It seems to be very decent lens. Especially speaking of build quality which is glass and metal with very little rubber and plastic. It is prime lens at 300mm with constant f/5.6. It is quite light. Has 3 rings. First to adjust focus. Second to adjust aperture. Third one is to open or close aperture. Second and third work together somehow.

Below you can find 100% magnification (NIKON D200) of BTS antennas. Image is reasonably sharp with little haze, blur or soap. Shot at f/5.6 which is wide open. Target at 100m.

Below you can find same target, but with COSMOS AUTO TELEPLUS 2x. Aperture goes from f/5.6 to something around f/11. I can’t see much more in terms of details. Image was taken handheld so it might be little movement.

I won’t test it against any kind of CA or coma or anything like that, just because it’s long, cheap and slow. But it gives pretty pleasant pictures at good light and that is enough. In case one would like to shoot in low light vision should spend then 10 – 100 more than the cost of this lens.

Still looking for some reason to use mentioned earlier teleconverter. Found some interesting example when it might be useful. Below please find two pictures at 400% magnification showing little crack in the ceiling of some building some 200m far. Second picture show that some extra details could be seen at 600mm instead of 300mm.

I may recommend this lens but please remember to mount it only when having good or very good light.