Resolution with DigitalCreative.info's Guide to Digital Photography and Image Manipulation

One term that you will come across frequently, and so need to understand, is that of 'resolution'. Don't be tempted to think, "Oh, this sounds 'technical', I can just forget about it!" Wrong! Understanding what it involves is fundamental to producing a good print or image on screen.

Image resolution refers to the actual size of the file that is being used to produce the picture; it is the number of pixels that exist and are being employed.

Monitor resolution, ppi, refers to the number of pixels that are being displayed, to the linear inch, across your screen; probably between 72 and 130 ppi although newer models can have a ppi resolution in excess of 200. This measurement is determined by the manufacturer and cannot be altered. In effect, a picture can look marvellous, viewed full screen on your monitor but, when printed at even postcard size, can be of extremely disappointing quality. The size of the displayed image on your screen bears absolutely no relationship to your image file or the size at which your image will come out when it is printed. This is all done in a separate dialogue box (which we'll look at in due course)

Print resolution, ppi, refers to the number of pixels which you decide should be employed for each linear inch of your printed picture. Using more pixels per inch at the printing stage means that fine details are sharply defined and different colours and tones change smoothly. It follows, therefore, that a small picture, for example 6" by 4" (approx 15cm by 10cms) will need a file containing a smaller number of pixels than a picture of, say 16" by 12" (approx 40cm by 30cms).

You will sometimes see reference made to dpi (dots per inch). This term refers to the printer's capabilities and indicates the number of dots of coloured ink that the printer will lay down for each pixel. The term dpi is not interchangeable with ppi, although you will sometimes hear them being used as if it is.For example, if your printer resolution is 1440 dpi and you choose a print resolution of 72 ppi, then the printer will place 20 dots for every pixel (1440 divided by 72). The following image file, printed at 72 ppi will deliver a print 70.2 cms (approx 27 inches) wide by 54.19 cms (21 inches) high. This resolution will not give you a good quality print, no matter how good the paper is.

If you would like a better quality print, from the same image file, make sure that the 'resample image' box isn't checked then change your resolution to a higher number, for example 288 ppi.

You will see that the number of pixels in the dimensions has remained constant. Although each pixel (which is now much tinier, in order to squash more into each inch) will have only 5 dots of of ink instead of 20, the tonal quality of the colours in the print will be much smoother. There will still be 1440 dots of ink for each inch of print (dpi) but, because the physical size of the eventual print is now smaller, the detail appears to be more clearly defined. If your printer offers a resolution of 2880 dpi then using this option allows you to make prints of even better quality. This difference is most noticeable when printing a monochrome print, and just using the black ink, as the lighter mid-tones are rendered more smoothly..

Incidentally, you can choose whatever resolution size provides the physical print size that you want to come out of your printer. On a 1440 printer you will get good results with a resolution between 200 and 300ppi. If you're not sure of what results to expect, and don't want to waste ink and paper, then make a small selection from your image and just print that, as a test strip.

It can be false economy to cram as many pictures onto your memory card as possible, using a low number of pixels in the camera because, although good image manipulation software gives you the opportunity to add pixels at the production stage, the software is inventing the colour information and it's never as good as it would be if gathered at time of taking. Better to have more pixels than you think you'll need rather than fewer. If, however, your images are only ever going to be used for projection on a screen or uploaded to a website, for instance, then a smaller number of pixels is acceptable. (this will be looked at in more detail at a later stage)

Editing software will offer you all sorts of automatic options to affect contrast, sharpness, colour saturation and added effects and these are all extremely useful to the complete beginner but, if you learn to take control of these changes yourself, you will find that (a) you will get better results and (b) there is much more personal satisfaction to be gained.