This is often confusing for many people. I often get clients who give me images from their websites and expect them to be good enough quality for print but this is not the case. Pictures on the web are generally 72 dots (pixels) per inch (dpi) whereas commercial print uses 300dpi or more. If the image is quite large on the website and appears to be good quality you can expect to achieve a similar quality in print if the image is reduced to about a quarter of its apparent web size. The best way to check is in an image editor.
The Resolution Formula to determine whether your image is good enough for use and not pixelate (break up into visible dots) is: Divide the number of pixels by the resolution.
eg: 2000 pixels / 200dpi = 10 inches
A large web photo:
850 x 562 pixels would give a reproduction size of 11.8″ x 7.8‚” at 72dpi
A full-page brochure photo:
4200 x 2790 pixels would give a reproduction size of 14″ x 9.3″ at 300dpi.
Careful with dots per centimetre‚ I’ve been caught out a couple of times.
As mentioned, large commercial printers require a resolution of 300dpi or more. If you use a large format inkjet printer or office printer, 150 dpi is perfectly adequate. Below this and there will be a discernible drop in quality. A test print would maybe be a good idea.
If you do start altering dpi and pixel dimensions remember that you can’t make an image physically bigger (up-sampling) without losing detail and quality. If you increase the resolution the size will come down accordingly. If you do need to up-sample for a billboard for instance, use Photoshop and increase the size in increments of 5% (with Bicubic Smoother).
Another important thing to remember that if you have a jpeg or gif (pronounced jiff), the file is compressed. If you edit and re-save it you will cause image deterioration. So always try to go back to the original file if you intend to change sizes and resolutions.
This is a rather simplistic view of it all which I’ve done on purpose as most descriptions seem to be overly complicated. If there’s anything you don’t understand or would like to know more please get in touch.