There are dozens of variations on the Spot the Difference theme available to download from Android Market, but Appy Fun's Foto Fun HD - Spot Differences is the best example we've found. Better yet, it's free.
This app works with Android versions 2.3.7 and later, on tablets sporting a resolution of 1024x600 pixels and higher. It makes sense to restrict installation to larger-screen devices, since it would simply be too difficult to see what you're looking at on a smaller smartphone display.
Most Spot the Difference Android games we've tried use low-resolution, pixellated images that make it impossible to pick out any variations – or worse, lead you to point out those that don't exist. The clue here is in the title: HD. Appy Fun's selection of images are crystal clear and, given enough time, it's quite possible to complete the game without resorting to cheating.
For those of us who have never played Spot the Difference, the concept is simple. Two similar images are displayed side by side onscreen, with one image altered in five areas. You have a given amount of time to point out these variations, and you then move on to the next image. You can use up to three hints within a game, should you get stuck, and any incorrect taps take five seconds off the clock.
Foto Fun HD - Spot Differences offers three modes of gameplay, with 50 levels in each. We started with the Beach mode, expecting to see pictures of sandy shores, palm trees and kids playing Frisbee. In fact, this is simply the basic mode, and uses the same images as the Challenge mode and intriguingly named Explosive (there are no explosions). In Beach mode, you get 60 seconds to complete each level. The quicker you do so, the more points you score.
Our main gripe with Foto Fun HD - Spot Differences is that it has a limited number of images. After only a few games, you begin to learn exactly where to tap the screen, without needing to inspect the image. The game then becomes less about using your eyes, and more about giving your memory a workout. At the time of writing, there was a single image add-on pack available on Android Market (65p). This could easily make Foto Fun HD a one-time wonder, uninstalled within hours, but Appy Fun's Challenge and Explosive modes take advantage of this limitation.
Challenge mode is the same as Beach mode, except that it reduces the amount of time you have for each level by five seconds for every three you complete. You won't notice this at first, but it quickly becomes difficult as the timer runs down.
Explosive mode takes things a step further, with the amount of time you have to play dependent on how quickly you completed the previous level. Every five levels you complete, 10 seconds are added to the clock.
A Save function is convenient, giving you the opportunity to finish the game at a later time. It's accessible once every three levels. There's also a Pause function, but cheaters should note that the image is removed from the screen during the timeout. And we promise we didn't use our Samsung Galaxy Tab 10.1's screenshot function to help us learn each image…
Once you've completed all three modes, a global scoreboard encourages you to start over and score higher. We can only assume that Appy Fun lumps in together all the scores from users of the free game and those with the paid-for add-on pack installed, since those at the very top of the board somehow managed to complete far more than 50 levels.