I've been talking to some people on Twitter about the technical feasibility of Apple one day introducing a smaller and lighter iPad. The general opinion I'm seeing is that it would require a change to the dimension of the screen in points, or the touch targets would be so small that it'd be impossible to tap them accurately. I'm going to lay out my argument for why I think it would be possible to shrink the screen size, maintain the same number of points, and maintain a perfectly usable UI.
We've heard a lot of rumours about a possible 7-inch tablet, but most don't go beyond speculation or loosely-sourced reports. iOS programmer Joel Bernstein, though, makes a compelling point about why a 7-inch iPad makes sense from a UI point of view.
In this post I'll be talking exclusively about "logical pixels", also known as "points", and which are equivalent to one non-retina pixel or four retina pixels in a 2x2 grid. I don't know if a smaller, cheaper iPad would be able to take advantage of pixel doubling, but it's really irrelevant to this particular discussion.
The iPad has always had a logical resolution of 768x1024, a 9.7″ (diagonal) screen, and a logical DPI (points per linear inch) of 132.
The iPhone and iPod touch have always had a logical resolution of 320x480, a 3.5″ (diagonal) screen, and a logical DPI of 163.
The iOS Human Interface Guidelines say "The screen size of iOS-based devices might vary, but the average size of a fingertip does not. Regardless of the device your app runs on, following these guidelines ensures that people can comfortably use your app. Give tappable elements in your application a target area of about 44 x 44 points."
44 points is 0.33″ on the iPad, and 0.27″ on the iPhone. By pretending the difference in DPI doesn't exist, and giving the same target size to both devices, they have essentially told developers to make tap targets 23% larger on the iPad than the iPhone. This has never really been an issue, as the iPad is a lot less cramped for screen real estate, and nobody minds a slightly-easier-to-tap button.
If the iPad's screen were scaled to 163 DPI, it would be 7.85″ diagonally.
This is suspiciously close to the 7″ iPad rumours circulating.
A 44 point target on a 7.85″ iPad would be the same size as a 44 point target on the iPhone (0.27″). Millions of people use the iPhone every day, and have little trouble tapping a 0.27″ target. As Apple has pointed out, their fingers do not change size when they move to their iPad.
So, through brilliant planning or sheer luck, Apple is in a rare position to modify the DPI of a touchscreen device without significantly harming usability. There are a few types of apps that would be negatively affected, and that would require some minor tweaks if the DPI changed:
Apps that need UI elements to be a specific physical size. Maybe your app displays a ruler on screen, and your ruler bitmaps have 132 pixels per inch. You would need to draw separate versions for the smaller screen.
Apps that have UI elements smaller than 44 points. These apps will go from being hard to use, to being slightly harder to use. Most developers adhere to Apple's guidelines, so I'm not terribly sympathetic to this issue, and I don't think Apple would be either.
Apps that require extreme precision. Since the touch sensor will presumably be the same as the one on the current iPad, it will have the same measurement error, except now this will be spread over more pixels. We're talking about a small error to begin with, though, so I doubt this will be a huge issue.
I want to be clear that I am far from certain that we'll ever see a device like this. Apple may decide not to target the low-end tablet market. They may not be able to get costs down to a point where they could sell it for an attractive price and still make a reasonable profit. All I'm saying is that it's entirely feasible from a UI perspective.