Creating Perfect Print Files #2
- Screen Resolution
We’ve all seen them haven’t we? – those printed newsletters, flyers or magazines with horribly ‘blocky’ looking images. Now, the casual reader may just assume something’s gone wrong with the printing, but to those of us who design and produce these kind of printed items everyday, those pixelated images really do jar with us.
So why do they happen and what can we do to prevent them?
Well if you’ve ever had an item printed only to find that one of your images has gone bad, your first reaction is often to say – ‘but it looks OK on screen’ – and indeed it does but as we said in our last blog, there are differences between on-screen and on-page image reproduction. When preparing images for print, you need to always remember the magical ‘PPI’ number. PPI stands for Pixels Per Inch – printing companies will generally refer to DPI – dots per inch – but don’t worry about that as, for all practical purposes, they are the same thing.
Of course, we all know what inches are (even if it is only those pesky Americans who still insist on using them!) but what about a Pixel – what exactly is that? Well, if you can think of an image as being made up of little building blocks, then a single pixel is one of those building blocks. If there are only a few blocks making up the image then the picture will lack fine detail, if the image contains a high number of pixels then a much finer appearance is achieved.
Another way of thinking of it is to imagine a mosaic – an image made up of lots of little coloured tiles. The smaller the tiles, the more detail can be seen in the finished image, the larger the tiles, the less detail.
So, when it comes to using images for printing, you’re ideally looking for images with around 300ppi. Of course it’s a little hard to know exactly what that means when you’re picking an image to use for a printed document so it’s often good initially to look at the file size itself. You can use the file size to act as a guide – so if the file is small at say just 200Kb then then it’s likely to have a low ppi reading when you bring it into your design document. On the other hand if you are working with a large file of let’s say 4Mb then the ppi is likely to be much more like what you need for the image to print well. Typically an image used on a website will be just 72ppi so if you want to take an image from your website, try to always source the original image file which is likely to be a larger file size that the reduced size used on the website (websites use smaller file sizes so that the pages load quickly).
Now, having said all that about trying to get images that are 300ppi or more, it’s important to know the distinction between ‘actual’ ppi and ‘effective’ ppi. This is the difference between the size an image actually is and the size it appears to be. So, it is in fact possible to use a low resolution image and give it the appearance of a high res image image. That’s to say you can work with a 72ppi image but make it look like an image of 300ppi or higher.
If you’re using Adobe InDesign then have a look at the ‘Info’ box. You’ll see that it tells you your actual and effective ppi reading. You’ll find the Info box in the menu that drops down from the ‘Window’ tab at the top of the workspace. Select it and move it to sit off at the side of your screen. OK, so lets say you have an area of 100x100mm into which you want to place an image – if the image is low res then when you fill the 100x100mm box you’ll find that the actual ppi is less that the 300ppi that you need. Let’s say it shows 75ppi and you want it to be 4 times that size. Well, the way to make it appear higher res is to reduce the area of the box – so if you reduce the box to 25 x 25mm and shrink the image to fit the box, you’ll see the ‘effective’ ppi’ figure increase up to a reading of 300ppi. Job done! The image will now print sharp and clear.
In reality you’ll often get away with a resolution of less than 300ppi – but that is a good standard to work to for printed documents that will be viewed close-up like like brochures, newsletters, flyers and so on. Items such as banners or posters that are viewed from further back are a little more forgiving and will allow you to work with lower res images.
Next time we’ll look in more detail at issues around how and why to embed fonts in your printed document artwork files and, as always if you have any specific queries or comments then just contact the customer support team on firstname.lastname@example.org