In article , dkirsner@u.washington.edu says...

>I'm planning on buying the Nikon diopters 5T and 6T in the 62mm size. I
>have two lenses that are 55 mm filter size and two that are 49mm. If I
>wanted to use these diopters on the the 49mm threads, would they still be
>effective since I would have to use 4 stepping rings between the lens and
>the diopter.

It will work, but it should be possible to use fewer rings. Also, though
I would not recommend step-down rings with anything but close-up lenses,
the Nikkor 52mm 3T and 4T achromats will fit your lenses with a 49->52
step-up, and a 55->52 step-down ring, and should not vignette.
Hope This Helps