Thu 29 Sep 2005
I must admit I like to challenge my students sometimes. Just mess a little with their brain. And one of my favourite subjects is the dpi setting in Photoshop. It might be old news to you, but there is still a strong belief that if you are going to use an image for web or TV, it should be 72 dpi. So here is what I say.
Statement 1: Dpi has no impact at all on filesize.
Statement 2: A setting of 3167 dpi is very nice for web. Or TV.
Do you feel the urge to shout "Wrong!!!!!!" ? A lot of people do… But let us examine it closer.
Dpi has no impact at all on filesize.
This is actually very easy to show. Open a picture and change the dpi to 300. Save it. Then change the dpi to 50 and save it again with a different filename. If you have done this correct, the files should be very close in size. There could be a few bytes difference due to the way the filesystem store things. But no dramatic difference.
But maybe you have a huge difference between the files? Then you forgot to uncheck the Resample checkbox. Remember, I said change dpi. Not resample the image. As soon as you uncheck the Resample checkbox, the pixelsize of the image is greyed out in Photoshop. And the only thing that happens when you change dpi, is that the print size changes.
A setting of 3167 dpi is very nice for web. Or TV.
Actually, a setting of 64 dpi is just as nice. Try it. Make a 300 x 225 pixel file at 3000 dpi. Then make a 300 x 225 pixel file at 50 dpi. Put them on a webpage and compare. Or take a look at my examples here.
The reason for both of these statement is the same. And very easy to explain. Dpi is all about print. Not about monitors or other forms of similar displays. It is actually very clear in Photoshop. Take a look at the area that has the print size and dpi. There is a border around it that separate it from the pixelsize. This is because on most monitors you can display a variety of resolutions and as the resolution change, the ratio between the physical dimention of the monitor and the number of pixels it displays per inch changes. 
Without any change to the dpi setting of the file. And on a TV in Europe, the size is 720 x 576 non-square pixels. This fills the TV no matter how big or small the TV is. So all that matters when you are going to output on a monitor is the number of pixels. Get the pixelnumbers right and set the dpi to whatever you prefer.
You might argue that if only the software could detect what monitor it is and how big it is, you could add a feature that will display the picture according to the right size. But this is a big if. I do not know about any software that can do this at the moment. The only software I know that really cares about dpi is software that is primarily meant for output to a printer. They will scale a picture on a page, relative to the size of the paper.
Please add any comments you might have.
November 8th, 2007 at 00:05
true, for now. Not very future proof though is it?
if you wanted your students to learn the true meaning of DPI get them to save various images in various DPI, upload them to the internet then save them from the internet and drag them into illustrator or photoshop, and see what happens.
one day soon screens will be good enough to display very high resolutions, then dpi will count - currently macosx is resolution independent, if you mess around with DPI it does count.
November 8th, 2007 at 22:34
If DPI will ever count on anything else than paper, then the whole display industry will have to re-invent their technology, and I do not see any sign of that happening any time soon.
As long as you can not know anything about the size of the display your reader is using, you can not control the size of anything on their screen. The problem is that an inch will contain different numbers of pixels on different screens even if the total number of pixels are the same. A 19 inch monitor at 1280×800 has a higher DPI than a 24 inch monitor at 1280×800 if you could ever use DPI about this. Also, each of these monitors can change the number of pixels displayed.
But who knows what the future brings? We need to teach our students what is real today. And putting a picture on the internet, downloading it and opening it in Photoshop will not change anything in the picture at all. Unless, of course you actually change something in that process.