Pentax SMCP-FA 250mm-600mm f/5.6 ED(IF)

This lens is very rare, but I got mine second hand as a bargain. It is a dream of a lens. I use it for wildlife photography especially for photographing Grizzlies in Alaska. The lens has a perfect zoom range for my application. There is no need to change converters with a 300mm lens, so I can react quickly. The quality is comparable to a couple of fixed focus lenses, but you have to carry only one lens. Sometimes I even use it with a Pentax Converter 2X-L with good results.

Sharpness and contrast. Smaller than a 4/600mm lens. No one else offers a comparable lens.

It is a large, heavy and expensive lens.

Pentax cameras and lenses, Nikon F90X, Nikkor 2.8/300mm.

Never needed.

Amazing! It's sharper than many primes (which is just as well; it weighs as much as a bagfull of prime lenses). It covers the range I use most photographing motorsports, so I don't have to switch between different lens/body combinations.

Sharpness & contrast.

The holder for the built-in filters has a tendency to fall out.
And, of course, it's big & heavy.

None

Too early to tell; I sent the lens in for a CLA (and to fix a minor problem that was there when I bought the lens, used).