A inexpensive UWA lens?

What about free, $0. Take whatever you already have and make a panorama of it then you can stitch the images together with either Photoshop if you already have it or Hugin which is free http://hugin.sourceforge.net/

110° angle with a 17-55mm f/2.8 lens on a Canon 7D

This photo is a stitch of 3 photos to get the angle wide enough. I would have needed an 8mm or 6mm (I don't even know if a 6mm exists) lens (on crop), then trim the floor and the ceiling. This is a 35Mpx file, and UWA would only result in a 10 to 12Mpx after the cropping.

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Thanks
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