This autofocus circular fisheye lens has an angle of view of 180 degrees in all directions. It is exclusively designed for digital SLR cameras with the APS-C size image sensor. When used in conjunction with digital SLRs with APS-C size image sensors, it produces circular image within the frame.
The F2.8 large aperture value produces a bright image in the viewfinder and allows photographers to take pictures in low light conditions, making it an ideal lens for Astrophotography and Aurora photography. Due to the quantifiable angle/area relationship it produces, this lens can be used for scientific and arts applications such as solid angle measurements of cloud distribution over the sky or vegetation distribution of the forest. Read the whole article here