What resolution should my artwork be?

This article addresses the issue of file size, and suitable resolutions for large format printing.

What file size do I need? This is a commonly asked question, to which the answer is: large enough so the print doesn't pixelate; large enough that you're happy with the detail of the final print; small enough that the file is manageable and transferable.

When discussing resolution we are discussing raster images or bitmaps, either in standalone form (eg. .jpg, .tif) or as components within other file types (eg. .pdf .eps ) If you are unfamiliar with raster and vector image types, we recommend reading the article on this critical issue before continuing. 

The first critical issue to address is what is "resolution"? Often you'll hear your image file resolution incorrectly defined as dpi, or dots per inch. Your image resolution is actually measured ppi, or pixels per inch. Dots/Pixels - all the same right? Well no. They're not, they're not inter changeable, and they in no way relate to each other. 

Pixels Per Inch - each pixel in an image is the smallest defined point of colour. A pixel may be any colour, so each pixel colour is defined by a set of numbers. In a CMYK colour space, a pixel will have four numbers to define its colour. In an RGB space, it will have three. The number of pixels per inch determines the level of detail within the file. To use an extreme example, if a file was at a resolution of just five pixels per inch, each pixel would be 5mm x 5mm. Close up the image would look like a mosaic. It the file was at a resolution of 200 pixels per inch, then each pixel would be 0.127mm x 0.127mm and close up the image would look like a photograph as you wouldn't be able to make out individual pixels. 

Dots per Inch - a printer's output resolution is defined in dots per inch (DPI). The important distinction between PPI and DPI is that a printer can only output dots in the ink colours it uses. (normally CMYK). When you think about it therefore, it's obvious that PPI and DPI are completely different and have no relationship to each other. DPI determines the number of dots a printer can put down in an inch in the colours it has, to fool your eye into seeing the coloured pixels the file asks for. A Mimaki JV3 for example will often run at 720dpi (more accurately referred to as 720x720). 

Often people make the mistake of thinking they need to create artwork at 720ppi as this matches the best output the printer can achieve. This is just wrong - the two things, PPI in your file, and DPI on the output machine are in no way linked or relevant to each other.

When creating artwork, you don't need to consider the printer's resolution (DPI) it is of no relevance to what you do. What you need to know is the normal viewing distance of the final print. For example - consider the 5mm x 5mm pixel we discussed earlier. We described the file as a mosaic. How far away would you need to be though before those 5mmx5mm square tiles, were not obvious? At that point, and all distances beyond it, the image will look sharp and detailed.

So there is no single answer to the question "what resolution do I need?". There are a few general guide rules we can highlight though. (all ppi values below are "at final output size.)

For crisp lightbox images for close up viewing, or high quality photo prints on gloss photo paper, 200ppi. 

For close up poster prints, commercial lightbox work (eg. fastfood outlet etc) canvas prints, other detailed imagery 150ppi.

For standard posters, roller banners, pop up stands, close up viewed items 100ppi.

For outdoor banners and other large displays viewed from a couple of feet 75ppi.

For anything viewed solely at distance 50ppi, or lower for very large items at extreme distances.

These are only guides, and there are many examples where perfectly acceptable prints have been produced from low resolution files. 

Seldom is it worth increasing the ppi. Upscaling can be used to "smooth" files that would otherwise pixelate, but at best this results in a softening of the image. Although the resolution does increase, the detail in the image doesn't. (If you're struggling to picture this, consider the 5mm x 5mm pixels, now divide each pixel into 1mm x 1mm pixels. So you've just increased the resolution by a factor of five, yet it looks identical. In fact, photoshop and other editors "blend" the extra pixels to smooth the transition between the colours, thus "smoothing" the image.