This adapter is simply the inverse of a teleconverter. The DPReview writeup on the EOS-NEX version of this adapter was a hodgepodge of misinformation, and even got the definition of f-number wrong. This adapter simply decreases the focal length, and because f-number is defined as the ratio of focal length to aperture diameter, the f-number decreases by the same percentage as the decrease in focal length. The change in focal length nullifies the effect of the crop factor, when using a lens designed for FF, on APS-C. BUT, if you use this with a lens designed with expectation of the crop factor, i.e., not intended to be used on a FF camera, the angle of light acceptance into the lens will be wider than what was intended in the design of the lens. At the very least you would not want to use the original lens hood, but you might find it necessary to crop off 1/3 of the longer dimension of the image, i.e., 1/6 from each side.