iPhone 6 Final Speculation
There has been a lot of speculation about the specifics of the soon to be announced iPhone 6 devices. There's too much smoke for there only to be one size of new device. It seems certain new iPhones will be announced with 4.7" and 5.5" screens in a couple weeks.
There have been extensive part leaks surrounding the 4.7" device and a launch, in volume, shortly after the announcement is expected. The 5.5" device has seen significantly fewer leaks and a delayed launch or a severely supply constrained launch seems likely.
Commentators have speculated in particular on possible differentiation between the two devices. Will Apple somehow justify a higher price for the larger screen device with a more attractive feature set? I think the answer in all cases except screen resolution is fairly obvious by looking backwards.
Last year saw the launch of the iPad mini with Retina Display and the iPad Air. Aside from a very slight processor clock speed reduction and difference in display color gamut, the two devices are identical. The larger iPad Air sells for a $100 premium over the smaller iPad mini with Retina Display. I expect that 99% of all buyers know nothing of the differences between these two models and accept a $100 premium for the larger device. Apple surprised almost everyone when they made the iPad mini with Retina Display so capable. I'll be shocked if they don't do exactly the same thing with two new phones.
The second reason for thinking this is more ephemeral. It just seems unlikely, and maybe wrong, for Apple to jump from it's long held position that smaller screens (in comparison to popular Android devices) are better, to a new position where the larger screen phone will be more capable. I'm not saying it's impossible, and Apple has belittled ideas right up until embracing them in the past, but I'll be surprised.
The screen resolution of the new devices is more interesting. John Gruber did a splendid job in a recent post of outlining all kinds of design considerations. If you haven't read it, please do so now and then come back. The post, in particular his speculation about the resolution of the 5.5" device, made me think about one thing, pixel count. Specifically the power required to power those pixels.
Frustratingly, Apple has repeatedly chosen to reduce phone thickness rather than increase battery life. It's a choice I disagree strongly with, but I expect the trend to continue.
I decided to build a Numbers spreadsheet with all previous iOS devices listed. I included screen resolution and battery information from all previous models and the best leak information available on the upcoming iPhone 6 models. I used this information to calculate a power/pixel value for each device.
A couple things jump out when looking at this data when graphed. The first is there were tremendous efficiency improvements after the iPhone 3GS and the iPad 2. The second is that recent devices, iPhone 5C and iPhone 5S, have power/pixel values of 0.00208 mAh/pixel and 0.00215 mAh/pixel respectively. In fact, since the iPhone 4, this value has been around 0.002 mAh/pixel.
John's speculated 4.7" iPhone 6 has a 0.00181 mAh/pixel value. This represents a 19% improvement over current models. This is a nice jump, but not unreasonable given previous improvements.
John's speculated 5.5" iPhone 6 has a 0.00106 mAh/pixel value however. This would be a 102% improvement and seems impossible. There simply isn't enough power to drive the pixels in this display without tanking battery life. Another display resolution mentioned by Mark Gurman breaks down to a 0.00178 mAh/pixel value for an improvement of 20%.
In conclusion, I expect the 4.7" and 5.5" iPhone 6 models to have a consistent feature set and for the 5.5" model to not get a 3x display. There isn't enough power to drive the display. Apple will be happy to sell the larger phone at a $100 premium based purely on size, just like they did with the iPad Air and iPad mini with Retina Display last year.