How to Make a Giant Halftone Poster at Home

Most printing presses can only lay down solid ink — a spot on the page is either inked or blank. Halftone is the trick that lets them suggest shades of grey anyway. The image is broken into a regular grid of dots, and each dot's size follows the darkness of the picture at that point. From a normal reading distance, the dots are too small to see individually, so the eye averages them into smooth tones.

The technique spread in the late nineteenth century and let newspapers print photographs alongside text; look closely at an old newspaper picture and you will still see the dots. Colour printing overlays dot patterns in a few ink colours.

Artists later turned the dots into a deliberate style. Ben Day dots, a related commercial printing process used in comic books, became an icon of pop art when painters enlarged them to canvas size. That is the look most people mean today when they talk about halftone: dots made big enough to be part of the design, not hidden by it.

Stretch an ordinary photo across a wall and every pixel gets bigger, so a crisp phone snapshot can turn soft at a metre wide. Halftone sidesteps the problem: instead of trying to reproduce fine detail, it redraws the image as clean, sharp-edged dots. A dot printed at 6 mm across looks crisp whether the source photo was huge or modest.

The second reason is how we look at big pictures. Nobody studies a wall poster from 20 centimetres away; they see it from across a room. At that distance the dots merge into the image, while anyone who walks closer is rewarded with an abstract, graphic texture. You get two pictures in one.

• Hides low resolution, so smaller or older photos still make bold posters.
• Adds a deliberate, graphic style that plain enlargements do not have.
• Forgives small misalignments between sheets, because the eye follows dots rather than fine lines.

Dot spacing is the distance from the centre of one dot to the next. Small spacing gives more detail; large spacing gives fewer, chunkier dots and a stronger graphic look. The right choice depends on viewing distance and poster size.

A practical rule of thumb: allow roughly half a metre of viewing distance for every millimetre of dot spacing. With 6 mm spacing, the picture reads clearly from about 3 metres; with 2 mm, from about 1 metre; with 12 mm, from about 6 metres. Treat this as a starting point rather than a law — high-contrast subjects such as faces and silhouettes stay recognisable with bigger dots than busy landscapes do.

It also helps to think in dots across the whole poster. A 76 cm wide poster at 6 mm spacing is about 125 dots across, which is plenty for a portrait. Drop much below 60 or so dots across and detail starts to vanish; that can be exactly the bold effect you want, but check the preview first.

• Hallway or desk wall, seen from 1–2 m: try 2–4 mm.
• Living room or bedroom, seen from 2–4 m: try 4–8 mm (the default 6 mm is a good start).
• Large hall, shop window or event backdrop, seen from 5 m or more: try 10–20 mm.

The shape of the dots changes the character of the poster more than you might expect, even though the overall tones stay the same.

RasterPoster is free, needs no sign-up and runs entirely in your browser, so your photo never leaves your device. It accepts JPG, PNG and WebP images.

Start by uploading your photo, then choose your paper — A4, US Letter or A3 — and portrait or landscape orientation. Set “Pages across” anywhere from 1 to 20; the number of rows follows the shape of your photo. Pick “Fit whole photo” to keep the entire image, or “Fill all pages” to use every sheet edge to edge.

Next, change the effect from “Photo” to “Halftone B&W” or “Halftone colour”. Two extra controls appear: “Dot shape”, with circle, square and diamond, and the “Dot spacing” slider from 2 to 20 mm. The hint below the slider shows roughly how many dots will run across the poster, and the live preview shows the sheet grid on top.

Halftone hides low resolution, so a modest photo is fine. Finally, choose 150, 200 or 300 DPI and download either a multi-page PDF or a ZIP of JPG images.

Every page of the PDF matches your paper size, so print it at 100% or “Actual size” — never “Fit to page”, which would shrink the sheets and stop the dots lining up. Each sheet has a 10 mm margin with crop marks, an R(row)C(column) label and a small map showing where it belongs.

The dot grid is laid out across the whole poster rather than sheet by sheet, so a dot cut in half by a page edge rejoins exactly when you assemble the pages. Trim along the crop marks, lay the sheets out in order and join them from the back with tape, or switch on “Overlap edges” to add a 5 mm glue strip for a sturdier join.

• A grid of 2 × 2 A4 sheets makes about 38 × 55 cm; 3 × 3 makes about 57 × 83 cm; 4 × 4 makes about 76 × 111 cm.
• Black and white halftone looks best on bright white paper, where the contrast between dot and background is strongest.
• Matte paper avoids glare, which matters for a poster seen from across a room.
• Use a craft knife and a metal ruler for straight trims; scissors are fine for smaller grids.