DIGITALIA FOR BEGINNERS
DPI AND SCANNING RESOLUTIONPixels (or dots) per inch sometimes confuse beginners to digital imagery. Ppi or dpi are the same thing. The term dpi, as is obvious, states a relationship between the number of dots an image is made up with and the measurement in inches that the image is displayed. An image, which consists of, say, 100 dpi, has 100 dots for every inch of width of your picture.
On your monitor screen anything from 72 to 100 dpi is suitable but if you intend to output your picture to an inkjet pnnter then you have to consider a higher resolution. Inkjet printers offer anything up to 2880 dpi these days so the gap between genuine photographic prints and digital prints is very close — unless you are in the habit of examining pictures through a magnifier! Let us assume you have a print from one of your latest holiday slides or negs., and you want to scan it and print it out yourself on an inkjet printer.
How do you work out he resolution required to print a good quality picture at a different size? On the monitor screen an image that is 300 dots by 300 dots at a screen resolution of 100 dpi (a figure chosen for ease of calculation!) the picture would fill a space 3 inches square. But print this out at a resolution of, say 300 dpi and the picture, although showing better detail, would only be 1 inch square. So to apply this calculation to work out the scanning resolution required for the size of picture we want to end up with, the best way is to start at the end and work backwards.
Let us say we start with a photo at the standard size of 6 inches by 4 and we want to enlarge it to, say 12 x 8. The lowest resolution we should consider is 360 dpi when printing, which should give, a normal viewing distance, a picture with plenty of detail.
Multiply the width of the finished picture by the resolution you have chosen to print at (in dots per inch), which we equal the number of pixels or dots required.
In our example we have 12 x 360, which equals 4320 dots. In other words, the scanned image needs to be 4320 pixels wide. So the number of pixels required divided by the size of the original photo will give us the resolution we have to set our scanner! In our case 4320 ÷ 6 = 720.
We have to scan our original 6 x 4 print at 720 dpi to get a 12 x 8 print printed at 360 dpi. If on the other hand we are using a picture in the CRC News, which is smaller than our original, the calculation still works. Our 6 x 4 original will be printed to a final size on the page of, say, 3 x 2. We want t to look really good at close distance, so let us print it at 720 dpi. Multiply 3 (the width of the finished print) by 720 (the resolution we intend to print at) gives us a required 2160 dots across the picture.
Divide this by the width of the original and we get 2160 ÷ 6 = 360. So we scan the picture at 360dpi. Easy, isn’t it? Just remember of course that each item in the calculation must be to the same unit. In our case we worked to inches. If you have been converted you may prefer to work in centimetres. Fine. If you like that sort of thing and think we fought Napoleon for nothing. But don’t mix the two. You mustn’t use inches for one measurement and dots per centimetre for the other.
Choose the one you are most comfortable with and stick to it through thick and thin and you won’t go wrong.