....... I had the Precambrian, pre stone age version ..... the 50-500mm F4-6.3 screw-drive non stabilised in the PK Pentax mount ..... it was only OK ish ....
.... watching Chris's video the OS model is clearly better than my Pre"cam"brian ..... note the word "cam" was already sneaking it's way into usage even then .. 🙂
Of course I was using it on a APS-C stabilised sensor .... so yes it struggled to sparkle ....
Personally, I would look at a used Sigma 150-600mm or the Tamron G2 version which is still serving me well as these two lenses were a big leap in IQ at an affordable price ... especially as they perform well on the APS-C sensor.
