New answers tagged lens-design
In the case you have things lying around, you can go this route: http://petapixel.com/2013/12/10/diy-adapter-allows-gopro-cameras-use-interchangeable-lenses/ However, I agree, that it might come in more handy (and most likely cheaper and with better image quality) to use a smartphone or some other camera.
The "mm" value of a lens is the measured conical point (in millimeters) from the back plane of the camera where light is focused. This relates into field of view, as the larger the number, the further away the conical point, the narrower the "field of view" that will be captured in the cone. There is no reason why someone can't come out with a 29mm prime ...
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