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I'm using a 13" laptop with FullHD (1920x1080) resolution and an external screen with FullHD resolution too, but of 22". It's quite strange to have a much bigger screen with the same "area space", and I was thinking about manually adding a custom resolution to linux config. I know how to do that, but I'm not sure about a good resolution to setup.

Any ideas? Any "don't do that please" answer? If yes, why?

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Not a full answer so a comment: If you exceed the maximum resolution of your screen and keep it in the same area (no 'overdraw'), the only way to do that is to leave pixels out. If you do that, the quality (especially with small text and graphics) will be terrible. – Nate Koppenhaver Sep 23 '12 at 2:00
The screen only has a resolution of 1920x1080 -- that's all the pixels it has. You can't make it display more pixels without buying a bigger screen. – David Schwartz Sep 23 '12 at 2:26
up vote 2 down vote accepted

LCD monitors have a fixed number of little rectangles called pixels (usually squares) that light up with different colours in order to make pictures. Each rectangle is made up of a red, a green and a blue smaller rectangle. You can see them all with a magnifying glass.

The resolution of the screen is the number of these multi-coloured rectangles in horizontal and vertical direction. That is why an LCD monitor only has one resolution it works with. From what you say, this appears to be 1920 by 1280 for your monitors.

Likely, the monitors have embedded electronics to rescale a number of different sized images to the correct resolution on the fly for you, which gives the appearance of working with a few other resolutions. This works very well for integer scaling factors. Some can tolerate it when the picture is scaled up, which means new pixels are introduced between the ones that are there on purpose. It works really badly when the picture is too large to display directly, especially if text is to be displayed. In part, this is because parts of letters disappear, lines next to each other are smudged together without space between, slim lines can just disappear. You really don't want that, especially not for extended time.

If the problem is the screen elements being larger on one screen than on the other, I suggest you solve the problem by placing the larger screen further away. This also lets your eyes focus on different distances, which some prefer for long sittings.

If you just want screen items to have a different physical size than they currently have, you could look into changing the display scale (often referred to as DPI, which is the unit). 96 dpi is the traditional value chosen, and 120 dpi is commonly available and will make screen items appear larger. I'm uncertain if different DPI settings between monitors is possible, though.

Adding custom resolutions under Xorg is done with the xrandr command. You might need to generate modelines with cvt.

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thanks for the clarifying answer. i already knew about LCD pixels but I don't know why, I didn't figured out that this wouldn't be a nice idea. haha the problem that I'm "facing" is that it's quite boring to see the same number of elements fitting in my 13" and the 22" screens. I'll try to place the screen further, besides the size factor there's that important ergonomic fact that you pointed out. finally, about the DPI: my laptop screen is using 138 dpi, and the external screen is on 96. So, yes, it's possible to have different DPI for different screens (: – igorsantos07 Sep 23 '12 at 5:41

"FullHD" is a consumer TV term, and is not a restriction on VGA, which is a computer specification. The typical GPU in a PC might be capable of resolutions such as 2560x1600. For example this GPU is only US$30 The typical problem has been finding a computer monitor that can display such a high resolution and doesn't cost 100 times the GPU!

However since the GPU in a laptop is connected to its display, the laptop GPU might be designed to offer no higher resolutions than 1920x1080. And your "FullHD VGA screen" could also be limited to 1920x1080. You will have to look up both specifications, since you didn't list the manufacturers & models. Until 4K displays become widely available, resolutions higher than 1920x1080 will tend to be expensive and/or scarce in both consumer TVs and computer monitors, but there is a limited selection of 2560x1600-capable LCD monitors.

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