Here's my story: I got really lucky and in a $25 grab bag full of stuff I got the highly regarded Kenko Pz-AF 1.5x Teleplus SHQ - autofocus, autoaperture... Very nice! I used it with my longest lens, the DAL 55-300 which I really like. Works great! Shots w/ the Kenko (at ~450mm) can be better than ones without it (at 300mm) and cropped, but as others have noted, a teleconverter will accentuate the weaknesses of the lens.
And now starts the slippery slope.
So, I'm thinking, how about getting a nice prime that will take full advantage of the Kenko! Something in the 200-400mm range can compete with the DAL 55-300. What's more, I'm finding it an enjoyable challenge to work w/ primes instead of zooms. Hmm... that DA* 300 f4 is out of my price range. Yes, I know the formula: good, cheap, or fast. Pick 2 of the 3, and so I'm happy to work with slow for now. I'm also hoping I can get lucky at $50 cheap.
I've been all through the lenses reviewed on the forum, and so for the cheap options, it looked like I would need to move into old M42s or T mounts. The Pentax Ks or Ms either got real expensive real quick or else had poor reviews compared to other lenses.
So, I got a nice
Tele Takumar 200mm f5.6 M42 that even came w/ an original Pentax M42>K adapter for about $30. It was given a 9.3 score for sharpness on the forum reviews, so I had high hopes. What a spunky little lens! So small for 200mm! Works better w/ a tripod only because it's so light! But... after quite a bit of testing, the DAL 55-300 certainly provides sharper pictures w/ better contrast, and its autofocus has a better success rate than my manual focus (using either viewfinder indicator or focus peaking on the K-30). Using it with the Kenko 1.5x TC wasn't worth it.
Next try was with a Tele Vivitar 300 f5.6 in T mount w/ both a M42 and K mount adapter for $25. It's a solid piece of equipment. It's useable handheld, but certainly better on a tripod. But... the DAL 55-300 provides as sharp or sharper pictures, better contrast, and autofocus. Again, the Kenko 1.5x TC doesn't provide much advantage.
So, does anyone know of a cheap telephoto prime in the 200-400mm range that can outdo the DA(L) 55-300? Can $50 get anything decent? Otherwise, somebody stop me!
Thanks for any suggestions.