The Horizon loupes are a very good item. I think they are a bit better than Peak Loupes--I have both Horizon and Peak--although I have not subjected them to any real tests. I use a Horizon comparator in the darkroom. I also have an 8x focussing with serrations along the inside the crystal which I have never used. It is still in the box. How many loupes do I need in the darkroom at any one time.......
Cheapest alternative is to use a $5 or less enlarger lens. These are usually triplets and some even Tessar type. They come at f/3.5~4.5 which gives you a good vision field. I use a 75mm (or 80mm?) for prints and a 50mm for negatives. You should look through the lens, its front on your side and the thread mount on the negative (or print) side.
Basicly there are 3 types:
1) 8x standing on you neg/slide
2) Full frame standing on your neg/slide, folding type
3) hand held
I have all 3, the Linhof and a Wista (1) (used on my focussing screen of my LF aswell, (2) for 35mm and 6x6, and (3) an old coated russian T3 2.8/80mm projector lens that I got for free.
I like them all and use them as it comes.
I own one of these Horizon models and find it quite decent; it covers a 35mm frame. I've also seen suggestions to pick up a manual focus camera lens -- perhaps an "obsolete" Canon FD 50mm f1.8 or the like.
Having "come of age" under the shadow of McCarthyism and Cold War paranoia where we were reminded at every turn about "those evil Russians," I am always amused to get out this loupe with "Made in Russia" prominantly engraved on the side.
I have two Horizon Loups a 6x and 4x (for 120). I also use the 4 times when focussing my Shen Hao. They are more than up to the task and represent great value for money. I also have a Horseman 7x loupes for the Shen Hao and find it just a little too strong as it tends to enalrge the grain on the GG to much.