Talking Pixels

DPI (Dots Per Inch) seems to be the most misunderstood image property out there.  Funny thing is, my investigation into this started with a conversation I was having with a coworker about a programming issue he was having that centered around the dpi of the screen.  Initially I said PCs are 96 dpi, and Macs are 72 dpi, but as I started looking into the validity of that statement, a world of misinformation and confusion was brought to my attention.

Even more confusion surfaced when I started looking into the dpi issue as it relates to photography.  Have you ever heard someone say, “save it at 72dpi for web presentation”?  Me too.  Actually, it would have been me if you would have asked me a few days ago.  However, I now know that it really doesn’t matter much*.  Ah, you noticed that asterisk; well it does matter in a few instances, but not where you’d think probably.

So here we go.  Why doesn’t dpi matter.  Well, it has zero, zip, zilch, nada, no effect on how an image is presented to the user when viewed on a computer screen.  The only thing that matters is the pixel dimension of the image.  That’s right, you heard me, the only thing that matters is pixel dimension.  You know, 640×480, 800×600, 3000×2000, etc, etc.  That’s what matters.  Here’s an example.  If the resolution of your monitor is set to 1024×768 and you try to view a 640×480 image, it will take up about 60% of your screen.  That’s true if you’re looking at a small laptop screen, or a projector displaying on a wall 60 feet away.  As long at the resolution of the output device is set to 1024×768, it’ll all look the same.

Still skeptical and need some proof?  Here ya go.  The three images below all have a pixel dimension of 625×750.  What varies is the dpi of each image.

They look pretty much the same to me.  What do you think?  And remember, this is your monitor, so there’s no way I can be doing any trickery here.  If you want to verify the dpi, click on each image, save the full size image, then view it in something like Photoshop to see the dpi for the image.

But what about file size I hear you say, 2500dpi must be hugh!  Well, no, it’s not.  They’re all about the same at around 180KB.  I think there’s a slight difference due to JPG compression, or perhaps I fumbled something when making these sample images(most likely case).

But what about printing?  You save web files at 72 dpi so they can’t be printed.  Well, that’s where part 1 of the asterisk from above comes in.  The dpi is really used to tell a printer, “Hey, print this image using this number of points per inch”.  In actuality, printers use something called PPI (Points Per Inch), and is the number of points that can be printed in an inch.  Now, depending on how small a printer can make those dots, you can get a higher resolution image.  From some empirical studies that I read online, 150ppi seems about the minimum you need to print a good image.  This means that you’d really have to be doing some pixel peeping to see and difference from a higher dpi image (like the well known standard of 300dpi).  So how does dpi of an image relate to printing?  Math.  It’s pretty simple actually.  To determine the ideal printing resolution of an image, and I did say ideal, you divide the dimension by the dpi.  For example, the files above, if you printed the 25dpi image you’d get a print that’s 25″x30″ (625px/25dpi = 25 inches and 750px/25dpi = 30 inches).  No one said it’d be a good looking print!  If you printed the 250dpi image, you’d get a 2.5 inch x 3 inch image.  And printing the 2500dpi image, you’d get a .25 inch x .3 inch print.

But why did I say that dpi is less important for printing?  Well, if you’re trying to be tricky and save that 2000×3000 image to 72dpi so people can’t swipe it, you just handed them a file that would print pretty good at 13″x20″, and great at 8″x12″.  Why?  Because changing the dpi of an image does not effect the image (unlike resampling).  All you really did was change a few bits of data in the header of the image.  The content of the image stayed the same.  So dpi does have some relevance when it comes to printing.  To protect your online images from being heisted, you need to combine a small pixel count AND a small dpi.  But even if you do all that, if someone is motivated enough, they can take your images and resample them to a larger size with some pretty sophisticated software to yield acceptable results for printing: this is especially true of canvas which supports really low dpi due to the characteristics of the material.

Oh yea, where does dpi REALLY matter?  Scanning images.  This is where dpi truly matters because scanning at a small dpi will result in a small pixel dimension, which results in a small file (viewed on a screen or printed).