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DPI is Danger

So twice in the space of a day I’ve been confronted by some dodgy information about DPI – Dots Per Inch – and about how it applies to the world. Let me first say, that I think that thinking about DPI is about the dumbest thing the desktop publishing industry has inflicted upon us. I say that because it’s a useless number on its own. Let’s say, for instance, that I scan something at 300 DPI. Can you tell me if it will fill a modern screen? No – you can’t. You can’t do that until I tell you how large the original scan was. If it was more than about 3 inches by 2.5 inches, it will fill a 1024×768 monitor.

The real answer when dealing with graphics is knowing how many pixels (individual dots) you need in order to fill the area you want to fill. DPI is the measure of the maximum resolution of the output in a given space. i.e. 300DPI means that you can fit 90,000 pixels in a 1″ square (300×300 = 90,000) If I want to be able to output at full resolution I need a file with a resolution of at least the number of inches times the DPI.

The good news is that people rarely talk about DPI when talking about digital photos … until someone with a desktop publishing background steps in the room. Digital cameras are measured in megapixels (or resolution not pixels per inch.) Occasionally you’ll hear folks talk about 72 DPI screen images – however, this is a misnomer on the PC since the resolution and screen size are not tightly coupled. My 15.4″ Lenovo laptops 1920×1200 screen has a different number of DPI than the 26″ external monitor I connect it to – but both have the same resolution.

Typically when you hear DPI, You heard about it from people talking about a setting for scanning. The interesting thing is that oversampling (scanning at a higher DPI than the output device could produce) can cause some real ugly scans – and some problems for trying to get an image to look right. If you know the resolution of the output device you should match that in order to get the best results. Having a bigger file with a higher resolution generally doesn’t do much harm – but it generally doesn’t do much good either.

So … PLEASE don’t talk about DPI. Talk about the resolution you need.

2 replies
  1. Robert Bogue
    Robert Bogue says:

    Hi Robert
    I disagree. DPI tells you something about the quality. Why not speaking about the quality of a photo independent of its size?

  2. Robert Bogue
    Robert Bogue says:

    DPI tells you absolutely nothing about quality. If I say I’m doing a 300 DPI (or 600 DPI) scan that says nothing. If that’s of a photo print I’m reading the actual maximum effective dots per inch — depending upon how it’s printed. If I’m scanning a negative or Microfiche, it’s no where near enough resolution.

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