Printing at true size

What scaling does to a ruling

A ruled sheet is a set of measurements, and a print dialogue is perfectly willing to change them. Settings named “Fit to page”, “Shrink oversized pages” or “Fit to printable area” all do the same arithmetic: they work out the factor that fits the page inside the paper in the tray, and apply it to everything on the sheet. The border scales, the type scales, and the ruling scales with them. Nothing warns you, because from the dialogue’s point of view nothing has gone wrong.

The trouble is that a spacing is not decoration: a 5 mm square is only useful while it measures 5 mm, and a quarter-inch square stops being one the moment it stops measuring 6.35 mm.

The two cases, with the real figures

Nearly all of this comes down to A4 and Letter, which are close enough to look interchangeable and are not.

An A4 sheet fitted onto Letter is scaled by a factor of 0.94074 — 94.1 per cent. On that print, a 5 mm spacing measures 4.7 mm. A quarter-inch spacing, which should be 6.35 mm, measures 5.97 mm.

A Letter sheet fitted onto A4 is scaled by 0.97267 — 97.3 per cent. There, a 5 mm spacing measures 4.86 mm, and a quarter-inch spacing measures 6.18 mm.

Neither is a rounding error you can shrug at, because the error is not on one line. It is on every line, and it accumulates: each successive line sits a little further from where it should be than the one above it. Anything that depends on the squares — measuring off the paper, working to a scale, counting cells across a plan — is working from a spacing that is not the one printed on it.

What to set instead

• Set the scale to 100 per cent, or choose “Actual size” where the dialogue offers that instead of a percentage.
• Turn off any option that offers to shrink or fit oversized pages. It is often on by default, and often a tick box some way down the dialogue.
• Set the paper in the dialogue to the same paper the sheet was drawn for. If the tray holds Letter, draw the sheet on Letter first rather than asking the dialogue to convert an A4 one — the paper is one of the numbers the drawing is made from.
• Where the dialogue offers to fit to the paper or to the printable area, take neither. Both are fitting, and fitting scales.

How to check: measure the bar

Every sheet from this press carries a measuring bar in the foot margin, printed at a stated length. On the metric sheets the bar is drawn 50 mm long and labelled “50 mm”; on the sheets ruled in inches it is drawn 50.8 mm long and labelled “2 in”, so it can be read against whichever rule is already to hand. It exists for exactly this: so a print can be checked with a ruler once it is out. A setting in a dialogue is a statement of intent; the bar is evidence.

Print one sheet. Lay a ruler along the bar and read it against the length printed beside it. If the two agree, the sheet is at true size and the rest of the stack will come out the same. Do this once for each paper and each tray you print from, and you can stop thinking about it.

When the bar comes out short

A short bar means one thing, and it is worth being precise about which thing: the file was right and the dialogue scaled it. The sheet is drawn from the measurement, so the spacing in the file is the spacing the file claims. Whatever went wrong happened between the file and the paper, so the fix is in the dialogue and not in the sheet. Redrawing it will change nothing.

The size of the shortfall tells you which. A bar that reads 94.1 per cent of its stated length is an A4 sheet that was fitted onto Letter: the 50 mm bar comes back at 47 mm, and the 2 in bar at 47.8 mm. One that reads 97.3 per cent is a Letter sheet fitted onto A4, where the same two bars measure 48.6 mm and 49.4 mm. Anything else, and the dialogue has a scale of its own set — often a custom percentage left in the box from an earlier job. Put it back to 100 per cent, print one sheet, and measure again before you commit the rest of the paper.