DPI is simply an acronym for “dots per inch”. It should be a simple concept, but people sometimes get twisted up in knots over it. Our printer manufacturers have not helped the situation.
What are “dots”?
Unless you are reading a printer spec sheet, dots just mean pixels. A pixel is a “picture element” – the smallest piece of a digital image. By convention, a pixel is a triplet of red, green and blue values. That sounds very technical, but it just means they are 3 values carried around together, say something like 95, 134, 47. By convention each value can have a range of 0 to 255. That is not representative of what camera sensors really do anymore, it is the convention. The convention comes from the 8 bit representation of color values way back in early times. The maximum value 8 bits of binary data can represent is 255. The practice has been established and perpetuated by Photoshop over the years. The actual data range we use is a subject for another post.
The image that gets stored in your computer is a grid of pixels. The camera I am using most often right new creates an image that is 8256 x 5504 pixels. That makes a lot of data!
A fuzzy quality metric
Your camera does not know the concept of DPI. As a matter of fact, DPI is a fairly useless term unless you are printing an image.
Have you ever had someone tell you they need a file that is 8×10 inches at 300 DPI? Unless you are sending the file to whoever does your photo printing, it really means they do not understand what they need.
At best DPI is a metric for the quality of an image viewed or printed at a certain size. The more pixels you have in a given distance, the better the image should look, in general.
But in most people do not even know the pixel resolution of their screen or printer. If you save a file in jpg format there is a good chance the default is 72 DPI. This was considered the “normal” screen resolution – way back. The main monitor I use is 219 DPI. And it is several years old. And I don’t really care, because that is a number that is never important to me. I never use it for anything.
DPI really doesn’t matter for the screen
One of the reasons DPI doesn’t mean much for most of us is that our computers scale images for viewing on our screen. And they usually do a really good job. But what most apps do is map the pixels available to the pixels of the screen. So if you look at a file that says it is 10″x10″ at 72 DPI, you will see a 720×720 pixel image. It will be however large 720 pixels measures on your screen.
When you view an image on your screen all that really matters is the resolution. The DPI number is generally ignored.
If DPI is not as important as many people think, then is resolution unimportant? No. Absolutely not. The number of pixels you have to work with is always an ultimate limitation of what you can do with the file. As is said in many things, size matters.
With plenty of pixels imaged through good glass you have the flexibility to print large, or to crop tightly or to create images of astounding detail. Also, massive numbers of pixels gives sharper edges and smoother gradients.
DPI for printers is a whole different thing
The major printer manufacturers have confused the issue for us. A printer ad may proudly proclaim it does 4800 DPI! This is technically correct, but not helpful. They are talking about the density of ink drops they can lay down on the substrate (paper). But printers do not print pixels.
A drop of print ink is not a pixel. You do not send the printer an image scaled to 4800 DPI!
A printer takes the pixels available in a given area and transforms them to densities of the subtractive colors needed to come close to reproducing the colors and gradations contained in the original pixels. This is a complex technology and I will not attempt to explain it here. Sufficient that you remember a drop of ink is not a pixel.
Where DPI means something
When you know how large of a print you wish to make then DPI becomes meaningful. The number of pixels available combined with the desired print size give us the DPI. DPI is a measure of the amount of information available for an inch of print.
For optimum printing the guideline is supplying a source file of around 250-360 DPI. This gives the printer driver enough information to do the ink transform we talked about above.
My Canon printer, for instance, has a “standard” resolution of 300 DPI. It can print well with a range of values, but this is considered optimal. This means that one of my image files of 8256 x 5504 pixels could be used to print an image of 27.5″ x 18.35″ with no scaling or loss of resolution. That is the size of this image at 300 DPI.
If I want to print one of my images at a more typical size, say 18″x12″ I could scale a copy of my master file down to 300 DPI. Or not, because Photoshop or Lightroom is perfectly capable of scaling it down when printing with little discernible degradation. One person even says that higher DPI gives better results.
If I have an image with insufficient pixels I could just try to print it. Printer drivers do amazing things. Or I could scale it up in Photoshop, which also does an excellent job within limits.
It’s those limits that you have to be able to estimate. If you have a 2 MPixel image and you want to print it poster size, well, your results probably will not meet your expectation. No free lunch.
So what about DPI? Don’t sweat it unless you are printing. Only if printing is it a meaningful metric. And it is only meaningful when you are taking about a particular image printed at a particular size.
There are plenty of technical issues to stress out about. This should not be one of them, unless you are producing high quality prints.