iOS's accessibility features are great, particularly on the iPad--as I've written elsewhere--but for disabled users, accessibility is more than just a niche set of options in the Settings app.
But perhaps the greatest accessibility feature is the most obvious part of a smartphone: the screen. As a visually impaired user, my effective use of these devices depends on the quality and brightness of the screen. In order for me to achieve optimal use--especially on the iPhone, the device I use the most--I've found that I really need a Retina display set to maximum brightness.
My eyesight is such that I see "pixels" naturally, meaning that everything I see is fuzzy. While no Retina display will ever be able to completely eliminate that fuzziness, such displays can drastically reduce it.
I used both the original iPhone and original iPad with great success with their displays set to full brightness, but seeing my iPhone 4's Retina screen was a total game-changer (in some cases, literally). It was the combination of the big, bright LED-backlit display with the high pixel density that made all the difference. Suddenly, even the smallest text was readable, and I was able to spot details in images that were previously indistinguishable.
Though the Retina display was rightfully a revelation to the normal-sighted, it was orders of magnitude more important to me. The screen must compensate for my lack of vision. Apple's non-Retina displays were great, but Retina is even better. And the better the display, the better I can use my phone--and better enjoy myself when I do.
Bright lights, big screen
The quality of the Retina display isn't the only factor at play where accessibility is concerned: As I mentioned, it's a combination of Retina and maximum brightness that makes usability truly shine.
But every Retina iOS device that I've owned--the iPhone 4, iPhone 4S, and iPad 3--has shipped out of the box with the brightness slider set approximately halfway. That doesn't work for me; I cringe when I power it on and start the setup process. My vision isn't good enough to see a screen that "dim"--I absolutely need it on full blast. So that's the first step for me: Every time I get a new iPhone or iPad, I immediately go to Settings -> Brightness & Wallpaper and drag that slider as far right as it'll go. Then I breathe a happy sigh of relief, knowing that I can comfortably use my device.
Having my iPhone's display at maximum brightness doesn't come without its pitfalls, though. The big problem with constantly keeping the screen so bright is that it absolutely kills battery life. So I must be judicious in using my phone, especially early in the day: I make concerted efforts to use it as little as possible, even during idle moments like standing in line somewhere or sitting on the bus.
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