Severe eye strain, I know it's fixable with one button, but don't know which
I have an iMac Pro too. I can no longer use it 
I use to be able to use it 24/7 hooked up a very particular way.
This was the way:
- Using the first release of Big Sur
- Hooked up to an old 27inch Apple cinema display with a mini displayport to usbc adapter (Could not use the imac screen). In mirror mode. I lowered the brightness on iMac and put thick cardboard over the imac screen.
- Turn off mac updates.
- Disabled font smoothing - https://forums.macrumors.com/threads/any-way-to-disable-font-smoothing-in-big-sur.2267601/
This would work for a little bit after a fresh install of of Big Sur. I did however notice after I installed and opened some apps it would cause eye issues across the whole OS and I would have to wipe the whole mac and do the steps above again to get it right. I made mental notes of which apps did this and not to use them.
Apps I found broke the mac for me: After effects, Adobe media encoder, VS code.
I can only assume the way I had it setup broke some form of the graphic acceleration / dithering. The Mac still run great and fast. Perhaps those apps enabled the graphic acceleration or something.
What I do know is it only worked with that copy of Big Sur. There must have been a bug that let it happen and it disabled dithering. Unfortunately I let the machine update and I think it has done a firmware upgrade. I can no longer get it to working state. Firmware update can not be undone. So my advice is don't update the OS if you had it working before.
So here's something interesting. I haven't made any progress with the iMac Pro and I've switched to working on my 2019 15" MacBrook Pro, which never before gave me any eye strain issues. It is now. They're mild, nothing like the iMac, but definitely something new that I never experienced on this machine before. There haven't been any updates to the OS or software on that MBP. I saw my eye doctor just a few weeks ago for my twice a year checkup and I'm totally good according to him. He's at an Ophthalmic Center filled with nothing but eye doctors and technicians so you would think they know eyes. They don't pretend to be computer display experts.
But here is the most interesting part. I hooked up the MacBook Pro to two non-retina external monitors: An HP M27ha and an HP E273. These are basic, semi-low end monitors. And I shut the MacBrook Pro lid so I'm just using those two monitors. And I still have the eye strain. Again, on the MacBrook Pro it's not anywhere near the eye strain intensity of the iMac Pro, but I do feel it immediately. I've adjusted the brightness and contrast on both of the monitors pretty low (not hard to see low….but low enough that bright white backgrounds shouldn't be causing any issue. And yet, it's there. I even tried remoting into the iMac Pro from the MBP using a third-party conferencing software and leaving the display quality low (non-retina) and sleeping the iMac display itself. Still hurts my eyes.
The fact that i'm not actually looking at the MacBrook Pro or the iMac Pro retina displays, I'm looking at only HD quality pixels on those low end external monitors, BUT generated by the MacBrook Pro or iMac, should tell me something or eliminate certain potential causes. But I'm not an Apple or computer engineer that can determine precisely what.
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brvideo He's at an Ophthalmic Center filled with nothing but eye doctors and technicians so you would think they know eyes
Ophthalmologists tell us we have dry eyes or sign us up for surgery. I would not rule out any eye problem until you've seen a COVD optometrist or neuro-optometrists that specializes in binocular vision dysfunctions. If your eye doctor did not use a Maddox Rod then find another -- I actually purchased on myself for $30 USD:
brvideo And I shut the MacBrook Pro lid so I'm just using those two monitors. And I still have the eye strain.
Even with sRGB mode? It is my theory but I've not had the equipment to test it, that macOS/drivers will always use temporal dithering, even on an external panel, and Apple introduced this circa 2013. I'd purchase a capture card and a Mac to test but that's an expensive effort. I could be proven wrong 
. The fact Amulet Hotkey had to release a ahkinject kext to disable temporal dithering might support this, but this was on AMD/ATI GPUs.
Are those monitors eye-strain free on Windows or Linux?
brvideo I also tried placing a black sheet over bad displays with the same results. The pain is lower but it still exists. I also have a skin sensitivity to some displays and it also hurts, not only my eyes. There is a kind of emission from the display that affects us and is not totally filtered by the LCD layer or by this black sheet.. As a result from a lot of tests I took, it seems related with the nits of the backlight. Higher is worst. And with PWM screens, when the backlight is on during the cycle, it is always at the maximum point of nits. I prefer warmer light (lower kelvins) than cooler (more kelvins).
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interesting. I know nothing about “nits”. Can they be adjusted and if so how? Is it just your brightness control? I never see a numeral value associated with brightness sliders on my machines software, just slash marks. On the monitor are we talking about the brightness control that goes 0 - 100?
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brvideo The first LED TVs came with both settings: backlight and brigthness levels. Backlights sets the amount of nits, which is the energy that emits the source of light that is behind the LCD layer. Instead, the brigthness and contrast settings are controls for blocking more or less of the light that comes from the backlight. After this years, a lot of displays came with PWM (Pulse Width Modulation) backlights, and only brightness settings, in which it controls the percentage of the time the backilight is on per every second. When it is on, it emits the maximum amount of nits the display is able to. You could see this real level by putting the brightness to 100%. This is the amount of light our eyes receive at a fraction of time, which is too much intense. In general, newer displays without PWM, comes with a fixed level of nits and you can only control the level of light by the brightness settings that acts on the LCD layer blocking the light from the BL.
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I'm in the process of borrowing a Window laptop so I can see if I have similar issues with it's screen and my monitors. I'm guessing not as I have had WIndows machines at the office for years with no problem, but we'll see.
In the meantime, I wanted to try and refocus on the specifics of my iMac Pro issue. While it's clear that I am one of the unfortunate who a sensitivity to some specific type of computer display phenomenon, I also know that I was able to use these machines with the same existing software and operating system without any issue at all for years until a week or so ago. My goal is to revert the machines back to that state rather than go down a rabbit hole of eye diagnostics. I've worked in broadcasting and computer training my entire career and have only experienced these eye strain issues with these two machines, which until about a week ago, I had no issues with.
To review, I originally had a severe eye strain issue with my 2017 iMac Pro. Using some technique that I can't remember (possibly SwitchResX, terminal commands, memory resets), I succeeded in eliminating the issue entirely. But when I recently opened the System Preferences on the iMac Pro, went to the Display tab, held the OPTION key and clicked the Scaled radio button to reveal the current screen resolution setting, I instantly noticed three things: #1 the colors got more vibrant #2 everything got slightly brighter #3 the text and icons got crisper and more defined. The size of the text and icons did not change. It was at this point that the eye strain instantly returned. Also, when I hook up external non-Retina monitors to the machine I still have the eye strain. I'm trying to figure out what these specific clues tell me about where the problem probably lies and how to get it back to the way it was. What adjustments can I make that will make the colors less vibrant and the text less crisp? So far, flipping through the various color profiles hasn't done it and all attempts to turn off dithering/font smoothing using either terminal commands or third party software haven't seemed to change anything. With respect to dithering/font smoothing, It's literally like they aren't doing anything at all…I see no difference visually, let alone eliminating the eye strain issue. You'd think that even if it didn't resolve the eye strain issue, I should see a visual difference when attempting to turn off smoothing/dithering. I've also used SwitchResX to change from Billions to Millions of colors and to choose only non-HiDPI resolutions.
Logically, when I held OPTION and hit the SCALED radio button, the purpose of that action is to show you all the possible resolutions that the computer can offer. When you don't hold OPTION and just hit SCALED, it just shows you a few options. So when I executed that keyboard move, i'm guessing that it changed whatever manual overides I made to fix the problem, and forced the screen back to only those resolution configurations that are pre-established in the Mac OSX, mostly likely to the one that most closely mirrored the pre-existing non-eye strain configuration I had, since the size of the text and icons did not change, just the crispness and colors, both of which got more intense.
One more clue. When I Screen Share my iMac Pro to my Plasma HDTV via Apple TV via HDMI, I still feel the eye strain on the HDTV showing the iMac Pro Desktop. Everything is slightly less crisp since it's a TV and not a computer monitor, and the colors may also be a little less vibrant, but the problem persists.
So any ideas on what to try to put it but it back the way it was?
Thanks.
brvideo I have a similar issue where I get an instant headache when looking a apple screens, and get terrible headaches in the center of my forehead. I downloaded SwitchResX and changed the resolution to 1536x960, 59.94 Hz with HiDPl disabled. I also changed the color setting from "billions of colors" to "millions of colors". These changes seem to help quite a bit.
I have been wanting to explore this matter further, as it seemed to deal with the issues of overbearing whites and harsh contrast on macOS which seem to affect others here as well. It looks like @brvideo has not been around in some time, but I would love to analyze his SwitchResX preferences files and the terminal history in ~/.zsh_history (from the time he got it working) to see if they would give us any leads.
Speculating about what the Displays preference pane would theoretically touch (when the inadvertent reset occurred), the following items came to mind: resolution, refresh rate, arrangement, color depth, color profile, and possibly HDR-related settings. Notably, the setting appeared to have a visual effect early in the boot process, suggesting that the setting was persistent in NVRAM. Noting a few interesting facts about the good settings in the story, 1) they appeared to require a reboot to go into effect, and 2) they were persistent through updates (until the interaction with the display settings), although it was not clear whether these were minor updates or if they included major macOS version upgrades.
Do any further ideas come to mind? To me, this sounds like a color profile or backlight setting issue (rather than something like dithering), or (less likely) related to the refresh rate. I have noticed that on several of my Macs, during the boot process the Apple logo will shift brightness a few times (some of which are more comfortable than others), so I wonder if he had previously managed to disable some part of the display initialization.
thorpee This would work for a little bit after a fresh install of of Big Sur. I did however notice after I installed and opened some apps it would cause eye issues across the whole OS and I would have to wipe the whole mac and do the steps above again to get it right. I made mental notes of which apps did this and not to use them.
This is quite interesting, counter-intuitive as it sounds. My problems began with a 2021 MacBook Pro 14-inch mini-LED XDR, but interestingly, when I got rid of it and used the Migration Assistant application to move my data back to my (previously safe) Intel Mac hardware, the strain seemed to follow albeit to a lesser extent. When I happened to boot another disk with a clean Monterey installation on that same computer, it looked "calmer" than my main Monterey environment, making me wonder if some strain-inducing setting(s) were migrated back with my data (although plausibly, my condition could just be deteriorating).
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macsforme You could use this command and start it before launching the display extensions to get a feel for what things it reads and writes. It's a lot. It even lists what plist values it uses. I actually think I saw some unknown font smoothing values there as well.
log stream --predicate 'composedMessage CONTAINS[c] "DisplaysExt"' --info --debug