This happened to me as well however mine occurred when I received my Samsung Galaxy S6 and has continued with my iPhone 6s+. I'm 39 and have never had vision issues; no glasses... never. Then I received my preordered Galaxy S6 in the mail. I opened it and turned it on. After about 5 minutes I felt a tension in my eyes and then I lost the ability to focus on the screen. I just couldn't make it happen.
I spent the next few days scouring the internet trying to see what the hell was going on, most of the posts are like these... **** talking and bad advice. So, since I didn't know any better I figured it was due to the new type of screen and my eyes would adjust.
Nope. I kept the damn thing for about a year because I couldn't budget getting rid of it. The hope was that once it was gone, and I didn't have to look at it - ever - my vision would recover. Nope. It's not just an issue with the device screen, I now have problems focusing at near and very near focal distances, no matter what medium... Screens are the worst though.
The culprit is quite simple; pwm. For many people the brightness of these new screens is becoming a bit much, most people can simply correct the issue by dimming, or using a filter like f.lux or night shade. For some people though, this doesn't do ****. The reason is Pulse Width Modulation. Turns out that some people's eyes can detect the flicker of the pwm display which makes the ciliary muscle go ape **** trying to focus with each flicker. True story. Many led displays operate via pwm. What that means is that when a screen is at maximum brightness, the individual pixels are transmitting *almost* continuous light. When you dim the screen, it doesn't simply reduce the voltage and thus render a dimmer image; what actually happens is that the off cycle is increased relative to the on cycle. ie. the valley's become wider in between the peak/plateau.
This is pwm. And pwm is a right nasty git... It trashed my vision, probably permanently. Now, if everything in the world is going LED, you're screwed right? Ish. PWM is a common technology on lower end screens, no matter how nice you think your device is, the reality is that the actual quality of that screen is much lower on these tiny devices than many larger, more expensive offerings such as TV's and large monitors. LED light bulbs are no different, some use pwm and some don't. Yay homework. After the Galaxy S6 I stepped into a iPhone 6S+, hoping my earlier comfort with the iPhone 4s screen would carry over.
If it did, I cannot say. I haven't regained the ability to focus without the aid of glasses so... No idea. Fact is, I can't look at a e-paper kindle or even real paper at close range anymore either; thanks Samsung. there are a great number of displays out there that do not employ pwm, or employ it at a frequency above the upper limit for human perception. While the information wasn't available when I made the switch, I now know the 6S+ employs pwm as well, but I haven't found a frequency to offer you, it does not kill my eyes as quickly as the Samsung did so maybe it's better?
I've included a link to a exceptionally technical review of the S6. I know this is a mac forum however it illustrates the pwm principles in play and offers digestible graphs. Point is, every hardware manufacturer out there knows explicitly whether they are producing pwm and at what frequency it will operate at. Please note, I'm not talking about the screen refresh rate, which for m iPhone is 60Hz I believe. I'm talking about the LED refresh rate... Two different things.
Like I was saying. It's a cost/benefit scenario for manufacturers. The percentage of the population who is sensitive enough to detect the flicker that a pwm screen produces is relatively small, even then it will impact people with varying degree's of severity; most people don't even know that's what's going on.
The big deal right now is blue light. Essilor has a blue light lens for digital users called Eyezen, others are coming out with their own flavors and nearly everyone makes blue filter coatings. Very few though are aware that pwm is even a thing. I imagine that in the coming years more and more people will become aware of it at the consumer level, and it will eventually become a big deal as the millennial digital power users begin to age. Until then, it is a ragged bitch trying to figure out which screens do and which don't - even within a brand. For example, in 2018 iPhone will likely be using a Samsung screen on the +/Pro model but not on the regular offering.
look around and do some reading. It's real, it's not some made up ****, and it won't get better by using f.lux or holding the screen farther away. It will get better - for some - by ditching the screen that is causing damage and stepping into one that doesn't.
Anyway, my two cents. Hopefully someone will come across this and maybe it will help. For those of you who don't like clicking links, I'll post the relevant text from the article under the link.
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What is pwm and why does it matter:
http://laptopmedia.com/highlights/w...-and-why-did-we-add-this-test-to-our-reviews/
Pulse Width modulation (PWM) is an easy way to control monitor brightness. When you lower the brightness, the light intensity of the backlight is not lowered, but instead turned off and on by the electronics with a frequency indistinguishable by the human eye. In these light impulses the light/no-light time ratio varies, while brightness remains unchanged, which is harmful to your eyes. If there is PWM, it is better to have a high frequency of around 20000 times per second instead of, say, 200. This is noticeable at different brightness reduction levels for different monitors. It is noticeable to a higher degree in LED than in CCFL backlights due to the former’s lower afterglow and higher nominal diode brightness.
Here are the waveforms of 4 displays. The first one doesn’t emit pulse-width modulation, which provides the option to work for extended periods of time. The second is a classic PWM. Its aggressive pulsations at a low frequency put extra pressure on the eyes and brain. The third also emits PWM, but it has an extremely high frequency, which is considered to be less harmful. The fourth one doesn’t flicker unless the brightness is <25%, so it is eye-friendly in practically every normal usage mode.
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Original Review:
http://laptopmedia.com/reviews/samsung-galaxy-s6-review-a-bunch-of-innovations-in-a-beautiful-body/
PWM
Although Samsung Galaxy S6’s display doesn’t have a backlight, the emitted light pulsates. In this particular case, there’s a difference between the S6 screen and the displays, which emit typical PWM-type flickering light. Those very same displays have a constant light intensity and the brightness change affects only the fill correlation of the impulses (light phases / dark phases), which changes as a fluent of the brightness slider position.
In Galaxy S6, when you move the brightness slider to 50% for example, the pixels are less bright (around 80%), and the dark phase increases to nearly one-third of the period. This is the preferred scenario, because that way the negative pulsation affect is reduced. However, the low frequency of 240 Hz is a disadvantage.
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