See my Guide and Review of the Leica M9 in DAP.
Wojciech B writes:
Having noticed recently that my eyesight is not as it used to be, I would need something to improve the focusing, therefore the question - for everyday photography which loupe would you recommend, and why? Would a landscape photography change your recommendation?
Leica offers both a 1.25X and a 1.4X viewfinder magnifier that screws into the viewfinder of the Leica M9/M8. Each comes in a tiny little case that snaps onto the camera strap for easy access. The 1.25X and 1.4X can even be stacked for even more magnification, which works great for critical focusing in tripod situations.
For the 16mm, 18mm, 21mm, 24mm lenses, use the 1.4X magnifying eyepiece: because the image is composed with a separate optical viewfinder, the viewfinder magnifier gives you a big aid to visual accuracy when focusing, while changing nothing in the image composition.
For 28mm, using a magnifying eyepiece is problematic, because the viewfinder frame lines will be cut off. If you’re willing to use a separate optical viewfinder, then use the 1.4X magnifying eyepiece as with the 16/18/21/24mm lenses, and get the Zeiss ZI 25/28mm optical viewfinder.
For 35mm, the 1.25X magnifying eyepiece works well, but does push the frame lines outwards, similar to the position when at 28mm.
For 50mm, the 1.25X is perfect.
For 75mm and 90mm, the 1.4X is ideal.
See Leica gear in my handy Leica wish list at B&H Photo.