LensTutorial is a free iOS app that demonstrates in a clever interactive way how your camera and lenses work together to create the precise photo you are looking for. While not designed for the iPhone's camera, where most settings are automatic and there is no optical zoom, there seems to a lot of interest in apps for photographers who want to move beyond the iPhone camera, as good as it is.
The app lets you select your camera by brand and model, and then shows you the effect of aperture, zoom and distance. The on-screen display is designed to emulate the markings on a lens, so it is easy to transfer what you learn in the app to your actual camera. Seasoned photographers know these things, but it is a mystery to many novices so the app really has some merit. You'll see depth of field, field of view and the effects of zooming. More than 3,400 cameras are supported, and the app supports a standard range of 18-135 mm ƒ/2.8 to ƒ/16 lenses.
Although the app is free, there are three in-app purchases at US$0.99 each. These purchases provide more versatility in aperture, distance and zoom lens specifications. For example, the aperture upgrade covers lenses from ƒ/0.75 to ƒ/64. I think someone interested in learning about camera / lens interactions will want the add-ins, and I'd like it better if the app was sold for $3.00 and included those in-app purchases.
Still, you can learn a lot -- even with the free version. LensTutorial requires iOS 5 or greater and is optimized for the iPhone 5 / 5s / 5c. The app is universal so it runs at full resolution on the iPad and iPhone.