I bought another 300 mm lens as an alternative to the Canon 300 mm L lens I already have. The main reason is to achive a lower vignetting. I have very positive experience from an earlier medium format lens. This is the Pentax 645 300 mm ED f/4 lens with good reputation. If I get some positive results from this lens I maybe replace my Canon 300 mm lens with this one.
Here is some photos and data about the lens:
http://www.astrofriend.eu/astronomy/...645-300ed.html
I bought it mostly because I'm curious about it and want to test it. It's more than ten years older than the Canon lens.
Lars