I linked to John Carey’s excellent iOS wallpapers the other day, but Carey just posted details on how he came up with an ideal size for the parallax effect to work. Noting:
I tested this by cropping images to Apples default wallpaper resolution then drawing a red box on top the pixel size of the iPhones screen resolution. I then loaded the resulting image into my iPhone and checked to see where the edges of the red box fell on screen. After trying a number of different cropped resolutions I discovered if I made any changes to the resolution set by apple the box would no longer line up ideally on the screen at a neutral position.
No wonder they look amazing — they weren’t just cropped down, they are considered for each device. Something that is evident when you compare the iPad and iPhone versions of the same wallpaper that Carey sells. If you just set the image and don’t fiddle with it at all the end result is a fantastic looking image. I cannot imagine how big of a pain in the ass all this testing and tinkering was.