The fisheye attachment lens is simply screwed onto the 58 mm filter thread of the respective lens. The strongly curved lens simulates a fisheye effect, capturing a significantly larger field of view. As with most attachment lenses, there is a slight loss of light. Additionally, depending on the aperture value and intended use, some edge softness can be expected. The macro fisheye adapter is not a complete substitute for a high-quality fisheye lens, but it offers a welcome alternative to often expensive and heavy lenses. The 0.42 indicates the factor by which the focal length of your camera lens is shortened. For example, a 20 mm lens becomes approximately an 8.4 mm lens (f/0.42). It's also suitable for close-up shots: always at hand. A practical added benefit is the macro lens included in the set. It typically remains attached to the attachment lens and provides significant magnification for close-up work. However, it can also be used without the attachment lens without any issues.