Viva la Resolution!
We've had some questions lately about resolution - dpi/ppi and web versus print - and while it seems from the outside to be a bit of a dark art, it's actually not that tricky.
Starting from the very beginning, DPI means 'dots per inch' and relates to printing. It is a measure of how many droplets (dots) of ink are deposited on to the paper or canvas in a linear inch. The higher the dpi, the more droplets of ink are used - and the sharper the detail and the more accurate the colours depending on the type of printer being used. (I won't go into CMYK but you can find an explanation here if you would like some more background). DPI of printers vary - from around 170 dpi to 1200 dpi, maybe even more for high end printers.
PPI means 'pixels per inch' and relates to TV monitors and computer screens. A pixel is a tiny square made up of a combination of red, blue and/or green light to create a shade of (RGB) colour. If you zoom in closely to a digital image you will see the tiny square pixels that combine to give you a your full sized image.
So as you can see, while DPI and PPI are often used interchangeably, they relate to quite different processes.
There's a bit of a myth that computer screens display at 72dpi: it dates back to the Apple Mac screens of the distant past, when their graphic user interfaces and the representations on their screens were set up as 1 dot (printer) should be represented as 1 pixel on a screen. At the time the printers' output was 72dpi which is why 'web ready' images tend to be set at 72ppi - so the input and output 'matched'. Windows blew that out of the water when their machines were set at 96ppi - and nowadays both are 'old hat' as most modern computer screens or TV displays are quoted as width by height. My monitor has a resolution of 1920 pixels wide x 1080 pixels high, and the screen is 20.5 in wide by 11.5 inches high. That means the dpi on my monitor is 1920/20.5 - or about 93dpi.
So what does that mean for your digital file?
Have a look at these two images:
Image courtesy of National Army Museum - Gorgeous isn't it! View it (and others like it) online at nam.recollect.co.nz
Which one is bigger? The 600ppi one? A nice high res tiff* that has been scanned at 600ppi must mean I can blow it up and print it at a much bigger size than the 72ppi jpg right?
Actually, no. Have a look at the dimensions - they are both 864 pixels wide and 1364 pixels high. Which mean they will both look exactly the same size on your computer screen, and both have the same limitations for printing.
File Dimensions and Print Size
Let me explain: If one pixel provides the colour information for one droplet of ink, then at a print resolution of 300dpi 300 pixels of colour information will print in one linear inch. This means that the photograph can be printed at (864 pixels divided by 300 pixels) roughly 2.7 inches by 4.5 inches. If printed at 200dpi it will be a little bigger - with a finished print (864 pixels divided by 200 pixels) of ~4.5 inches by 6.5 inches. Note both results are enlargements of the original image which was tiny; about 1.4 inches by 2.25 inches (864 pixels /600).
Don't Panic! (in large friendly letters)
If you receive a jpg file to use for printed material, and it says its resolution is 72ppi, don't panic because it's the dimensions of the file that matter more. Look at the number of pixels (width and height) you have to work with and that will give you a rough guide to how large you can print the file before there is some noticeable deterioration in your printed image. There are some other considerations as well - for example some printers can do some magic to allow printing at a larger size, and posters that are viewed from a distance can often 'get away' with being enlarged as our eyes will smooth the image and fill in the gaps for us. But if you have an image that is 3000 pixels wide, it will give you a 10 inch wide print (at 300dpi), regardless of whether it is 600ppi tiff or a 72ppi jpg.
So, why bother with tiffs then?
Don't get me wrong, tiffs definitely have their place. They are an open standard (the storage algorithm is included as part of the file), plus they use a 'lossless' form of compression making tiff files the format of choice for 'preservation' or 'archival' digital images. Converting to jpg, even if retaining a 100% quality ('uncompressed'), will mean some subtle information is lost - but the advantage is the file size is greatly reduced. And of course it shows up on your website!
If you're still unsure, the NZMS team would be happy to help you with your print requirements, or answer any questions you might have. And if you like getting your hands dirty with some of this technical knowledge, you might be interested in some of the case studies and white papers on the NZMS website - for example:
*Note a tiff is not a file format that will display on the web - the example image is a screenshots of the two images sided-by-side on my monitor then converted to jpg for use as illustration of this concept.