John Gruber, who runs the popular website Daring Fireball, recently expressed the following thoughts with respect to Apple's (NASDAQ:AAPL) ProMotion technology (Apple's marketing term for high refresh rate displays) and this year's iPhone models (emphasis added):
Last year when True Tone was introduced with the 9.7-inch iPad Pro, Phil Schiller said something to the effect of "Once you get used to True Tone, you can't go back." I optimistically took that as a sign that the iPhone 7 would have True Tone. It did not, and the reason is probably that True Tone requires additional hardware sensors on the front face to pick up the ambient light temperature, and the iPhone has less room for additional sensors. But with ProMotion, I'm really hopeful that it'll make its way into this year's new iPhones. It doesn't require additional sensors -- only a super-fast GPU (which the iPhone will have) and intricate software support in iOS (which work Apple has already done for the iPad Pro).
While it is true that moving from displays that can refresh their contents at a rate of 120 Hz won't require the use of additional sensors, it's important to understand that making this transition on the iPhone -- should it happen -- would be far from trivial.
Here are some of the technical and economic challenges that Apple would face in trying to pull this off.
Building fast iPhone displays
It's not easy to build displays with high refresh rates and I suspect that Apple saw a noticeable increase in display costs in going from the previous 9.7-inch and 12.9-inch iPad Pro models to the displays on the new 10.5-inch and 12.9-inch models.
Furthermore, it's worth noting that while Apple and its display partners seem to be able to supply these high refresh rate displays for the iPad Pro models in reasonable quantities, Apple iPad Pro unit shipments are substantially lower than iPhone unit shipments.
If Apple and its partners face yield issues in building the high refresh rate displays for the new iPad Pro models, then the resulting iPad Pro shortage would be a bummer, but it wouldn't dramatically impact Apple's business results.
If Apple chooses to go with a technology that proves too difficult to build for the iPhone and that decision ultimately leads to severe iPhone shortages, that could and likely would seriously impact Apple's business.
It's not just manufacturing difficulty, either. I suspect that Apple and its display partner(s) had to invest substantially in research and development to build these thin, power-efficient, and fast displays suitable for tablet use.
Building such displays for smartphones would present additional challenges. Displays that can refresh their contents more quickly likely consume more power, and so Apple would have to figure out how to build small, high-resolution, fast displays that won't have too negative an impact on battery life.
Apple implements a variable refresh rate technology in the iPad that can slow down the refresh rate when the display is displaying "slower" content. Such a feature would likely also find its way into the iPhone if the iPhone were to get a high-refresh rate display, but this would once again add cost and engineering complexity.
Speaking of power consumption, there's another point worth making: If Apple implements a 120 Hz display on the iPhone and includes a graphics processor powerful enough to drive such a refresh rate, then I'd expect that in applications such as games, the processor (and other subsystems) would be stressed much more, leading to increased power consumption and decreased battery life.
Why? If a display is limited to 60 Hz, then that means the hardware inside of the device need only work hard enough to render images at 60 frames per second. If the display can update itself twice as many times per second, then if the hardware will likely work harder to try to draw that many frames per second.
Makes sense for next year's premium iPhone
Given the above, I think a high-refresh rate display on next year's version of the upcoming premium OLED iPhone would be the right way to go.
The technology is likely to be both too risky and expensive to implement across Apple's entire iPhone lineup in the near future, but as a "killer" feature on a premium iPhone that Apple would presumably charge quite a lot for, it could make perfect sense -- if Apple can keep battery usage in check.