The 72 DPI myth

Too often I hear that “if someone wants to publish pictures online, they should be 72 DPI”. People always say “300 DPI for print, 72 DPI for screen”. This is just plain wrong, and here's why:

The DPI value indicates the dots per inch resolution used to print things out. With 300 DPI there are enought dots in one inch to make the pictures look sharp, with 72 DPI artifacts are clearly visible.

But on the Internet, the resolution is irrelevant! Let's make some tests:
Our test picture is 500x500 px, 300 DPI. The filesize is 259,4 KB.
Now let's change the resolution to 72 DPI, leaving the physical dimensions at 500x500 px.
Still 259,4 KB!
What if we change the resolution to 2 DPI? (Don't try to print that out ;)
Yep, the filesize is still 259,4 KB.

You can download the files and see that DPI won't make any impact on filesize, it will only affect the printed materials. If you want to reduce the filesize of a picture, you will have to reduce the pixel dimensions.

The font used to “test” is the free Regular Cargo by Nik Thoenen.

6 comments:

jeremy said...

That's what I've always said to my co-worker, she never believed me! I never got around to actually proove her wrong, so thanks ;)

liz said...

*diggs*

SARAH FRANCE said...

Very nice blog! I love typography - and it is great to see where others get their get their inspiration from. Thank you. Sa.

Anonymous said...

Oh THANK YOU, THANK YOU for FINALLY pointing that out, that 72 dpi talk is just MISLEADING! Keep up the great work!

Graham said...

I think it depends whether you have the 'Resample image' box ticked in the image size window - if its unticked then you're not changing the amount of pixels contained in the image so when you place it on a web page, it is effectively 72 dpi and hence the file size not changing. If you tick the 'Resample image' option, then it will remake the image with a new amount of pixels in relation to the dpi and the images physical proportions. It is in this latter situation where having less than 72 dpi be too lo-res, even for web use.

I think this is correct, though I'm mostly a print design monkey and could be wrong…

nicetype said...

You are correct Graham, with physical change of pixel amount also the change of file size will take place.

You are wrong though, when you say that an image on the web would effectively have 72 DPI. This is related to the device the image is shown on. For example there are 13" laptops with resolution of 1024x700 and 1280x800. The same image would have the same pixel dimensions, but different physical size on the monitor and thus different DPI.

This test was aimed at the misleading talk that the DPI alone is responsible for the file size, which is not the case.