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Images and all dimensions in px or pt that I export for web assets appear to be bigger on another monitor than they appear to be on my monitor. For example, an image of 800 pixels height looks like 1200 pixels on another monitor.

Is there a possibility this is caused by the dot pitch on my monitor, which is very low compared to other screens (Dell 2515 – dot pitch 0.216). Has anyone experienced the same problem?

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A pixel is a point sample. A point sample has no size (read a pixel is not a sqare3). The pitch between samples on a digital display device unspecified. When you make a image 800 pixels wide you get a randomly large image.

The technical reason is that: Images degrade on anything but 1 pixel = 1 pixel ratio. Beyond this the operating system does not even know how big the monitor is. So what happens is your shipped with x samples and the monitor displays those x samples at x samples in its display how large that happens to be. This is how computer imaging has worked for 50 years.

This unitless system is what you want for most situations. Since the opposite of this would be all systems would be scaling images left and right*. Imagine how frustrated a user with a mobile device would be if your image was shown at the same size as on a 21 inch monitor. In reverse the guy with a 50 inch monitor is not going to be happy with images that scale as big as a mobile device.

So at the end of the day:

  1. Even if you specified a image x inches wide,
  2. and the monitor and software would be able to comply with this.

You would just end up with a mess because some monitors would be displaying only very few pixels at that size while others couldn't span that size. And nearly nobody would be happy. Forcing me to view a image at a specified physical size makes just about as much sense as trying to force me to switch to a different amount of pixels on the monitor. As this would mean id have to buy a new monitor for every webpage, and that is not going to happen.

* And apple actually did this, and now people have all sorts of confusions about developing for the web.

  • I lol'ed at the "different monitor for every webpage". Good answer though. – PieBie Jul 11 '16 at 17:59
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A pixel is not a fixed size. 800 pixels on one monitor will be a completely different size to 800 pixels on another monitor. There is no way around that, it's just how screens work.

If you really need to see things at close to actual size (assuming you're working in physical units) you could do some calculation and view your image at the correct size, but since you're working with web assets you can forget that.

  • Thanks for your answer. Then the dot pitch don't take under consideration? It's irrilevant for this problem? – Vittorio Jul 11 '16 at 15:59
  • Yes, it's very relevant to the question. The dot pitch and PPI (which is often called DPI but should really be PPI) are what measures how big a pixel is. But in the world of digital media there is nothing you can do about it so theres no real need to worry about it at all. – Cai Jul 11 '16 at 16:10
  • @Vittorio theres no guarantee that the systems will report right pitch. Indeed most systems do not know what the pitch is. Even Photoshop does not know what size monitor most users have and adobe actually implemented this decently. But alas the monitor wont tell and expecting the user to do so is a waste of time and effort. On top of this users are free to have custom zooms. – joojaa Jul 11 '16 at 17:16

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