I thought I had that problem once. So disappointing to look through the viewfinder and it never comes into focus. Turns out I had bumped the diopter adjust on the viewfinder.

At $100 I would pass. I have two manual mirror lenses and find them difficult to use. They're manual focus and depending on the camera likely will be manual exposure as well. My Sony camera doesn't have the controls as well placed as my old film camera for quickly adjusting the exposure. I don't mind the manual focus, typically my left hand is on the lens barrel twisting the zoom ring anyway. The most annoying part is the very shallow depth of field. Although the lenses will focus as close as 10 feet, the depth of field is about half an inch. A head-on shot of a butterfly at 10 feet will have the tail out of focus. Shoot a blooming flower and the stem will be out of focus. For distance shots my 70-300 zoom will crop and get an image as good as the 500mm mirror lenses. Those donut shaped spots are really frustrating too. Even on distance shots, I find dark donuts in the sky. I've been thinking of converting one of them into a coffee cup.

I spend a lot of time photographing our dogs outdoors. I have a "rain" camera, a Fuji S2940 that I don't mind if it gets trashed. Unfortunately I get a lot of dog butts. The auto-focus is slow and the subject is usually out of the frame before the camera focuses. Don't have that problem with my old Sony A390 or SLT-A55. The auto-focus capture speed isn't published on most cameras.