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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
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
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.
See my hand-picked Nikon list.