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It seems the terms DPI and PPI are misunderstood by more people than actually understand them. I'm not just talking about average Joe trying to print a holiday snap, I'm talking about graphic designers, photographers and even the guy who runs my local print shop—people who really should know. It honestly baffles me.

The common misconception is that an increased DPI will inherently improve image quality and likewise increase file size. What people don't understand is that it has no tangible effect on an image file itself, only its effectible print size (or PPI does, hence my next point). Add the fact that DPI and PPI have become interchangeable in recent years (no, they are not the same thing)...

Why are these terms so misunderstood? (Bonus: What can be done about it?)

  • Not sure if this is really answerable, but as a comment: There are some who claim that these terms (and other things) are taught incorrectly. See dpiphoto.eu/dpi.htm for example. Many people regard a/b = c as maths, so have a look at tvtropes.org/pmwiki/pmwiki.php/Main/EverybodyHatesMathematics and tvtropes.org/pmwiki/pmwiki.php/Main/WritersCannotDoMath (Warning: TVTropes) So what can be done: improve education (the solution to most, if not all of this planet's problems) – Michael Schumacher Apr 30 '16 at 19:23
  • I cannot find the word "effectible." Can you provide a synonym or the meaning of the word? – Stan Sep 12 '16 at 15:14
  • @Stan "Adjective: effectible ‎(comparative more effectible, superlative most effectible) — Capable of being done or achieved" A Google search for "effectible" gives an entire first page of definitions. – Cai Sep 12 '16 at 15:21
  • Many thanx, Silly me. I used a couple of dictionaries. – Stan Sep 12 '16 at 15:45
  • No worries, apparently it's use is "archaic", which I didn't realise. – Cai Sep 12 '16 at 15:56
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Those terms are largely misunderstood for two reasons:

  1. Most people do not really understand what units are. Units have been taught to most by memorization, and quite lot of it, without much deep thought put into the subject. Since this kind of learning worked for most purposes for other units then they think it works for this too.

    From this follows that people tend to ask for a good value and just fill it in without thinking much about it. This means rules of thumbs became things set in stone because that's just how units are approached in general. Besides its easier and it sort of works.

  2. Many users have no intuitive feel in what an image is, or how images are presented technically. For most people a image is just a bunch of squares, which is somewhat true albeit a bit misinformed interpretation. The technical aspects are below the radar of people.

    From this follows that people are unable to understand when the DPI or PPI is a valid concern and when not. The practice of sometimes calling this the resolution binds the information to quality which makes it worse. For most people its just a higher value better quality kind of thing. A bit like more megaherz used to mean better processors or more magapixels make a camera better and so on.

The first of these is very hard to compensate when the problem was already seeded in elementary school. The second is much easier to address. Except Apples way of doubling images on retina screes makes the problem much worse and visible. The concept of where the terms apply is pretty easily explained:

  • Defining how densely to show images pixels only matter on devices where you can control the rasterization process and size of media. Usually only in printing.

    It is somewhat meaningless on a digital display since you can not really affect the pitch, except if your actually buying the screens for this purpose. Also its superfluous to deal with a conversion value form pixels to physical units and back to pixels.

What the value means is also quite easily explained:

  • How big do you want the output to be. With the caveat that this only applies to in situations where you can control the medium which in general only means print.

It would be much easier if people would primarily think of how many pixels they have and then only use the conversion to real physical units to verify that your size makes sense.

TL;DR

Too much work to understand easier to just guess. Think pixels first then conversion to physical units if you need it. If you need to prepare for print convert your desired size to pixels and then stop thinking about this value.

  • 1
    There are various uses of "resolution" because it is a generic word, like measure. What are you measuring? Distances? Time? Weight? – Rafael Apr 30 '16 at 21:36
  • @Rafael yes offcourse but since people want to simplify things and dont understand units its a problem. i dpnt think it would br such a confusing term if we used print resolution, or print density. Of just skipped it entirely and asked print size. – joojaa May 1 '16 at 5:28
  • Dots per inch and Pixel per inch are diferent units, that measures diferent things. "Oversimplification" is (many times) what make people having a wrong understanding of things. :o) – Rafael May 1 '16 at 10:56
  • @Rafael I am aware of such semantics but its not so clear cut DPI is perfectly valid in many different circumstances. Wheras PPI is much more restrictive definition. Ive long sice stopped caring and bringing in a controversy about this helps nobody. – joojaa May 1 '16 at 11:03
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...increased DPI... only its effectible print size.

Nop. That is PPI. DPI is a printer capability unit.

I think the root of the problem is that the dpi is widley known because for a time was an important issue to buy a printer. This unit was adopted as image quality.

A scanner was thought as just the input part of the same ecosystem. The people just thought about this as "Oh. I put a document here (input) and I can have a copy there (output) and thought the units were the same.

From there it all went to hell. Some software designers used wrongly dpi.

And the only place where people can see this tiny unit is on a dialog box that people do not understand.

Bonus: What can be done about it?

Actually I am preparing a series of videos on the subject I will post on youtube. It is taking time because it has some animation. They are going to be very short. (I will be happy if later someone helps me later with the grammar and spelling for the translation into english)

  • "Nop. That is PPI" That was kinda my point.. there is a misunderstanding of what the terms mean and that they are the same thing. – Cai Apr 30 '16 at 21:41
  • I would be very interested to see those videos and more than happy to help out! – Cai Apr 30 '16 at 21:41

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