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Please explain the following:

If I use my Dell monitor in landscape mode I am OK. If I use the monitor for a few hours in portrait mode, my eyes will have something like scanlines for a few hours.

My eyes are pretty much OK and I don't wear glasses. This is not a medical question but rather:

How this is possible, technically speaking? Does a monitor got a different horizontal refresh rate and a different vertical refresh rate? Also the monitor I use doesn't use PWM.

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I haven't the slightest idea what you are talking about. –  Robert Harvey Jan 7 '14 at 21:38
What to explain? –  Luka Jan 7 '14 at 21:38
The monitor in horizontal position doesn't hurt my eyes. The monitor in vertical position does. Why and how? –  Luka Jan 7 '14 at 21:39
Your answer is here: programmers.stackexchange.com/a/103599 The viewing angle of a monitor is much shallower in its vertical dimension, and this causes your eyes to see two slightly different images when you rotate the monitor vertically. –  Robert Harvey Jan 7 '14 at 21:40
I see thanks for the answer. I use a Dell U2713H (pretty expensive) and I am amazed by the fact it produces this issue. –  Luka Jan 7 '14 at 21:44

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