A 1334 x 750 display on the 4.7-inch iPhone 6 would results in 326 PPI (pixels per inch) -- the same amount as current iPhones with retina displays. This finding corroborates Daring Fireball's John Gruber assessment that the 4.7-inch model would pack a 1334 × 750 display while the 5.5-inch model would feature a 2208 × 1242 resolution display with 461 PPI.
MacRumors did an analysis of the the Connect to iTunes screen and found that a 1334 x 750 resolution display was likely.
The method relies on the fact that the "Connect to iTunes" image does not completely fill the display on the iPhone 6, with the Lightning cable ending above the bottom edge of the screen whereas on current iPhones it extends all the way to the edge. Assuming this "letterboxing" is due to the image not being optimized for the larger iPhone 6 display, it would correspond to the image filling an area equivalent to a 4-inch screen centered on the device's 4.7-inch display.
By our calculations, the border areas not covered by the image together suggest that the iPhone 6 display carries approximately 17.5% more points in the vertical dimension than a current 4-inch display. This would move the current 568-point height of the iPhone 6 (1136 pixels at 2x Retina) to 667 points (1334 pixels assuming 2x Retina) on the iPhone 6.
Assuming the aspect ratio of the screen remains the same as in the iPhone 5s, which by all indications it does, this would mean a 667 x 375 point (1334 x 750 pixels Retina) display for the iPhone 6.
A lot of assumptions are made before reaching this theory, however more and more evidence is pointing towards a 1334 x 750 display for the 4.7-inch iPhone 6. Many iClarified readers also pointed towards the gear icon that flashes just before the Connect to iTunes display. It is likely that this unit is running internal diagnostic tools used to test the devices before iOS is flashed on to them.
All of this is speculation, and we won't for sure until Apple takes the stage September 9 to announce two iPhone 6 models and its highly anticipated wearable device.
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