The iPhone and iPod touch look very similar to each other, and the iPad looks like a large version of the other two. They all run the same operating system and run mostly the same apps, so what exactly are the differences?
Well, the iPhone is the only one of the three which can be used as a phone. It also comes with both Wi-Fi and 3G connectivity, while the iPad is available as a Wi-Fi only version and a Wi-Fi and 4G version. The iPod touch has only Wi-Fi.
The iPhone is also the only one which has a built-in GPS receiver, although it is possible to buy an external GPS receiver for all three. A GPS receiver is needed for satellite navigation.
One more way in which the iPhone 4S differs is that is has Siri, the "voice activated intelligent assistant" which features heavily in the adverts.
Other differences include:
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The screen size on the iPad is 25cm, with the other two having 9cm screens. The overall size and weight of the units is consequently different.
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While all three have 32GB and 64GB storage options, the iPod touch has an 8GB option while the others have 16GB versions.
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Battery life is notoriously difficult to be exact about, because it can vary so much depending on what you do. The Apple store says that with the iPad 2, you have up to 10 hours of watching video or listening to music; with the iPhone 4S there's 10 hours of video playback, 40 of audio playback or 8 hours of talk time on 3G; and the iPod touch gives you up to 7 hours of video playback and 40 or music playback.
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The iPad 2 starts from £399, the iPod touch from £169 and the iPhone 4S from £499, although it's also possible to get the iPhone on contract from a number of companies.
One interesting development within this space is the take-up of the iPhone or iPod touch by blind people. These aren't considered tablets in the mainstream because of the small screen, but the operating software and screen reader are the same in these as in the iPad, and if you're not bothered about the screen, then they make very good - and very portable - replacements. With the addition of a small Bluetooth keyboard - or even, if you have the money and read braille, a 20-cell braille display with input keys - you've got a machine that you can do all your email and web work on and it all fits in your handbag or coat pocket!
For low vision users of any of these devices who require the built-in magnification, using the on-screen keyboard is problematic. It is possible to use a Bluetooth keyboard with it, and an iPad dock is also available.