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I have a 13 inch screen and my friend has a 27 inch screen. If we both download the same photo from my website, the file size and photo dimensions are the same (70ko and 300x450 pixels).

However when we open the photo in our respective softwares, his opens to 9x14 cm and mine opens to 6.5x10 cm for the same quality!

Also, he can scale the photo to around 30x45 cm before you start seeing pixelization, whereas around 10x15 cm mind is already so pixelated it's basically useless.

I don't understand how there can be such a difference. If the file size is the same and the dimensions are the same, why would there be a difference from one screen to another?

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    Where are these cm dimensions coming from? You're measuring with a ruler? Also, I don't understand quite what's going on here. How are you scaling the images? Same software on both machines? 13 inches sounds small for today unless yours is a tablet. – Octopus Dec 21 '15 at 19:45
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    @Octopus - 13 inch screens are very popular in laptops. I.e. Macbooks, Dell Inspiron and XPS, Lenovo Thinkpad, etc. – dpollitt Dec 21 '15 at 19:48
  • Please post the actual image, and a screenshot of a portion of 'yours' showing the pixelation at 10x15cm. – Roddy Dec 21 '15 at 22:20
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I suspect you are mixing a couple of things here - and so far the good answers have touched on it.

What is "image quality"?

  • colour accuracy (we ignore it here)
  • sharpness of the image - you are asking about this it seems

Assuming we can ignore colour accuracy, the number one way of improving image quality is resolution - hence the bigger you want to print, the more resolution you need. Of course the image has to be sharp - hence it can be advantageous to upscale an image prior to printing at a larger format as this allows you to say sharpen the upscaled image more to account for a loss in sharpness from upscaling.

If we now return to a computer screen, perceived image quality will depend (aside from colour accuracy) primarily on pixel size, the smaller the pixel, the sharper an image will look. (A common monitor will be around 96DPI/PPI, though today's high resolution screens are often higher.)

So if you now take the same image to two monitors of different pixel density (PPI), on the monitor with the higher pixel density the same resolution will be displayed on a smaller space with smaller "pixel points". E.g. 500 pixels will cover over 5 inches on a common PC monitor, while on my (Fujitsu) IGZO screen laptop at 262PPI it would cover just 2 inches. THIS is the size difference you have observed.

Nowadays, anything below 1920x1200/1080 for a 24" monitor is insulting, while high end monitors will offer "4K" (marketing blurb for 3820x something) or higher resolution. However, some high quality products will also offer high resolutions on smaller screens, my laptop offers 3200x1800 on 14", about 262PPI. Having said that, many smartphones are in similar pixel density territory, my BlackBerry with 1280x800 pixels on a 4.5" screen has a pixel density of around 348 if I am not mistaken. (Some newer phones again offer higher pixel densities.)

Incidentally, common advice for typical 10x15 photo prints is to print at 300DPI as it is hard (if not impossible in the case of ink) to distinguish pixels at such a dot/pixel density.

Now coming back to monitors: In some, but not necessarily all cases, users of high resolution monitors may employ "DPI scaling" (as Windows calls it). This will result in image or video content being upscaled (though I'm not sure if browsers will take files "as is" if they are scaled using HTML which is otherwise considered bad practice) which will result in "fuzzyness" for most images. (Mainly because images are just upscaled and not sharpened again.) An exception to this is text or vector graphics which are just re-rendered at a larger size if scaling is required.

  • Thank you for such a detailed answer, that was extremely helpful, I really appreciate. – jeremy radcliff Dec 21 '15 at 21:38
  • @jeremyradcliff Happy to help :) – DetlevCM Dec 21 '15 at 21:48
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    I agree with everything you've said here except this doesn't really explain why a 30cm image looks great while the same image at 10cm looks pixelated. I am completely baffled as to how that could be. I do not believe pixel density is the issue here. My guess is that the software being used on the system with the smaller screen is inferior. – Octopus Dec 21 '15 at 22:08
  • @Octopus Key word scaling - different scaling algorithm. (So yes, what you suspect and what I touch on with DPI scaling). – DetlevCM Dec 21 '15 at 23:14
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    Specifically in the case of the OP, I would suggest that one viewer/computer smoothes the image while enlarging (effectively upscaling as most upscaling algorithms that we actively use smooth the output too) while the other does not. – DetlevCM Dec 21 '15 at 23:42
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Displays can have differing resolutions as well as differing physical sizes. See: https://en.wikipedia.org/wiki/Display_resolution

In other words, you have to consider how many pixels the display has. If the displays are the same physical size and the pixels are of different sizes, you will have a different resolution. Since you are comparing different size displays, the resolution may be similar but of course the size of the pixels would differ.

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It is the screen resolution not the physical size of the screen that determines the viewing quality.

My Galaxy S5 cellphone has a 5.1" screen but the resolution is 1080 x 1920 pixels.

HD TV's started out at 720 x 1280 then went to 1080 x 1920. Now we have 4K TV's with 2160 x 3840 and 8K with 4,320 x 7680. All these resolutions come in varying sizes.

More info here: Resolution to Monitor Size Chart

enter image description here

  • I just used the chart to visualize the variety of screen resolutions. – Mike Sowsun Dec 21 '15 at 20:29

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