Adding to my collection of Olympus OM mount lenses that I can use on both my older OM-2sp and my Fuji X-E1 cameras is a Sigma 600mm f8 mirror lens. There are a lot of write ups already you can find on the Internet about this lens so you can search them out and read the reviews. My impression so far with this lens is that it works very well on my digital camera, better than my Minolta 500mm mirror lens. Hand held at 1000/sec or higher gives fairly sharp results all things considered but the more daylight you have the better. For the price you can find these on ebay for it is a pretty good deal if you want a long focal length without spending a lot of money.