DPI: When you DO care

In my last entry, I said that you usually don’t care about DPI.  You should definitely read that and understand it before you dive into this entry.

Now that I’ve told you why you don’t care about DPI, I’ll tell you the one time that you do– when you’re printing an image.  That’s the one time that DPI comes into play, and when you get to it you’ll really care.

For print resolution, there are really two things you care about– dots and inches.  Dots are easy– that’s the number of pixels you have in each direction of your image.  If your file starts out with a resolution of 3600×2400, you have 3600 pixels/dots on the long side, and 2400 pixels/dots on the short side.  That’s pretty obvious, right?

The second number is inches.  If you’re going to print your photo at 8×12, there are 12 inches on the long side and 8 inches on the short side.  I know, I know, but I’d really rather state the obvious and get it over with rather than having some poor guy in the back scratching his head.  Besides, if you’re not American you probably grew up with the metric system, and you think an inch is something that should be scratched.

Ahem, OK.

We were talking about DPI.  Right.  As I said earlier, DPI stands for Dots Per Inch, and it’s a measurement of how much to stretch the image out when it goes on a piece of paper.   Working with DPI requires some really simple math.  Duck, because I’m going to hit you with a formula.  Ready?

Pixels / DPI = Inches

That wasn’t so bad, was it?  That’s really the core of what you need to know about DPI.  Let’s walk through it.

Remember our 3600×2400 image?  Let’s figure out what happens if we print it at 300DPI.  For the long side, we have 3600 pixels.  If we divide that by 300DPI, our printed image will be 12 inches.  For the short side, 2400 pixels divied by 300DPI will be 8 inches.   See?  That wasn’t so hard.

Now, try it yourself.  What happens if you print it at 200DPI?  Go ahead.  I’ll wait.  Here’s a pretty picture… you can think of it as the equivalent of on-hold music.


You’re done?  Did you figure out that it would be 18×12 inches?  If so, give yourself a gold star.  You did the math correctly.  If not, let’s walk through it.  3600 pixels divied by 200DPI = 18 inches.  2400 pixels divided by 200DPI = 12 inches.

Most printers will have some native resolution that they want you to send them files in, often 240DPI or 300DPI, but check the doicumentation for the device you’re printing on.  If you just send the image to the printer at that resolution, you now know how to figure out how big the print will be.

Often, however, you want to go tthe other way– you have an image, and you want to print it out at a fixed size.  In order to do that, you just swap the equation around a little bit:

Pixels / Inches = DPI

It’s the same equation as before, only we’ve rearranged the terms ala Algebra 101.  You do remember Algebra 101, right?  If not, just trust me.

What this means is that if you have a 2400×3600 image and you want to print it out at 4×6, you can do a little bit more basic math.  In particular, 3600 pixels / 6 inches = 600DPI.  Since DPI is essentially always the same horizontally and vertically, it works out well that 2400 pixels / 4 inches = 600DPI.

(Note:  Everything I’m writing today assumes that the aspect ratio of your image is the same as the one that you want to print at.  If you have a rectangular photo and you want a square print, you’ll need to start by cropping or padding the iamge to the right aspect ratio.  That’s not a topic I want to cover today.)

So what happens if you have a 2400×3600 file, you want a 4×6 inch print, and your printer really insists that you send it 300DPI files?  In that case, you have to resize your image.

The mechanisms for doing this will vary greatly depending upon what image manipulation software you’re using, and resampling is an art unto itself.   One way to do this is to do the math yourself and figure out how many pixels you need (in this case, 6 inches * 300 DPI = 1800 pixels on the long side, and 4 inches * 300 DPI = 1200 pixels on the short) and then resize the image to those pixel dimensions.  In Photoshop you cantake the easy way out and let Photoshop do the math– resize the image, set the DPI and the inches on each side, and let it figure out how many pixels the resulting image has to be.  Other image manipulation software is probably similar, but I’m not familiar with the exact mechanisms.

Most consumer-grade printers have drivers that are pretty smart about resizing images on their own.  My Epson R1800 will let me send any size image to it, and I can change one setting to tell it to fit the image on whatever size paper I’ve chosen.  That’s simple!  You still want to understand the basics, though, so that you know if your image will turn out OK. The lower the DPI, the fuzzier your image will look when it’s printed.  200DPI or more should certainly give you acceptable images, and for very large prints you can go lower– people stand farther away from big prints, so they won’t notice if the print isn’t completely crisp and perfect.

If you’re resampling the image, you can generally make it smaller without losing a lot of image quality.  There’s a limit to how much you can make it bigger, though.  When you make an image bigger (called upsampling, among other things) the software has to guess at what information it should fill in.  Newer programs are pretty good at this, but they’ll never produce anything as good as if you’d captured more pixels to start with.

Does this all make sense?  I hope I’ve left you with a better understanding of DPI.

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