A very good lens. Belongs to a family of unit-focusing telephotos with similar 5-element designs, including AIS 200/4, AIS 180/2.8 ED, and the super-telephoto ED lens heads for AU-1 focusing unit. The simple optical structure with relatively few glass-air boundaries ensures good light transmission with high contrast.
Close focus distance is rather long at 5m, giving 1:9.1 magnification. It actually focuses slightly beyond the 5m mark so the magnification is about the same as the IF-ED version which gets to 1:8.8 at 4m (the IF-ED version has focal length shortening at close range which is why the magnification is not greater even though it focuses 1m closer).
I sometimes use extension tubes to increase magnification. Due to the long focal length, a lot of extension is required to much difference. The PK-13 (27.5mm) will give reduce the focus distance to 3.1m, with magnifications between 1:14.5 and 1:5.6.
As others have mentioned, focusing is heavy due to the big focus helix required for a unit focusing lens, and slow due to the long 260° focus throw, but focus action is smooth and precise.
The tripod collar and mount is solid, and the whole lens is stable in use. At slow shutter speeds the lens does require good support - I lost many twilight shots due to poor tripod technique!
The lens has 7 straight aperture blades. When stopped down the shape the aperture opening is noticeable compared to shorter lenses, it would be nicer with 9 rounded blades, but that's a minor point.
