How to print a photo mosaic

The download is one JPEG, 9600 by 12000 pixels, in a 4 by 5 upright shape. It contains 40 tiles across and 50 down, and each tile is 240 pixels square. Those numbers are the same for every page, however many photos it holds.

The download comes with unlocking a page, which costs $19 once. It takes up to a minute to prepare, and you can download it again whenever photos have been added.

Print size is pixels divided by pixels per inch. Printers commonly ask for 300 pixels per inch for something that will be looked at closely, and accept less for a large piece seen from across a room.

At 300 pixels per inch the file is 32 by 40 inches. At 240 it is 40 by 50 inches. At 200 it is 48 by 60 inches, and at 150 it is 64 by 80 inches.

Smaller prints simply use more pixels per inch than the paper needs: a 16 by 20 inch print works out at 600 pixels per inch, and a 24 by 30 at 400. The file is not the limit at those sizes.

With a mosaic, the more useful question is how big each tile ends up. Divide the print's width by 40.

At 8 by 10 inches each tile is 0.2 inches across, about 5 millimetres: the main picture is clear, but the individual photos are specks. At 16 by 20 a tile is 0.4 inches, about 1 centimetre. At 24 by 30 it is 0.6 inches, and at 32 by 40 it is 0.8 inches, about 2 centimetres.

If the print is meant for people to walk up to and search for themselves, go as large as the wall and the budget allow. If it is mainly to be seen as one picture from a distance, a smaller print does that job.

Sizes in the same shape are 8 by 10, 16 by 20, 24 by 30, 32 by 40 and 40 by 50 inches, or in metric 40 by 50 and 60 by 75 centimetres. Order one of these and nothing is cut off.

Poster sizes such as 18 by 24 or 24 by 36 inches, and A-series paper, are a different shape. The print shop must then either trim tiles from the edges or leave white bands. Ask for "fit, do not fill" and accept the white margin, or choose a 4 by 5 size and a frame to match.

Give them the original downloaded file, not a screenshot and not a copy sent through a messaging app, which may shrink it. Tell them it is 9600 by 12000 pixels and the size you want.

Ask them to print it as supplied, with automatic enhancement, sharpening and cropping switched off. Ask to see a proof or preview with the full edge of the picture visible.

A matte surface cuts reflections, which helps when people look closely at small tiles. Your print shop can show you samples.

We can state the pixel size of the file: 9600 by 12000 pixels. How a print looks also depends on the printer, the paper and the inks, which are the print shop's side of the work. Each tile is made from a phone photo that someone sent, so a blurred or dark original gives a blurred or dark tile at any size.

Download the file before the page's end date. The page and its photos are deleted 30 days after uploads close, and the file cannot be made again after that.