Magnifying Eyepiece for DSLR
For Nikon DSLR users (most models), I recommend the following (some items are for some models and not others).
Eyepiece items in general
Nikon offers a range of eyepiece and eyepiece accessories, including diopter-correction eyepieces and other. See my list of eyepiece and eyepiece accessories.
Below are a few that are particularly useful. Not shown below are various anti-fog eyepieces, very helpful under some conditions.
Nikon DK-17M magnifying eyepiece
Nikon DK-21M (bottom)
The DK-17M eyepiece screws into D3/D3s/D3x and similar Nikon bodies, providing 1.2X magnification, which I find helpful for manual focusing.
Get the DK-21M for other Nikon models (D200, D300, D40, etc). I've found both of them a plus for my use.
Eyeglass users might not like either magnifying eyepiece because it could cut off things like focus confirmation and other data at screen bottom.
More stuff
See my hand-picked Nikon list.
For timely, in-depth research and reports on photographic equipment and technique, please subscribe to Diglloyd’s Advanced Photography.