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Spots, dots and pixels just what are dpi and ppi? - part 2 of 1 2 3 4 5

by Mike McNamee Published 01/06/2011

So much for image resolution, now what about printer resolution? The numbers take an immediate leap upwards. Typical printer resolutions are 1440 dpi, 2880 dpi and now (Epson R30000) 5760dpi. The two types of resolution numbers should not be confused or interchanged. Photoshop deals in pixels per inch, printers deal in dots per inch!

A typical inkjet printer might have between four and 11 colour cartridges. Simplistically, if a six-cartridge printer fires two dots per colour to create a pixel then the dpi needs to be 12x the ppi. Sadly the reality is more complex! The printers change the size of the dots (variable droplet technology as Epson call it) using larger ones in areas of heavy colour. Then, to compound matters further, the printer fires the dots in a random (stochastic) pattern to break up the lines of dots and make for smoother transitions (dithering). All we can say for certain is that the dpi required is always many times larger than the ppi, and you should not get them mixed up.

The standard, higher-quality settings on an Epson printer call for 1,440dpi and 2880dpi in the driver settings. For this reason the preferred ppi values are either 180 or 360ppi because this divides into 1,440 eight times or four times. In practice, though, you will be hard pressed to notice the difference between using say 360ppi and 300ppi, the dithering engine accommodates the difference quite well and masks a poor match. At 180ppi things are a bit more of a struggle as there are fewer pixels to play with and, on architectural shots, you might notice the difference on corrugated roof cladding or angled roof lines. For landscape and portrait shots you will be hard pressed to detect any difference.

Having always lectured that 360 and 180 are preferred but noticed little difference in general practice, we set out to make an accurate comparison. We chose four image resolutions 180, 220, and 360 ppi and then, to really work the system, 354.6ppi! We used the architectural subject shown and examined the indicated areas in full-size prints. It is difficult to put numbers to the findings so a narrative verdict will have to suffice! The findings were as follows:

1. At the two higher resolutions the sloping roof line was cleanly rendered, at the two lower values it was jagged.
2. At the two higher resolutions the shadow noise on the wall was smoother. It was crisper at 180ppi than 220ppi indicating perhaps that the print engine was slightly smoothing the detail, in other words it was slightly softer.
3. The thin joint lines were visually smooth at both the high resolutions but divided up by the lower resolution interpolation into 18 small streaks at 180ppi but only 11 streaks at 220ppi, in other words the lower 180ppi value was slightly better.
4. The corrugations in the roof were true at both higher resolutions but disturbed at lower resolutions, more so at 180ppi than 220ppi. This result reverses the proposed trend.

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1st Published 01/06/2011
last update 21/07/2022 08:46:23

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