Fisheye

Today I have been helping (possibly hindering) our steelworkers and when not doing this, clearing out more of my things. Some years ago I made my own fisheye lens to fit on my old Nikon F3 and today I rediscovered it (I have been looking for it for years). Once I found it I could not resist using it, so here is a picture of the sky, including the rear of our house, at one edge.

Fisheye lenses were originally designed for meteorological work, measuring luminance levels and cloud cover, to do this they must cosine correct the image, so the lens distorts according to a very specific formula, the length at the image is directly proportional to the cosine of the off axis angle (i.e. Y = cos X). My homemade lens does not do this perfectly, but it is very close, made up from a cheap wide-angle lens, together with the front elements from a cheap wide-angle convertor, which produced extreme barrel distortion and was unusable for the purpose for which it was sold. The combination of these, with appropriate spacing between the elements, produced very good results as can be seen here; the coverage angle should be 180 degrees, but is actually slightly less than this at about 173 degrees.