Is it worth buying fish eye lenses?

I really love how pictures look with those lenses. I tried making my own with the door peephole thingy. It was such a fail… Lol. BTW, how much do these lenses cost? Thanks!

One more thing. Does the iPhone have a filter that does the fish eye? Because I've seen those pictures, or is it just an app/filter already built in the iPhone? Like, how do they do those pictures on the iPhone?

First we have to define what a "fisheye" lens is.

It is a lens that sees 180 degrees or more of the scene.

Some produce a full circle of image in the frame. Others show 180 degrees from corner to corner of the frame

Here is an example of a full circle fisheye shot.

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If you are using a cell phone like a iPhone, you need to look at some add-on lenses like this

http://www.iprolens.com/

Whether is it worth the cost to you, buying a fisheye, only you can determine that

It's kind a one trick pony. Cool results but you can't use it all the time. Believe me, there's an old gentleman who every week at the local photographers club I go to, shows image after image taken with his "fish eye" lens. If what you're after is artistic flare and you want to produce a different way of looking at things, it's a great tool, but just like any other tool, if used the wrong way or in the wrong situation, problems will arise.