A super wide macro lens adapter can be used to give a regular lens a fisheye effect.
Products sold as super wide macro lenses are often actually wide angle or fisheye lens converters. These converters are used to shorten the effective length of a lens, and give a wider angle of coverage when attached to a macro lens adapter.
Lens Converters
Lens converters decrease or increase the effective length of a single lens reflex (SLR) camera. Those with a converter number of less than 1.0 are wide angle converters, which decrease the effective length of a lens, and give it more of a wide angle effect. Those with a converter number of 1.0 or greater are teleconverters, which effectively lengthen lenses, making them more like a telephoto lens.
Fisheye Converter
Super wide macro lenses usually have a conversion number of less than .50, which means they effectively double or more than double the width of the angle that the lens covered. For example, a 0.42x converter attached to a 35 millimeter lens will give it an effective coverage angle equivalent to a 15 millimeter lens. The lower the conversion number, the greater the fisheye effect, which at its most extreme makes straight lines appear curved, and produces a circular image.
Macro Function
Many super wide macro lens kits can be separated into two parts, with the macro adapter that makes up the rear end of the lens adapter employed to give your camera's lens a closer focusing distance. This can be useful for taking extreme close-ups, for example, of flowers or insects.
Tags: wide macro, effective length, lens adapter, macro lens, wide angle, conversion number, converter number