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Resolution vs Size

A common question we’re asked at the design stage is if files being supplied are suitable for use on a sign.

Recently we were asked to consider a 4x2.4m wallpaper while being supplied a 255x197 pixel photo. If you haven’t already picked up the potential problem, it’s best that you read on.

Those paying attention will remember from our previous [Rasters vs Vectors] article that vector artwork is our first preference for logos, text and graphics as it can be reproduced at any size without loss of quality. But for photographs and images (eg. including most files you’d download off the internet) there are 4 things we need to consider to ensure a great result:


Photos displayed on your monitor are comprised by thousands of coloured boxes (pixels), that blend to give the image that you see. The total number of these pixels, divided by the display size, gives a DPI/PPI  figure (Dots Per Inch/Pixels Per Inch). Once the resolution of an image has been reduced, that information is lost and it can’t be recovered or enhanced.

Eg. iPhone 6 camera

iPhone 6 camera = 8 megapixel = 7 990 272 pixels  = 3264px wide X 2448px high

The same image posted to Facebook = 1024 x 683 px (1/3 of the resolution!)

Iphone Photo Magnified


The final size that the image will be produced is an important consideration, as a photo on a brochure will have greatly different requirements to one being used for a wallpaper. The number of pixels in an image is fixed, so increasing the size reduces the resolution proportionally.

Eg. The iPhone 6 photo produces an image 300dpi @ 27.6x20.7cm (just smaller than an A4 page).

If you were to enlarge the image 10 times to suit a large sign, then the image would be

30dpi @ 276x207cm


Consider the differences between viewing a brochure at 30cm and a billboard at 30m. The greater the distance you’re viewing from the less observable the resolution will be. If you ever get the chance to safely stop on the highway near a billboard, walk up close and you’ll probably be surprised how low the print resolution is! But from its intended viewing distance you’d never know.

In the mock-gallery below, it’s difficult to pick the difference between the 300dpi and 30dpi images from this 3m distance. But if you reduce the quality further or approach the wall closer, you will start to see the loss of quality.

Rose Resolution

We use a simple formula to evaluate at the design stage whether the end result will be of suitable quality at its intended viewing distance:

SOT Distance vs DPI Table


This is less to do with quality and more to do with legalities. The images you find in a Google search probably don’t have appropriate licencing or permissions to be used on your signs. We are able to purchase images licenced for commercial use, and they’re typically much higher resolution as well.

Hopefully you can now understand that the wallpaper I mentioned at the beginning wouldn’t look much good with the supplied image. In fact at only 1.6dpi, at a normal viewing distance you’d probably have difficulty deciphering what the photo even was. We’re here to help make sure you get a great result. If in doubt, just send us the best image you have and leave the calculations to us!