Enhance, enhance, enhance!

As this first blog post of 2014 is being published rather later than intended, instead of talking about New Year’s Resolutions, let’s take a look at image resolution.

It used to be that writers only needed to concern themselves with words, but now that practically everyone owns a scanner or a phone with a camera, and enhanced ebook formats for tablets allow us to include our own photographs and illustrations in our books, we all need to know something about the way in which digital images are displayed.

Simply put, a digital image is made up of lots of coloured dots or pixels, which are spread out regularly like the squares on a piece of graph paper or the stitches of a cross-stitch embroidery. A pixel – a picture element – is the smallest point that can be identified and addressed on a screen: it can’t be split into parts and can therefore only be a single colour.

Historically, standard computer monitors had a resolution of around 72 ppi – that’s 72 pixels per inch, so to fill a square inch of your screen, you’d need an image with 72×72 pixels, and to fill a four inch square you’d need 288 x 288 pixels.

Obviously the more pixels in an image, the bigger the file size. So if you want to send a photo via email or use an image on a webpage, it’s best to keep it small so that it doesn’t take too long to load. But if you make it too small, the computer won’t have enough information and will either show a tiny image or the missing data will have to be invented.

comparison of hi- and low-res images
If your image is too small for the purpose, it will appear blurred.
Left: 300 ppi image; Right: 72 ppi image enlarged to 300%.

At 72 ppi, an image that is 144 x 144 pixels only contains enough information for a two inch square. This means that if you want to display it as anything bigger, the picture will look blurred, as the computer software will have to “guess” what colour dots to use to fill in the spaces. Displaying it as a four inch square, for example, three quarters of the information would be invented. (Incidentally, this means that most TV CSI-type procedures are unrealistic: the boss asks the techie to enhance an image and the computer software supplies estimated data to fill in the blanks; it can’t retrieve information which isn’t there, it can only make a guess based on the logic programmed into it.)

Once we start talking about printed images, we need to work with higher resolution – usually 300 ppi – if the pictures are to appear clean and crisp.

At screen resolution, four inches is 288 pixels, so if you try and print a four inch square picture you’ve downloaded from a webpage, the resulting picture will either be big and fuzzy on paper, or high quality and about the size of a passport photo.

So far, we’ve talked about 72 ppi for displaying images on screen; but monitors and displays are improving and tablet screens – where your enhanced ebook is likely to be seen – tend to have much higher resolutions, typically ranging from 150 to 250 ppi, although some displays are now just as hi-res as print. So although a small picture can be scanned and used successfully alongside a story you post on your blog or webpage, it may make a very disappointing full page illustration for an ebook.

This also means that if you’ve got images which you had optimised for web, they won’t be good enough for use in your enhanced ebook when it’s read on a tablet.

It’s tempting to think that you can simply ‘size up’ the image, using the program you used to optimse it earlier. But this is still essentially asking software to interpolate missing data. In fact, you need to go back to the original image and work from that. If you scanned an image, you’ll need to rescan it at 300 ppi resolution. It may look huge on your computer monitor but that’s the size you’ll need to work with for a high-resolution tablet display.

The easiest thing, of course, is to leave it to an expert. Just remember that in real life even experts can’t work miracles.

Of course TV experts are another species; fans of CSI and other such series may enjoy this video compilation by Duncan Robson:


We’ve been working with images for print and digital collateral for many, many years. If you want your publications to be every bit as good as they can be, why not talk to us?