The reason why fisheye lenses were initially developed was to photograph the entire night sky within a single image (horizon to horizon in all directions). The fisheye constitutes one type of lens, the other type - rectilinear or normal lenses (with out the fisheye distortion) will vary in the amount of sky captured.
- Fisheye - Fisheye lenses originally were circular - which means that on the frame of the image, you will get a round circle, 180 degrees, horizon to horizon in all directions. This is having the image circle projected on to the sensor to fit on to the sensor or be smaller than the sensor. There is another type of fisheye - full frame, that enlarges the projected image on to the sensor such that you get the 180 degree coverage from corner to corner diagonally. This is much more common and preferred by most folks, since it fills the entire frame. Note, I said corner to corner diagonally, and not edge to edge. What you loose are the edges (top, bottom, left and right - the area down to the horizon), since you are essentially placing a rectangle (the sensor), within the sensor circle, rather than placing the image circle within the frame of the sensor. Shooting the sky with the "squared" version, you will really not detect the fisheye effect of the lens. The fisheye lenses will offer the fastest apertures, since they are primes and essentially uncorrected for distortion.
- Rectilinear - Rectilinear or normal lenses are not 180 degrees wide. They vary by focal length. The Sigma 8-16 is the widest available and is 117 degrees wide (edge to edge) at the 8mm mark. The longer the focal length the narrower the field of view. Wide angle rectilinear lenses tend to not be really fast - f2.8 at the fastest and usually f4, and are corrected for distortions (trying to keep the straight lines straight). Therefore their optics are more complex, more expensive. Zooms just add to the complexity (and that is why you usually see wide angle zooms with zoom factors of 2x (across the focal length limits) as opposed to 4x for the more normal focal lengths.