Stop Shooting Auto!

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.

I see this question, or some variation on it, over and over again:

“I just got a new camera, and it shoots at 72DPI by default.  How do I change it?  Won’t 300DPI give me higher-quality images?”

It’s a common question, and a very common misunderstanding about DPI.

First, let’s start with the camera.  Your camera’s sensor has some native size, some fixed number of recording points (remember the sensor gnomes?) on its sensor.  Each recording point records one pixel of the image.  Every image that’s shot with the camera has that size no matter what.  Using the Canon Digital Rebel XTi as an example, the sensor is 3888 pixels wide and 2592 pixels tall.  If you multiply those numbers together you get 10,077,696, which is why it’s called a ten megapixel (ten million pixel) camera.

Remember that.  As long as you shoot at the XTi’s maximum resolution (and by that I don’t mean DPI, I mean L or L+RAW) you’ll get an image that is 3888 pixels wide and 2592 pixels tall.  Some cameras let you shoot smaller images, by changing the size from L to M or S.  If you do that, you’re shooting at whatever size that particular setting gives you, but it’s still independent of DPI.

So what’s DPI?  It stands for Dots Per Inch. For our purposese, a dot is the same as a pixel, so it would be pixels per inch. (There is a technical difference between dots per inch and pixels per inch, but for purposes of this discussion we’ll treat one pixel as being the same as one dot– it’s one unique spot of data in our digital image)  DPI is not really a part of the image itself, and doesn’t change anything within the image.  Rather, it’s a number that’s stuck onto the side that tells a printer how big it should print out the image– imagine someone handing the file off to be printed, and sticking a post-it note on the file so that the printer will know how big to make the image.  If the image is 72DPI, the printer will print out 72 dots in every linear inch.  At 300DPI, the printer will print 300 dots in every linear  inch.

If you’re working with an image on the computer, and never printing the image, DPI is completely irrelevant.  A 3888×2592 image at 72DPI is exactly the same image as a 3888×2592 image at 300DPI or 1DPI, or a million DPI.  Really.

The thing about DPI is that you can change it arbitrarily, and it won’t affect the image.  As long as it stays 3888 pixels wide and 2592 pixels tall, you can rip off the 72DPI sticky note and change it to any other number, and you still have exactly the same data in your image.  That means that you don’t really care what DPI the camera sticks onto the side when you first take the picture– changing it in-camera won’t change the information that you capture, and it’s easy to change it later if you want to print the image out.  Plus, if you’re printing the image you probably will want to resize it, crop it, or make other adjustments.

And that, in a nutshell, is why you probably don’t care about DPI.  Unless you’re printing the image out, DPI is a completely irrelevant concept, and one you basically just shouldn’t worry about at all.

Next up, I’ll explain how DPI works when you’re printing.