Resizing your images: The difference between PPI and DPI
What is the difference between PPI and DPI? Digital colour printing at home has revolutionised photography (well it has for me, has it for you?). It's now no longer necessary to drop a roll of film at a lab and then wait days, even weeks to get a set of prints back. However, with great power comes great responsibility. Making a print at home means understanding a few important concepts. Such as the difference between PPI and DPI and what that means to the potential size of a print.
All digital images are made up of pixels. This image is 600 pixels across by 480 down. The pixel resolution ultimately determines the size that a print could be made without an unacceptable drop in quality.
PPI first. PPI is short for Pixels Per Inch (if you think metric you can also use PPC or Pixels Per Centimetre). The PPI value of an image determines how many pixels in that image will equate to one inch in a print. If an image resolution is set to 300ppi and the image is 3000 pixels across the printed image would be ten inches wide (3000/300=10 inches).
A 3000-pixel image at 300ppi will create a print ten inches across...
However, an image's PPI value is an entirely arbitrary number. It can be anything you like. An image's PPI value can be changed (in Adobe Photoshop for example) without altering the number of pixels in the image. The difference between a 3000-pixel image set to 300ppi and one set to 150ppi is that the former will create a print ten inches across, the latter will create a print twenty inches across (3000/150=20 inches). If you had a sufficiently large printer you could even use 1ppi if you had a mind to. This would make a print 3000 inches across from a 3000 pixel image! Try finding somewhere to hang that!
whereas at 150ppi the print would twenty inches across. To change the PPI value in Photoshop select Image > Image Size, uncheck Resample Image and change the value next to Resolution.
Changing the PPI is therefore a very powerful way to alter an image's print size without needing to resize the image itself (by adding or subtracting pixels). However, there's a catch. By lowering the PPI value you spread an image's pixels across a greater area of paper. This lowers the perceived quality of the print, particularly if you look closely at it. But here's the thing. Most people don't put their nose right up against a print to find its flaws. Well, most normal people anyway. Photographers are the exception that proves the rule...
The bottom image has been rescaled to simulate a print with a low PPI. Close to there's a big difference between top and bottom. Now try looking at the two from across the room. Can you still see a difference?
Selecting the right PPI value is therefore a trade-off between the size you need a print and the viewing distance of that print. The greater the viewing distance, the lower the PPI value that could be used without flaws in the print becoming apparent. Don't believe me? Next time you come across a billboard take a close look. You'll be surprised at how rough and low-resolution the image is. This is because a billboard is designed to be looked at from a distance of ten or twenty feet not one or two.
The inset image shows how the main image would be dithered to reproduce it as CMYK image in a book (which uses the same principle to simulate a greater range of colours that would be possible otherwise).
Now for DPI or Dots Per Inch. DPI specifies the number of dots of ink a printer can make across one inch of paper. A printer has a limited number of ink colours: cyan, magenta, yellow and black. Inkjet printers use a technique known as dithering to simulate the huge range of colours found in a digital image. Dithering is the placing of very, very small dots of ink in varying proportions in a precise pattern. This pattern fools the eye into seeing a huge range of colours rather than just cyan, magenta, yellow and black. The higher the DPI of a printer, the finer the pattern and the more seamless the image will look.
Printer resolution needs to be high because ink has a nasty habit of spreading when it hits paper. Therefore the dots of ink have to be very, very small indeed to stop this being a problem. Inkjet printers typically have DPIs between 300-600. If you alter the print quality (from Photo to Draft say) the printer alters the number of drops of ink squirted out for every inch of paper. The better the quality setting, the more precious ink is used, but the smoother the print will look.
Now if you'll excuse me I've just got to pop out and get ready for the sun to set. Something's you still have to wait for, but I'm not complaining.