#2. "RE: DG-2 2X Magnication Eyepiece" In response to Reply # 0
Livermore, CA, US
I used and liked one of these on my D50 which had a really small viewfinder (although IIRC it was DK-21M). I find it didn't work so well with cameras like D300 and D7000 which have larger viewfinders to start with, as it cut off corners of the viewfinder.
For manually focusing macro work, the other poster is 100% right - use magnified live view.