DPI vs PPI and Why it matters (or not!)

This is a slight rework of a post I made on an old blog back in 2011, with a bit of an update to elaborate on a couple of points.

Originally this was in response to an article written by a particularly well known photographer claiming to “bust” some myths that his article actually end up reinforcing and propagating (which might explain why his post now seems to have magically vanished from the web).

I’d had a little back and forth on Twitter with this person regarding his new article and why PPI doesn’t matter when resizing images for digital display (meaning, on the screen, on the web, via digital projector running off a laptop, via a mobile device, whatever) which he claimed was a myth, and that it does matter.

So, firstly, for those that do not yet know, I want to explain the difference between DPI (dots per inch) and PPI (pixels per inch).

Let’s start with PPI, as that’s the easier one.

PPI is a measurement of how a digital image relates in physical dimensions in a print.  For example, if you had an image that was 1800×1200 pixels, and you wanted to print it at 300PPI, it would print out to a 6″x4″ image. If you suddenly told your computer to print that same 1800×1200 pixel image at 150PPI, then it would now be 12″x8″ image. If you told it to print it at 600PPI, it would become 3″x2″.

PPI is the masurement that you enter into Photoshop or whatever application you’re using in order to tell it how large you want the image to be when it comes time to actually print it out, and only when you want to print it out.

DPI, on the other hand, is measurement of the detail your printer can print and, as an aside, it’s not always an accurate indicator of the quality of a particular printer.

For example, a CMYK dye-sublimation printer that prints at 300dpi, is going to be about the same level of detail and quality (unless you want to get really pedantic and technical) as a conventional CMYK ink jet printer that prints at about 1200dpi. This is basically down to how the technology works.

Dye-sub printers lay varying amounts of individual colour & black on top of each other in order to produce the final colour result.

Ink-jet printers print coloured and black dots next to each other to achieve a similar result. So, for each “dot” on a dye-sub (remember, they all lay on top of each other), you would need at least four “dots” from a standard inkjet (one of each cyan, magenta, yellow and black), but, it does not change the physical dimensions of the printed image.

This is why DPI and PPI are not technically the same thing, although, “DPI” seems to have become a generally accepted term when they really mean “PPI”, which is what causes some of the confusion in the first place.

A “Pixel” can be made from 1 “dot” (colours laid on top of each other, like a dye-sub printer) 3 “dots” side by side (Red, Green & Blue, like your TV or computer monitor), or many “dots”.  Some professional inkjet wide format photo printers can have 12 or more ink more ink cartridges to give the full range of tone for each pixel.

So, with that out of the way, why does PPI (or DPI, if you’re that way inclined) matter when you’re producing images for digital display, and not going to print? Well, in short, it doesn’t.

When you resize an image from within Photoshop using specific pixel dimensions, regardless of what you set your PPI to, that image will always have those same pixel dimensions and filesize.

For example, here’s an image of the lovely Raj that I shot in 2010. This was shot on the Nikon D300s, which has a native setting of 300PPI. So, after my usual post processing in Photoshop, here’s the original 2848×4288 image scaled down to 200×301 pixels at 300PPI, along with a screenshot of the Photoshop image resize dialogue box. This image file is 26.2KB.



So, let’s have a look at the standard “give it to me at 72PPI” request.  The following image was, again, resized from the original 2848×4288 pixels to 200×301 pixels with a resolution of 72PPI.  This image file is also 26.2KB.



And finally, I did the test again, this time resizing the original 2848×4288 pixel image down to 200×301 pixels at a whopping 10,000PPI!  Oh yes, this file was also 26.2KB.



As you can see in the screenshots of the Photoshop dialogue boxes, when you’re outputing specific pixel sizes, altering the PPI does absolutely nothing to the dimensions of the image, nor the filesize.  If you don’t believe me, feel free to load up each of the three images into Photoshop and check the filesizes and PPI settings for yourself.

The observant amongst you will notice that the “Document Size” dimension are different in each of the dialogue boxes shown above, while the pixel dimensions remain the same.  This is due to the fact that Document size relates entirely to the image in its printed form not how it is displayed on screen.

The REALLY observant amongst you will also notice that the PPI “Resolution” input is in this “Document Size” section of the dialogue box too, and not in the “Pixel Dimensions” area.

PPI has absolutely zero effect when it comes to having specific pixel dimensions.  It ONLY relates to the size of the image when printed.

Please feel free to comment below if you have anything to add.

Header Image : ‘macro pixels url cliche’ by Chris Dlugosz available on Flickr, used under a Creative Commons Attribution 2.0 license.