Getting a board printed
A poster is the one place a collage stops being a picture on a screen and starts being an object with a size. Two numbers decide whether it works.
DPI is a ratio, not a quality
Dots per inch is not a property a file has on its own. It is the relationship between how many pixels the file contains and how large it is being printed. The same 3,600 by 5,400 pixel export is a twelve by eighteen inch print at 300 DPI, an eighteen by twenty-seven inch print at 200, and a twenty-four by thirty-six inch print at 150 — identical file, three different posters, each one slightly softer than the last.
Which is why the board asks for both together. Change the DPI here and the paper size stays where you put it while the pixel count moves, because the paper size is the thing you actually chose.
| Finished size | 150 DPI | 200 DPI | 300 DPI |
|---|---|---|---|
| A4 — 21 × 29.7 cm | 1,240 × 1,754 | 1,654 × 2,339 | 2,480 × 3,508 |
| A3 — 29.7 × 42 cm | 1,754 × 2,480 | 2,339 × 3,307 | 3,508 × 4,961 |
| A2 — 42 × 59.4 cm | 2,480 × 3,508 | 3,307 × 4,677 | 4,961 × 7,016 |
| 12 × 18 in | 1,800 × 2,700 | 2,400 × 3,600 | 3,600 × 5,400 |
| 18 × 24 in | 2,700 × 3,600 | 3,600 × 4,800 | 5,400 × 7,200 |
| 24 × 36 in | 3,600 × 5,400 | 4,800 × 7,200 | 7,200 × 10,800 |
| 30 × 40 cm | 1,772 × 2,362 | 2,362 × 3,150 | 3,543 × 4,724 |
How much resolution a photograph really needs
The board can be 300 DPI and still print badly, because a board is only as sharp as the photographs on it. A frame two inches wide at 300 DPI wants 600 pixels of original behind it. A twelve-megapixel phone photograph has four thousand, so it has room to spare. A picture saved off a messaging app at 800 pixels wide does not, and blowing it up to six inches is where a collage poster usually disappoints.
- Check the widest frame first. Zoom the board to one hundred per cent and look at the largest photograph. If it is soft there, it will be soft on paper.
- Small frames forgive a lot. An inch-wide cell at 300 DPI needs 300 pixels. Almost anything clears that.
- Screenshots and app downloads are the usual culprits. They are small on purpose, and nothing recovers detail that was never saved.
Margins, trim and the edge of the paper
Every printed sheet is cut, and cutting is accurate to a millimetre or two rather than exactly. That has two consequences. Anything you want kept — a face at the edge, the corner of a frame you are fond of — should sit at least five millimetres inside the finished edge. And if the printer asks for bleed, make the board larger than the finished size by twice the bleed and let the outermost photographs run off into that band, so a slightly generous cut still lands on picture rather than on white.
This board draws one margin guide and nothing else: no trim marks, no crop marks, no registration targets. Those belong to a layout program and to the printer’s own workflow, and a rough approximation of them here would be worse than none.
What to send, and what to say
Send one file, as PNG where the upload allows it, named with its own dimensions — the export here does that for you. Tell the shop the finished size you want and the resolution the file is at, in the same sentence: “7,200 by 10,800 pixels, to be printed at 24 by 36 inches, which is 300 DPI.” That one line removes almost every conversation that follows.
Say what paper, too, because it changes the picture more than most people expect. Matt paper eats a little contrast and is kind to a busy collage. Gloss holds saturation and shows every fingerprint. Somewhere between the two — satin, lustre, whatever the shop calls it — is where most photographic collages end up.
Colour on this board is approximate by design. Everything is converted to sRGB when it is read, because a board of mixed profiles would shift colour at the boundary between two photographs, which is the most visible place a shift could possibly land. A CMYK scan is labelled where it comes in and converted roughly. Proper colour management for a print run happens at the print shop, with their profile and their paper, and nothing done in a browser tab replaces it.
Common questions before ordering
Is 300 DPI always necessary?
No, and for a large poster it is often wasteful. 300 is the figure for something held at reading distance. A poster looked at from two metres is indistinguishable at 200, and at 150 for anything above about A1. What matters is that the pixel count and the paper size are stated together — a file is never simply 300 DPI, it is a number of pixels that happens to be 300 per inch at one particular size.
Should I send PNG or JPEG?
PNG unless the shop asks otherwise. A photographic collage at poster size is a large PNG — a hundred megabytes is ordinary — but it is exact, and nothing in it was thrown away to make it smaller. JPEG at high quality is perfectly printable and a fifth of the size; use it when the upload is the problem, and set the quality high rather than merely acceptable.
What about bleed?
This board does not add bleed and does not draw trim marks. If your printer asks for 3 mm of bleed, make the board 6 mm wider and 6 mm taller than the finished size and let the outermost photographs run into that extra band. Anything you care about keeping should sit at least 5 mm inside the finished edge.
My file is too big to upload. Now what?
Drop the DPI one step and export again — 300 to 240 removes over a third of the pixels and is invisible on a wall. If it is still too large, JPEG at high quality will take it down again. Getting a file under a specific number of kilobytes is a different job with its own tools, and this board is not one of them.