Loupes © 2004 KenRockwell.com
35mm full frame: I use the 8x Peak model 2018 loupe or the 4x loupe I got as a promo from Fuji Film. I think the 4x Fuji loupe is made by Kenko. For absolute checking of lens sharpness I use the 22x Peak model 1964
Medium Format: I use the huge Kenko 3x 6x7 Loupe that sells for $140. It barely covers 6x7 and covers the smaller formats very well. I prefer it to the more expensive offerings from Mamiya. Mine came with a nice padded drawstring bag.
4x5: I have no loupe that covers the whole film. I use the loupes above.
You can order any of these cheap from Adorama.com.