A few weeks ago I purchased a used Pentacon 6 mount 300mm 1:4 lens on eBay. This is a medium format lens built for the Pentacon 6 camera system, but comes with an adapter to M42 Pentax mount, which is then easily adapted onto EOS EF.
This lens is huge. There really is no other way to explain that. Weighing in at around 2.3kg on it’s own, it’s not something you’re going to want to hand hold for long, and definitely not something you want to drop on your foot. Anyway, the main reason I went with this is because I wanted something a bit longer than the Carl Zeiss Jena 135mm for shooting birds and other wildlife, but really didn’t want to fork out the huge amount of money for a Canon EF native mount one of any decent quality. I’d read good things about this lens online, especially with how well it compares to the modern lenses.
One of the theoretical advantages of using a medium format lens such as this on a crop factor camera like the 60D (or even full frame 35), is that you are only really using the very center of the image circle, which, generally speaking is where the best image quality is. Whether or not that would work in practice was a different question.
Another interesting thing about this lens is the aperture mechanism. It has 19 blades, which results in an almost perfectly circlular opening at all stops. It’s a little bit of overkill in some ways, but it’s pretty and kinda fascinating to watch it in action (excuse the poor video quality, can’t be assed setting it up properly).
What better way to test a lens I purchased specifically for wildlife, than by shooting wildlife?
Anyway, the only problems I found with this lens were reasonably bad chromatic aberration under some lighting conditions, and the rather large turning distance from one focus extreme to the other. It takes a fair bit of time and turning to switch from minimum focus distance to further away, so you’d better not need to switch in a hurry. That’s to be expected from a manual focus lens of this size though.
It surprisingly also makes a good portrait lens, provided you have the working distance.
All in all, I think for the usage I had in mind for this lens, it’s going to be more than adequate.