The two most common sizes for photo printing are 6 x 4 inches (15 x 10cm) and A4. 6 x 4 prints are popular because this is the traditional size for snapshot prints from 35mm film, but in fact it is not an ideal size for digital prints. The negative of a 35mm film print is 36 x 24mm, which is a 3:2 aspect ratio, so for this the 6 x 4in print size (also 3:2 ratio) is ideal. However most digital compact camera have an aspect ratio of 4:3, the same as the majority of computer monitors and TV sets, so in order to print digital images out on 6 x 4in paper either a strip at the top and bottom edges of the photo is cropped out, or wide borders are left at either side of the print.
Most digital SLRs and some digital compact cameras can shoot in the 3:2 aspect ratio, which is better for snapshot prints, but if you want your prints to look good you can always crop and resize them using a photo editing program. Likewise A4 is not an ideal size for digital prints either, and for the same reason. A4, in fact all paper sizes beginning with an A, have an aspect ratio of 1.414:1, so again for best results and borderless prints you should crop and resize your photos before printing. Oddly, none of the major manufacturers make a photo printing paper with a 4:3 aspect ratio.
How big is big enough?
For perfect photo quality, your digital images should be printed at a resolution of around 300 pixels per inch, or 120 pixels per centimetre. This means that if you want a photo quality 6 x 4in (15 x 10cm) print, your digital image needs to be at least 1800 x 1200 pixels, which is roughly 2.2 megapixels. A4 paper is 21.0cm x 29.7cm, so for perfect photo quality A4 prints your digital image needs to be at least 2520 x 3564 pixels, which is just under nine megapixels. Even if you routinely print all your photos out at A4 size, you still donâ€™t need a 12-megapixel camera.
Incidentally, donâ€™t confuse image resolution, measured in pixels per inch (ppi), with print head resolution which is measured in dots per inch (dpi). Some people seem to treat these two as though they are interchangeable whereas in fact they refer to very different things. A printer with a 1440dpi print head doesnâ€™t mean it will print your photos at 1440ppi, it just means that the individual ink dots that make up the pixels of your 300ppi photo are smaller and closer together, and thus more closely approximate a smooth even tone.
How to optimise photos for printing, how to add borders, and simple framing.