Ever had an image that looks amazing on your desktop, but looks like a dog’s dinner in print? If so, you should find this simple guide useful.

Resolution, dpi, CMYK, RGB, it’s easy to get lost in jargon and the science of it all, so we have attempted to keep this post as practical and straight forward as possible.

Topics covered include:

– Screen resolution
– Image size
– pixels and DPI
– CMYK and RGB

If you feel the urge to get more technical, please see the links at the bottom of the page.

What is screen resolution?
Screen resolution means the amount of pixels your monitor displays. A monitor with a screen resolution of 640 x 480 pixels will display 640 pixels (width) and 480 pixels (height). There are different screen resolutions you can use depending on the physical size of your monitor. Generally speaking, the larger the monitor the higher the screen resolution it can display.

Here’s how to check your screen’s resolution: On a PC choose Start – Control Panel – Display – Settings – screen resolution, on a Mac, choose Apple Menu – System Preferences – Displays.

Screen resolution v Print
Compared to print, your computer monitor is a low-resolution medium. Monitors display approximately 72 to 92 pixels per inch of resolution, whereas most four-colour printing uses anywhere between 150 and 300 dots per inch (the print equivalent of pixels – number of dots of ink printed per square inch).
Even though your monitor has a significantly lower resolution, the difference in quality between screen and print may not be immediately obvious.

Ink v Light
Four colour printed images use four printing inks; cyan, magenta, yellow, and black (CMYK) and depend on reflected light. In contrast, your monitor displays images using a combination of Red, Green and Blue (RGB) light to deliver millions of colours with a greater range of contrast and colour intensity.

This is why, even though your monitor is lower in resolution, it will display colour images that look as good, if not better than those at an equivalent size in print.

How big can I print an image?
In reality you can print any image as large as you wish. However, the larger an image is printed, the more clearly you will start to see the pixels that make up the image. As a rule of thumb, the image quality will be determined by the resolution of the image and the resolution it is being printed at.

For example, if you have an image that is 300 x 100 pixels and it is being printed at 200 dpi (dots per inch), here is the calculation: 300 ÷ 200 = 1.5, 100 ÷ 200 = 0.5, therefore if the image is printed at 1.5 x 0.5”, you will get exactly 200 dots per inch.

File size and resolution
There is a direct correlation between the resolution of an image and its file size. The higher the resolution, the bigger the file. File size is generally referred to as ‘Kb’ or ‘K’ (kilobytes) or as ‘Mb’, ‘M’ or ‘Megs’ (megabytes). For the sake of argument, a megabyte is equivalent to 1000 kilobytes.

A practical image size guide
The following sizes are a rough guide only, but should help in judging whether a raw image file is large enough to reproduce well.

20Kb – 100Kb: viewed on screen as a ‘thumbnail’ image
100Kb – 250Kb: viewed on screen as a header (approx 700 x 400 pixels)
500Kb – 1Mb: print (approx 25mm x 25mm)
1Mb – 2.5Mb: print (approx A5)
2.5Mb – 4Mb: print (approx A4)
4Mb – 10Mb: print (approx A3)
25Mb – 100Mb: print (larger poster formats)

Please note, if you are supplying Jpegs, these are compressed files. Small Jpeg files (up to 250K) are generally used online and will be left compressed. When Jpegs are opened in Photoshop to be edited or retouched ready for print artwork, they are uncompressed and the resultant file size will be significantly larger.

There is no formula for judging how large a Jpeg will become when uncompressed as it is dependent on the complexity of the image. If in doubt, send files to your design team who will be able to help.

For a more technical take on this subject, visit:
www.bowhaus.com
www.photo.net