The diopter correction piece for the rectangular Nikon viewfinders is a nice one, and it enables my somewhat poor vision to see clearly without my glasses getting in the way. However, while some eyecups may fit to the regular eyepiece (sans diopter corrector), the edge of this piece seems a bit thinner and will preclude a few rubber eyecups from fitting and may require trimming. I am not sure at the moment, but it may be possible for an eyecup with a plastic mount to attach to the diopter corrector if you do prefer using them.Aside from that and a tendency to fog at times, it's a pleasing item for those who wish to shoot without glasses.