For me it’s six to seven steps above complete dark, on MacBook Air M1.
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Keep in mind that not all monitors have the same brightness - even the same model. I’ve had three LG5Ks (as I wanted to check the backlight for evenness) and one of them was noticeably brighter than the other two (the one I kept! 
)
For me I am usually between 50% and 75% or up to 100% for some video sources or if it’s very bright outdoors… but it should depend on the brightness of your room and what’s comfortable for you.
If you’re finding things uncomfortable, or too contrasty, it could be a sign of underlying health issues (or even something like borderline diabetes):
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On my machine, there are 16 brightness levels, so level 7 is equal to 43.75%. This machine’s screen has 400 nits brightness. The Apple website says that they tested this machine’s battery “with display brightness set to 8 clicks from bottom”. It means it’s a brightness level considered optimal for most of the users.
My screen’s brightness is often below 8 clicks (50%), because I often work in low ambient light, or maybe this habit results from our old misconception that using computer screens at lower brightness levels is good for eyes.
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Thank you for the link!
I’m taking this test right now. Adjusting display gamma isn’t easy. 
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One thing more. During the test I had to keep the screen brightness at 50%. For the first few minutes it felt a bit too bright, but now I feel like this brightness level is also comfortable for me.
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Well on this monitor at work my phone light sensor says it’s putting out 155.46lux, so sure? I like monitors dim but with good color accuracy.
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I installed a free app on my Android phone called Light Meter. It reported 7 lx at 43.75% brightness (7 clicks from bottom) and 10 lx at 50% brightness (8 clicks from bottom).
I kept the phone at my eyes distance from the screen.
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Ah I did my reading ‘at’ the screen as that’s what the manufacturer will report as well.
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By at the screen, did you mean that you almost touched the light sensor to the screen?
At 50% brightness, from one to two cm distance the reading was around 62 lx, which decreased to around 58 lx when I touched the screen.
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About 1cm away but basically yeah. I have no confidence in the accuracy of a phone lux sensor as it’s to adjust screen brightness and not for accurate measuring though, lol.
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50% on mac, full-on Thinkpad.
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I really need mine to be very low, or else, I get headaches. 
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Please take the free Visual Contrast Sensitivity Test @AstonJ suggested to me, to see if it’s because of some underlying condition or it’s just a habit.
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Thank you. Will look into this.
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Depends on the environment. On my MBAir, if I’m in the living room, then it’s usually at 8 or 9 out of 16, if the lights or on or it’s day and I have the blinds open. Once in a while I can’t sleep, come downstairs in the middle of the night, and do something on the MBAir. with the lights off, screen at 1 out of 16, and keyboard backlight on (it’s usually off). If I’m on the back porch during the day, though, it’s gotta be 16/16.
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