Why DPI does not matter

One of the most common question on web boards asks what DPI images should be saved at for viewing on the web.  Most likely the original poster read somewhere that web browsers all view images at 72 dpi, saved an image at 72 dpi, and it looked bad.  Here's the secret - DPI doesn't matter for viewing images on the web.

You want proof?  Here are 4 image all 200 pixels by 200 pixels. 



Notice something?  Yeah you do.  They're all exactly the same size on your screen.  They are all 200 pixels square. If you want, download them and look at the DPI.  It matches what the picture says. 

Confused?
DPI is a printing term.  It means dots per inch and how many dots a printer can put down in a single inch.  But we're not printing.  So don't worry about it. 

I'm not 100% sure about printers, but assuming you don't change the pixel dimensions along with it changing just the DPI of an image doesn't add any image data.  The DPI is just a small bit at the start of the file.  I have had printers in the past that have done better if I sent it a 300dpi image set at the correct linear dimensions rather than a, for example, 150dpi image with twice the linear dimensions but scaled in the printer software. 

Now, let's say you have a 6mpx image which is roughly an 8x10 @ 300dpi.  Changing it to 16x20 @ 300dpi will certainly add more pixels, but 75% of them will be interpolated by software and were not part of the original image.  Photoshop does a pretty good job at guessing what each new pixel should be, but they're still guesses. 

To confuse matters even more, no current printer that I can think of actually prints at 300dpi.  The $100 Epson I have sitting on my desk prints at 720x1440, but that doesn't mean I should send it an 8x10 @ that dpi.  At one point I had read that most printers handle 300dpi images better than other dpis, but I don't know if that's true or if it was if it still is. 

Reading back through that I don't know that I really answered the question, and dpi is certainly a confusing term for many people, my self included.  The reason for the article is that I've seen a lot of web postings asking questions like "I heard that I'm supposed to save images at 72dpi for my website but they look bad.  Should I save them at 300dpi instead?"