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The difference between PPI and DPI
Article Details

Last Updated
22nd of August, 2011

PPI - Pixels Per Inch

DPI - Dots Per Inch

Unfortunately, in the world of imaging, PPI and DPI are used almost interchangeably. They are two quite different things and the confusion of the two leads to more confusion than almost any other single thing in digital imaging.

The first things to understand is that in digital imaging, the only thing that really counts about a file is how many pixels are in a file. Terms like megapixels, DPI and file sizes in megabytes only confuse the issue. In the end, all digital images are simply X pixels by Y pixels big (by Z bits of colour data but we can ignore that for now).

This is the only absolute measure of the quantity of information in a file (nb it has nothing to do with the quality of information in a file!)

In almost all cases, unless you are talking about physically how your printer is laying ink down on the paper, you are actually dealing with PPI - pixels per inch.

Dots Per Inch is an old printing term and has almost no place in modern digital imaging. DPI is a measure of how many tiny, tiny droplets of ink a printer is laying down in its dither pattern to form one inch of a print. Most Epsons, for example, operate in Photo mode at 1440 DPI (can be as low as 720 DPI and as high as 5760 DPI). You tell the printer which mode to print in in the driver - this is usually camouflaged by the use of modes 'Photo', 'Best Photo', 'Photo RPM' or 'Fine', 'SuperFine'. In the bigger printers you can usually choose the DPI directly as, say, 2880 DPI. Incidentally, almost all printers operate best for general photographic usage at around 1440 DPI. The higher DPI modes like 'Photo RPM' are useful if you're printing really high key (i.e. all light toned) shots, but next to useless for general printing. In fact, they are worse than useless, they are positively damaging - way too much ink is layed down on the paper, resulting in seriously impaired shadow detail. And of course the more ink you use, the more ink you pay for (an Epson R800 running in 'Best Photo' mode is less than half as expensive to run as one left in 'Photo RPM' mode for instance, and in 99.9% of photos, you'll get worse quality from the 'Photo RPM" mode!).

Pixels Per Inch is the key term. It is a description of the logical number of pixels from your original image (X pixels by Y pixels, remember) that will be used to tell the printer to print one inch on paper. Assuming a sharp original shot with good technique (see resolution discussion below), the higher the PPI, the better the quality print you can achieve - this is testably true even well beyond most claims of 360 PPI being the most you need ... 600 PPI images can easily be seen to be much sharper again if this data is available at good quality from the original file).

PPI is a logical term - changing the PPI of a particular file does not in any way affect the file itself - it is simply a decision about how many pixels of the available pixels you will use to print an inch on page. You can choose any number you like - from 1 to infinity. The de facto standard for high quality, photographic printed images is 300 PPI - that is, for each inch of the printed image, there must be 300 source pixels to use.

This is why the 'resample' check box, in the Image Size dialogue, is the single most important (and dangerous) control in Photoshop! When you resample an image, you are actually changing the number of pixels in your image (i.e. changing the value of X and Y) - adding some or throwing them away. You should only do this if you are making an explicit and informed decision to do so, because no single other thing will affect the quality of information available to you from your file as this!

An example will make all this clear:

Say we scan a 35mm tranny at 4000PPI - this will result in a file that has 5400 by 3600 pixels.

We now want to make a print of 12 by 8 inches. This means the PPI we have available from our file for a print of this size is 5400/12 = 450 PPI (or 3600/8 = 450). We are choosing to use 450 pixels to represent one inch of our print.

The printer driver will now translate those 450 logical pixels into 1440 physical dots per inch (DPI) and produce a very high quality print for us.

If we wanted to know the maximum print size we can achieve at good quality, and we know from experience that given a sharp original outputted on an Epson inkjet, 240 PPI is sufficient, we can calculate the maximum print size by taking the total number of pixels available to us (5400 on the long edge) and dividing it by the PPI required to give us, in inches, the size of the print:

5400/240 = 22.5 inches
3600/240 = 15 inches

So, our final print will be 22.5 by 15 inches, with 240 pixels used to represent each inch. And of course, the printer will actually use its 1440 dots per inch to actually print that image to paper.

Hopefully that is clear - it can be a bit confusing at first. Just in case, you can read another version of the same stuff here to get a different perspective on it.

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