I am looking for an app that can take continuous photos. In other words, I should be able to set the interval (e.g. every 3 seconds) and for how long (e.g. 2 minutes) and it would just snap photos.
Does something like that exists?
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Try Triggertrap, it has time lapse and other options to trigger a photo. It works with the iPhone's camera or, if you purchase the appropriate cable, you can hook it up to a dslr. The free version is limited but I think provides the functionality you're looking for.
I've used iTimeLapse a couple times and it seems pretty good.
If you can update to iOS 8, then you can have this Time Lapse feature for free from Apple.