This is an Ultra wide lens and is based on 35mm or full-frame format. It is wider than 24mm. Ones of this type will give a distorted image compared to a non-fisheye which will keep perspective normal. Generally they can focus much closer than a standard or regular wide-angle lens. In some cases, the filter is either built-in or screws in to the rear of the lens. If you use this lens on a less-than full-frame camera, you will have to multiply the focal length by a factor based on the exact format you are shooting. In most cases, this is about 1.5x, but can vary slightly. There are focal lengths in this type available – 8mm, 15mm, 16mm, 17mm, and 20mm.