What’s wrong with printing pictures from the web – they look OK on screen?

It’s mainly a question of resolution, and the difference between screen display and ink on paper.

The resolution of an image is the amount of tiny dots/pixels that an image is made up of – lower resolution simply means there’s less information in an image. For web, or viewing on screen, a lower resolution picture will often display well as the maximum pixels displayed is 72 per inch. For print we need at least 300 per inch – which means a much bigger file to start with! As a rule of thumb, a file of less than 25Mb (opened in Photoshop) will not be of high enough quality to print on an A4.

Screen display is a completely different technology to ink on paper and can give misleading impressions. Screens work in the RGB colour space, whereas most colour printing is created with CMYK inks (cyan, magenta, yellow and black – or “key”), so the images need to be converted before printing. Additionally, you view screen images with light pouring through them which makes everything appear very bright, whereas on paper you are working on a non-luminous surface. Our screens at L&B are calibrated to display as near as possible to printed results but we still need to judge from experience how the printed job will appear. That’s also why proper proofs are so important (see What are my options for printed proofs?).

Digital photos and scans – bitmap files
For bitmap files, the resolution will determine the end result. Resolution refers to the amount of pixels per inch (dpi) or per cm of the image – the more pixels there are, the subtler the colours and the smoother the end result. However, if an image is enlarged, the dimensions of the image change but the amount of pixels stay the same... so an image measuring 5cm width with a resolution of 300 dpi degrades to a measly 150 dpi if you blow it up to 10cm across.

An example of a high res image – clear and crisp with no pixilation.
An example of a low res image – the pixels are visible.
An example of an up-sampled image with extra pixels added.

Increasing the resolution on a bitmap image means that the computer adds more pixels to the picture. It has to guess at what colour these new pixels should be, and “interpolates” by adding colours which are related to the adjacent original pixels. The result is often soft and blurry as can be seen in the example above.

A rule of thumb is that an image needs to be at least 300dpi AT THE SIZE IT WILL PRINT. All image editing software (eg Photoshop) will easily tell you the dpi, so we can check this for you. Additionally, our flightcheck software will highlight any images which are under print resolution – but it is better to identify them before we get to this stage!

Logos
Please see our topic “What’s the difference between a vector file and a bitmap?”.

How to send us images

It is vital that images which are to be printed are supplied in a suitable format, to ensure a good result. There is little that can be done to improve a poor quality or low resolution image and – while it may look fine on screen – it will scupper the end product if printed. The two main types of image files are bitmaps (scans, digital photographs) and vectors – please see our topic “What’s the difference between a vector file and a bitmap?” for a fuller explanation.

We can take files up to 10mb by email – for larger files we suggest You Send It where you can transfer files up to 100mb through a simple web download without charge (just sign up for the free account).