A gray, cloudy view through a reversed lens usually does not mean the ring is the wrong size. If the ring mounts correctly to the lens filter threads and camera mount, the fit is probably fine.

With a reversed lens, the focusing distance becomes extremely short. You often need to place the subject only a few centimeters from the lens before any image will come into focus. If you are too far away, the viewfinder can look like a uniform gray blur.

Another possible issue with reversed-lens use is aperture control, since many modern lenses do not stay open when removed from the camera normally. That often makes the image very dark, though, rather than gray and cloudy.

So the most likely cause is that your subject is simply not close enough. Try moving very close to a well-lit subject and adjusting the camera-to-subject distance slightly until detail appears.

To choose a reversing ring, match the ring’s thread size to the lens’s front filter thread size and the camera side to your camera mount.