Wednesday, 9 March 2011

DPI vs. PPI - And why it matters, or not. :)

Hi folks. Yes, I realise I said I was going to post more, but things don't always work out the way they expect, so no more promises from here on in. I'll post as and when I feel I want to post something. That maybe twice in one week, or I may go 6 months between posts. But anyway.

I had a little back and forth with Scott Bourne on Twitter (@ScottBourne) regarding a new article on his site about why PPI doesn't matter (which he claims is a myth, and it does matter) when resizing images for digital display (that is, on the screen, on the web, via a digital projector running off a laptop, whatever).

Firstly, for those that don't yet know, I want to explain the difference between DPI (dots per inch) and PPI (pixels per inch).

So let's start with PPI. PPI is a measurement of how a digital image relates in physical dimensions in a print. So, for example, if you had an image that was 1800x1200 pixels, and you wanted to print it at 300PPI, it would bring out to a 6"x4" image. If you suddenly told your computer to print that same 1800x1200 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 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, however, 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 the technology used. 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 four "dots" from an injket (one of each cyan, magenta, yellow and black). But, it does not change the physical dimensions of the printed image.

So, that out of the way, why does PPI 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 last year. 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 2848x4288 image scaled down to 200x301 pixels at 300PPI, along with a screenshot of the Photoshop 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 2848x4288 pixels to 200x301 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 2848x4288 pixel image down to 200x301 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 (on paper!) not 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.

Feel free to comment below and tell me how I'm wrong.  I don't lock the comments on my blog posts just because some people might disagree or have evidence to disprove my statements. :)

1 comment:

  1. Cool post. Here's another take on it, that I think mostly agrees with you (though perhaps not with your distinction of DPI versus PPI?), while also presenting it a little differently, which might help some readers (different ways of saying the same thing is always a good thing, right?)

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