under 20 — comfortably within reach, and indistinguishable from two metres away. 300 DPI in Pixels covers where the standard came from and when to leave it behind.

Deciding the distance first also settles the arguments earlier. “Is this file big enough?” has no answer. “Is this file big enough for something read at arm’s length?” has one.

Proof a detail, not a thumbnail

Both habits fail the same way — silently — which is why the last step is to look at something.

Do not proof the whole image scaled down; that hides exactly the softness you are checking for. Print a 4 × 6 inch crop of the busiest area at the intended density, on the intended stock. Foliage, fabric weave, hair, small type. Five minutes and a sheet of paper against a full-size run.

If you cannot print, view at 100% zoom — one image pixel to one screen pixel — and remember you are looking at it far closer than the viewer will. If it holds up there, it will hold up on paper.

The one line, for completeness

Everything above rests on the same identity:

pixels = inches × PPI

Worked examples at every common size, and the reverse conversion, are on How to Convert Pixels to Inches. The arithmetic was never the hard part — doing it after the crop, and looking at the result before you commit, is.