Photo Printing

Prints

Digital images may be used to make prints for photo albums but also larger prints for wall pictures for the home. To ensure a quality print requires some understanding of resolution and colour management.

Resolution is related to the number of pixels needed to make an image adequate for the human eye.
Printed images are generally made up of many tiny coloured dots. The size and number of dots and the viewing distance from the image determine if the image is adequate for its purpose. A poster on a billboard is designed to be seen some meters away so can have dots large enough to be easily seen at 20cm. A high quality photographic print for a magazine will need very tiny dots as it is viewed at 30cm but the dots should not show even at 10cm.

The measurement most used for printing quality is pixels per inch or ppi. For a high quality magazine print, 300 pixels per inch may be used but lower ppi may be used for larger prints without losing much quality. Apple has designed 'Retina' displays that are supposed to be matched to the human eye at about 220 ppi. Some prints may still be acceptable at 150 ppi.

This again brings in the issue of the size of camera sensors affecting pixel quality. An 8 Megapixel mobile phone image may not be able to be printed as large as an 8 Megapixel compact camera for this reason.

It should also be mentioned that computer screens are generally a more modest megapixel size (perhaps 2-3 Mp) so that many digital images are downsized to be able to be displayed. This applies to images used on web pages which are often deliberately sized to 72 ppi to allow good appearance and small file size for quick web page loading.

Colour Management was mentioned briefly in Digital Imaging. The problem is that the colour you see on a screen may not be standardised so that a printer will not then print the same colour on paper. Professionals are very careful to standardise screens regularly and have a standard setting for each printer and each printing paper used. This is usually too much for the casual user but it can explain how prints that look alright on the computer screen or camera screen do not look right when printed. Colour is a very complex subject and way beyond our purposes here.
A brief discussion can be read at this link

The best advice is to send an image file you want printed to a commercial printer to determine if they can print to the quality you want on the print size you desire. They will be able to tell you if your print will work out. For instance, canvas prints may be able to accept a slightly lower pixel size as the image does not need quite the same sharpness as a gloss print.

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