Originally posted by SpecialK I post this for giggles. It is a 4-element, 2-section, 800mm "tuber".
I bought it used and sold it for less.
New owner said it was sharper than his (unknown) mirror, and thanks.
Never really got above "fairly poor" images.
The bottom image has a very dreamy quality to it. Kinda reminds me of the Pentax 85mm soft focus lenses. It's nice but not in a great big 800mm lens. Portraiture is hard when you are a hundred meters away from your subject.
I recently acquired an SMC Pentax 1.7x AF Converter and was pleasantly surprised when it worked with my SMC Pentax A 400mm f/5.6 lens. You dial in the focus on the lens to "close enough" and the converter will then fine tune it to "spot-on." By way of contrast(!) a Tokina RMC 400mm f/5.6 does not work. The converter just hunts back and forth and then gives up.
The converter is a good option, but only the best glass will work with it. In other words, if it doesn't say SMC on the lens barrel you are on your own. Even then, a maximum aperture of f/8 or f/11 might not cut it. I don't have any other super telephoto lenses so I can't give any opinion.
My LBA side really wants an 800mm or 1000mm but my practical side wants to know what it will be used for. The 400mm lenses see little enough use as it is.