Getting a Digital Photo Ready for Print: dpi and ppi – Digital Photograpy

What Pixels Per Inch (PPI) and Dots Per Inch (DPI) mean and how to work with them when preparing a digital photo for print. There are a number of confusing issues surrounding the number of pixels it takes to produce a print from a digital camera. Solving this problem requires using complex multi-variable differential equations that can easily overwhelm even today’s most powerful desktop PCs. And if you believe that then I have some beachfront property to sell you here in Arizona! It’s really quite easy to figure out and there are a few simple guide lines that can help you get the Biggest Bang Per Pixel, or B2P2 – that’s pronounced “Bee-two, Pee-two”.

Just Print it Baby
The best way to figure out what your photo will look like when it is printed is to simply print it. Then, you’ll know. Of course printing takes time and costs money. If you just bought that expensive photo paper then you’ll want to make the best use of it and that ink ain’t cheap either. So, experimentation is great but let’s keep it to a minimum. If you make prints of your photos pretty often it will become second nature to you in no time. If you don’t make prints very often then be sure to keep notes handy so that you don’t make the same mistakes overr and overr.

Probably the single biggest point of confusion comes from Dot’s per inch and Pixels per inch, that’s DPI and PPI. DPI is the number of tiny little drops your printer can spray in a one inch square. When you’re buying a printer this is a very important number to keep in mind. And, for a variety of reasons, it’s also pretty worthless. That’s because experts usually can’t tell the difference between prints from a 1440 vs. 600 DPI printer. Companies measure their printer’s capabilities differently because they use different technologies to print. 300 DPI is probably enough and 600 is plenty. This isn’t like the processor wars between Intel and AMD. There is such a thing as enough. Instead of looking for huge DPI counts look for things like paper sizes the printer can accept, ink types/durability and cost per print. I’ll talk about pixels for a bit and then come back and show you a test to tell you if your current printer is good enough for your needs.

Pixels, pixels everywhere
This is where things get tricky so I’m going to give you lots of examples and analogies along the way. Your digital images are made up of pixels. If you plan on printing your photos then you want as many pixels as you can get. Think of it like this: a digital photo is like a mosaic. The more tiles you have and the smaller they are the more realistic your mosaic will look. The tiles are like the pixels in your photo and the smaller they are is like the ppi – one more time for you acronym-a-phobics: that’s Pixels Per Inch. The more pixels you have per inch the finer the detail will look. Think of ppi as pixel density. The more you pack in an inch the harder it is to tell the difference from one to another and that’s what you want.

You should know that as long as you are viewing your photos electronically then the ppi makes absolutely zero difference to what you see. It doesn’t change the file size one single bit. PPI will only come into effect when you print. So what does that mean? Basically this: if you aren’t going to print your photo then ppi is totally irrelevant

What would you do with a printer if you had one?
Digital Equations
Megapixels =
Photo: width x height
Photo Print Size:
Photo width / ppi by
Photo height / ppi
Print width * ppi = pixel width
Print height * ppi = pixel height

But what if you are going to print? Now how many ppi should you have? That depends on the quality that you are looking for and of course how large you want your print to be. This is where you start judging one value against another. What you need to know is, “what is a minimum ppi value that will produce a photo realistic print?” Yeah, that means, “how many ppi do I need to make this damn photo look good on my wall?” I’ll give you a number to start working with: 200 ppi. I’m basing this on a study by the magazine Popular Photography & Imaging. They asked both armatures and professionals to judge a selection of prints and determined that 200 ppi was usually sufficient. The experts liked to see somewhere between 250-300 ppi but some people are just too hard to please.

Using this as a baseline, let’s figure out how large a print you can make from your digital camera. Take the pixel width and height of the photo and divide it by 200. My Camera (Canon D30) is a 3.1 megapixel camera; it produces photos of 2160 pixels x 1440 pixels. To get the photo dimensions I divide 2160 pixel by 200 ppi and I get 10.8 inches and doing the same thing for the pixel height I get 7.2 inches. So I can print a 10.8 x 7.2 inch photo at 200 ppi. If I want to go up a bit in quality I can print a 8.6 x 5.7 at 250 ppi. The size of the photo gets smaller as the pixel density – ppi – gets higher; it’s like squeezing the photo down in size.

And you can do this in reverse. If you want to print a 8.5 x 11 at 200 ppi you’ll need a 4.34 megapixel camera – one that can take 2550 by 1700 pixel photos. And if you want to get the same size print with a ppi of 250 you’ll need a 6.8 megapixel camera. To move up to an expert quality of 300 ppi you’ll need a 9.8 megapixel camera. Depending on the print output you desire you now know how to figure out the number of pixels you’ll need.

That’s all she’s got Cap’n! I can’a give ya any more pixels.
What about, gasp, adding pixels? Just the thought of adding pixels to a photo brings to mind an image of a sleazy man, wearing an overcoat, standing in a dark alley, saying, “hey buddy, you want a pixel boost?” In days gone by I would say, “never” but times are a changin’. The only reason you would want to do this is to increase the print size of a photo. There’s no point in adding pixels to increase the ppi. It would be counterproductive at best. Interpolating a photo in any photo editor just runs an algorithm that makes a best guess at what the added pixels should look like. Some algorithms work better than others. Photoshop 7 does a great job of interpolating an image up in size by as much as 200%. Beyond that you’re pushing your luck. There are programs out there, such as Genuine Fractals, that can add even more than 200% to a photo size but results are mixed.

Here is what the Image Size dialog looks like in Photoshop 7. If you want to add pixels to your image you’ll want to make sure that you have Resample Image checked. Of course “Constrain Proportions” should always be checked unless you want to make someone look fat/thin. Now you can change the Resolution and Width/Height under the Document Size. The Pixel Dimensions will change accordingly. You can use them to keep track of what percentage increase you have made so that you don’t over do it. Remember, 200% is pushing it but you’ll have to experiment.

How good is good enough?
I mentioned earlier that I’d show you a way to test your printer’s quality. Try this: uncheck the “Resample Image” box. Now increase your ppi – resolution – up to 300 ppi or even more. You’ll see that your print size drops. That’s okay. You don’t need a big printout for this. Break out with the good paper and make a printout of this photo. (Make sure your printer is set to “Best Quality”.) Okay, take a look at it. If your printout isn’t very good then you need a better printer.

Iceberg off the Starboard Bow!
If you’ve noticed that your photo printouts don’t have the same colors and brightness as the photo you see on your monitor then join the crowd. This is a problem with monitor settings and color spaces. If you think all this ppi, dpi, resolution, image size stuff is confusing then you just wait until I talk about color spaces. We’ll need Adjustment Layers to tackle this problem and that’s the next set of articles I’ll be covering.

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