Eyepiece mangnifyer recommendation
On the D90...
Nikon DK-21M 1.2x is easy to mount but is not very good optically.
I tried the Tenpa 1.36x but got rid of it because it cuts corners and the bottom line too much. The ca 1.2x magnifiers are acceptable regarding this.
I'm using the Nikon DK-17M in Tenpa's adapter.
I think Tenpa's 1.2x version might be fine too.
I can't use the cup because one needs to squeeze the eye very close and the cup gets in the way. It is not a good combo with glasses either.