Pixels At A Distance

Posted on July 31, 2015 at 12:00 AM

IMG Pixels 1b

This is always a hot topic in digital print... just how many pixels does an image need to get a good result? Rather than let you rely on taking our word for it, we've run a series of prints so you can see for yourself. We have done a range of prints from 10ppi to 300ppi to demonstrate exactly what a print at a specific resolution will look like. The results are quite interesting.

We've been here before. First up, I'd like to note we ran a similar experiment back in the early 2000's. It was to ascertain some real numbers for use in our popular file preparation form (here) that assists designers to create great files for the very best printed output. After running those tests we were sold on the idea that the data required in your file should be dictated by the distance the print will be viewed from. It was way more important back then because transporting large files was way more of a mission than present. Good old Zip discs were the most popular storage along with CD's and DVD's. The aim was to get a file as small as possible literally so you could save it onto something that could be taken to the printer.

Gigabytes of files. Interestingly even with the ability to cart around a tiny USB device with massive storage capabilities, the pixels vs viewing distance is as important as ever. The difference now is things have changed over time in the other direction. The internet and digital capture devices have created a trend for smaller files. Most of the time the question these days is not "is my file too big?" But is instead "is my file big enough?"

Step one. We created an image with features to judge the quality of the print such as differing text sizes and various hard edges that will exhibit plenty of pixel steps. The majority of the image started as vector and we also included a high resolution raster image that had plenty of detail for even the highest 300ppi chart.

This test was for bitmap/raster/photo files, not vector which can scale to any size without loss of quality. One by one individual files were rasterised from the original file to represent resolutions from 10 to 300 ppi.

A note on ppi. In the past it was referred as dpi both in print and software. Photoshop now uses pixels per inch to represent the quantity of data in a file. The printer we used for this test was a high end HP Latex 360 running at a processing resolution of 600dpi (dots per inch). These results would be different on a lower resolution printer.

Step two. After completing all the prints we set them up on a wall for evaluation with the trusty human eye, nothing scientific going on here! Using marks on the floor going back to a long viewing distance of 20 metres, we evaluated each one next to it's neighbour to see at what point the two looked indistinguishable. This proved harder than first thought as some were too far away to make out the text regardless of quality. Saying that, we thought it would be good to note a recommendation of readable text height per resolution.

So how far away does 10dpi compare with 300dpi? Well we were pretty convinced at 20 metres you could barely tell them apart. So if you have a photo that would give a measly 10dpi at finished size, don't give up on it as it's pretty good if you can guarantee no one will get closer than 20m to it.

Our champion 150ppi. This is the resolution we recommend for most applications. It's only when you want a super sharp image for in your face viewing that you need more pixels in the mix. You get a really nice image for most applications and the files are not ridiculously huge or a pain in the neck to manage.

It was surprising how fast the images started to look the same. It was the fine text that really gave away which ones were the smaller files (note the text in the black box under the camera on the 10ppi chart above). In the majority of cases if you are wanting a photo printed you can get away with very limited file data for some very nice results. 

Interestingly we found that 200ppi is about the maximum you want to go. The difference between 200ppi and 300ppi requires some close attention, specifically looking to find clues that they are different resolutions.

Here are the results:

Raster images and text printed at 600 dpi 

Resolution Distance (acceptable) to (same as 300ppi)
10 ppi 12 to 20 metres
30 ppi 4 to 6 metres
50 ppi 2 to 3 metres
80 ppi 1.5 metres
100 ppi 1 metre
150 ppi .5 metre
200 ppi less than .5 metres
300 ppi less than .5 metres

One important thing to remember is this test is not using vector artwork. Another is that we don't account for fudging the files with techniques like interpolation where you add new data into the file to remove the pixelation artificially. We did not test for this so although the techniques do soften the edges from awful pixel staircases, we don't know how different they would be compared to a file that has all the right pixels in all the right places.

Where possible always use vector, especially when it comes to text; that way you know it's a safe bet for any size or viewing distance.

We have the images available here in our showroom for you to look at and see for yourself.

Tags: technology, Articles, quality control;