I've been using a Dell P2317H 23" with a LG IPS panel 1080p 60hz 250cd/m² for 6 years with 60% brighness without any issue in my office PC with Win11 21H2. The same way I've been using my Samsung 49KS7000 TV with VA panel, the old Samsung LCD monitor in other PC, smartphones with LCD and OLED, tablet, etc. All devices that with screens that I had during my hole life never gave me strain, fatigue, headache and any other issue. I go to the ophthalmologist between 6 to 12 months and my prescription to myopia and astigmatism is the same since 2013. I can read anything well with glass and without them at close distance.
So, I bought the famous Dell S2721DGF 27" IPS QHD 165Hz 400cd/m² G-Sync/Freesync Premium Pro to make that great upgrade in everything possible but as soon I turn it on it makes my eyes burn. Like many of people here I had strain, fatigue, headache, difficult to focus. I felt very strange because it never happed with any screens in my life and I calibrated in different ways, started reducing the brightness until I get only 15%. I used it with 165, 120, 60Hz, with and without Freesync. Tried everthing I could.
I read a lot trying to understand what was happening and discovered that it uses a NanoIPS LG panel that uses red phosphorus to amplify the color gamut and the saturation goes crazy. Many people don't feel anything but some are very sensitive. This particular monitor cannot emulate sRGB because of Freesync specification nad I did it partially with AMD Adrenalin that softens it but it was still unusable. The different resolution, the wild brightness, the possible flickering (it's flicker free but Rtings discovered that it fickers with 20000Hz frequency) and other stuffs can be the reason. It's complicated and it was sad but I returned it.
So, I decided to buy a Dell S2421HN because I thought that could be somenthing like my P2317H. It is IPS, same 1080p resolution, only 0.8" wider, 250cd/m², but it has 75Hz with Freesync. It was cheap and gave it a try but I had problems too. I put it side by side with P2317H and calibrated to be similar, adjusted the brightness to be the same and it bothered me. With my old one I could read, scroll and read and I feel nothing. With the new one I could read but I feel my eyes lost like with them trying to focus on something. When I scrolled and tried to read something my eyes can't focus immediately and I had to wait something about 1 second to refocus and read. Very weird and with my one one is instantaneous. I was very disappointed and returned it.
Last try. I bought a P2423D the is a QHD 24" 300cd/m² with 60Hz, no Freesync or any other stuff. I thought, the other two was S series and 21 models, let's try a P series that I already use and model 2023. Maybe the very high ppi would be nice. Well, it was less problematic that the other two but still it's very different when compared to the old one about focus and even sharpness. Using it was not so terrible like the other two, but the rest of day the eyes hurt. Of course, tried different calibrations, put them side by side, matched the brighness even reducing more to see what I feel.
I tried to do something more and plugged the monitor in my Lenovo Ryzen 5500U with Vega graphics with Win11 21H2 laptop but it didn't change much thing. Before I really consider return it I will plug it in my old core i5 computer with Win10 and Intel graphics and my other computer that uses a NVidia card and Win11 21H2 to see if I feel something different but I really don't have faith that will make any difference. All this new monitors says that are flicker free, low blue light emission, TUV certification, strain reduction and all this stuffs, but what I can say it that crap LCD from the past never caused this king of stuff without any kind of certification. There are a lot of them aroud us.
I researched a lot to find anything and found more things about S2721DGF because it's very popular and has and agressive color gamut that really bothers some people. The other two don't have strain complains and I couldn't find who manufactures the displays. I found more general complains about what can cause strain and I ended up here. I saw a interesting post about polarization and tested it. My P2317H blocks horizontal light and the P2423D vertical but old Samsung monitor blocks diagonal, my two Samsung TV vertical, my S20FE with OLED diagonal, my other LCD phone vertical. So, it doesn't seem that polarization plays a role in this problem.
Guys, I'm really lost. Now I have this monitor working, and I can buy a new one and return it and continue to use it. But as soon as it doesn't work anymore I will be screwed because I don't have a minimum idea about what I can use and I will won't be able to return as I can do now. One option would be go to stores and phisically see every monitor but here in Brazil is difficult. I went to shopping mall, different stores and they don't have this kind of stuff. Specialized computer stores operates online.
Sorry to build a wall of text and maybe repeat a lot of thing that people already said. I have until next friday to return it to Dell and I have to thing about it and also try to figure out what I can do. Thank you fo this great forum.