And I want to respond to the comment that suggests losing the diaphragm. I disagree. I'm a professional photographer and just ran tests, I don't notice any loss of light. In this type of test it is necessary to be very rigorous, there are several possible approaches and I will give you one of them. 1/ Eliminate any outdoor light source that is constantly changing. I used LED lighting so I let the temperature rise nicely. 2/ To check the aperture of the lens you want to use, take a lens with an external aperture ring (I used a Nikon 50mm f/1.4D) on the adapter ring side. In this case it is important to set it to the closed position. It is the lens that takes control of the aperture. I set it to an aperture of 5.6. 3/Don't leave the sensitivity on auto (I set it to 400 ISO) 4/Take a photo of a very specific scene (you must have the same frame for the second photo) 5/Remove the ring and set a Fuji XF lens on the camera (I used the 16x50mm kit lens on the aperture first, or manual, of course the same aperture here 5.6). 6/ Take a picture of the scene (identical framing) and compare the speeds between the two photos. With a Fuji X-T10, the photo with the ring was darker, but the speed was faster. At the same speed, the photos are the same, so no loss of light. Now you should also know that each lens can have real differences in aperture compared to advertised values. Also, be careful when comparing speeds with another device. It is very likely that you have differences. Fuji, for example, cheats with the announced sensitivities. If it tells you ISO 200 you are actually at +- 120 ISO. Worse still, with an Olympus OMD EM10 I notice speed differences that can easily be doubled with a Nikon D800 (so 6400 iso Olympus = 3200 ISO on a Nikon D800 for example). Anyway, when a manufacturer announces that his device starts at 200 ISO and at 100 ISO there is a loss of dynamics, I understand that he is cheating on his 200 ISO, it is 100. Coming back to this ring: Personally, I don't find any flaws in it , except maybe its weight (130 g).