Meopta Meopix 55 mm iPhone 4/4S Adapter

It is $60 for what is essentially just a piece of plastic, but it does work well, and is much more compact, light weight, and easy to use than more elaborate digiscoping adapters. Professional photographers will not be impressed, but I've been pleasantly surprised with the quality of photos I've been able to get just using an iPhone.
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My iPhone 4 fits very snugly in the adapter - in fact almost too snugly as it's a little difficult to remove. If you have a bumper or case on your phone of course you'll have to remove it first.
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I'm using it with a Swarovski ATM 25-50w lens, and it fits very well. You might have to nudge it ever so slightly to get it perfectly centered over the lens, but after that it's fine. The phone and adapter together are light enough that you don't really need a tripod rail, although it will droop slightly if you don't have the head locked down.
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In all I'm quite pleased with this, I would purchase it again.