I was asked recently by the editor of a website I write for to please make sure the images I send him are saved at 72 DPI. As soon as I saw this I thought "uh-oh, here we go..". I politely agreed to make the change to the setting (it's easy enough) but asked what the reasoning behind this would be. I had a pretty good idea what the response would be, but I wanted to make sure. A few minutes later I received the response that I feared. He said it would help the images load faster when viewed on the web site.
I was correct in my guess. The editor was under the impression that a reduced DPI setting would decrease the image file size which in turn would facilitate faster loading times for images from his web site. This is not an uncommon misconception but one I felt I would like to clarify. This editor is intelligent and a great guy to work with so I wanted to help him understand. This setting would not affect the images in the way he thought they would and while it would not harm anything being set differently, it adds an extra step in his workflow at times that he doesn't have to take.
I explained that this was an unnecessary step in his process and it would only affect images when they are printed. I even sent him a couple of example images that were both 800X600 pixels in size. One set to 300 DPI and the other set to 72 DPI. I showed him that both image files were the same size in KB and that both were compressed at the same level. He agreed and said to me,
"[I've] Been doing it for nearly 15 years now! I wonder why that was the recommended dpi back in the old days...",
I thought about this for a bit and wrote my response as follows:
If I'm not mistaken it comes from Apple's default settings on their monitors in the early days. Monitor sizes didn't vary much back then and so the 72 DPI standard came from being able to render fonts clearly on a given size screen. Windows based PC's actually used 96 DPI as their standard (I remember seeing this as an option to adjust on windows 95) and you could change it to display larger screen fonts. This was meaningful back in the 80's and 90's when monitors were 14" 's standard so the displayed content was actually close to the 72 DPI so your screen and printed material looked nearly the same.
As monitor sizes became larger and larger and more varied, this became more and more irrelevant where on screen display was concerned. For desktop publishing it was still nice to have the exact DPI of your monitor set up as you could easily tell what the final printed product would be and how legible a particular piece of text and associated images would display. These days so much is displayed on monitors that these needs have kind of flown out the window. We have such a variety of displays and devices that this has ushered in the idea of "responsive" templates that adjust now for various mobile devices as well as PC screen size and/or resolution.
So yes, there was a need at one time to keep an eye on this for proper display on screen, but not really anymore, as it has no effect on the way images are displayed. (800 X 600) pixels will render the same on screen no matter what the DPI setting as the computer will always show a 1:1 ratio of image pixels to screen pixels (not including sub-pixel elements). It is however very relevant when printing as the dpi setting greatly affects the final image resolution at a given physical size.
If you go into the image size settings in photoshop, you'll see that changing the dpi will affect the pixel count depending on what the physical dimensions (in inches for instance) . Of course though you'll see that if you change the dpi and then readjust the pixel dimensions back to the original, the physical size will now change, but again this won't be realized until you print the image to paper or other physical media.
At this point I thought this would make good blog entry for those of you who may actually care about these sort of things. I'm sure I'm not the only one, right?
P.S. I do realize that PPI is probably the more accurate term when discussing images on a screen but that's a post for another day :)