Edit: The abbreviation dpi is not used correctly here. It should have been ppi. To all and everyone – whenever it say dpi in this post, please read ppi Thank you to the readers that has pointed this out. And a word to everyone that feel like nitpicking – it is all semantics in this context. Neither ppi or dpi matters to someone doing a graphic for web or TV. Back to our regular program:
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.
Main picture: Photo: Miika Silfverberg (CC BY-SA 2.0)