Originally posted by WMBP I'm not sure where y'all are finding these cheap, long Pentax primes. The 400 f/2.8 is selling right now at keh.com for (USA) $3400. The 300mm f/4 is much more reasonably priced ($350 - one tenth the price of the 400!).
I don't think people are referring to the A*400/2.8, but rather, the M400/5.6 and other 400/5.6's, which tend to be in the same price range as the K300/4. The A*400/2.8 - because it is f/2.8 - is an *enormous* and *extremely heavy* (somewhere in the 10-15 lb range) chunk of glass. Plus it's an "A" lens (always more expensive than M), and most importantly, a "*" lens, which denotes the higher level of quality and always comes at a high price.
By way of comparison, the M400/5.6 is only slightly bigger and at most slightly more expensive than the 300/4. I don't see any M400/5.6's on Ebay right now, but there is a K version for $449 "BIN".
Quote: I bought and tried the Pentax 300 f/4 (from keh.com) but didn't feel that the image quality was an improvement on the Tamron - and the Pentax prime was a lot heavier lens, without auto-focus. Returned it to the seller.
I'd say the main reason to use the prime would be if you needed the extra stop of speed (f/4 versus f/5.6), or wished to use it with a TC. Again, as discussed in the current thread on zooms versus primes, simply comparing them at the things they can both do well does not tend to show any advantage to the prime; you need to consider the things the prime can do tha the zoom cannot. Although had you tested in a high contrast situation wide open or close to it, I'm guessing you would have found the K300/4 a *ton* better than the Tamron at controlling purple fringing - the Tamron is one of the worst lenses around in that department. I'd also expect color rendition to be better from the K.
But the Tamron is indeed reasonably sharp and a fine deal for the money - in many situations it will produce images about as good as any other lens in that focal length range.