Originally posted by photolady Which is the sharpest lens I can get that doesn't weigh a ton?
I can't comment on the Pentax 80-320, having never used it, but I did own the Tamron for a year or so. The one merit of the Tamron is that it is light. According to some resolution tests I checked before buying it, the Tamron supposedly has better resolution than either the Sigma or even the DA 55-300. Even if these tests are valid (I rather doubt they are), however, they don't tell the whole story. At 300mm, the Tamron produces photos of low contrast and somewhat pale colors. Perhaps it does produce better resolution at 300mm then some of the other consumer grade telephoto zooms; but that doesn't mean it produces better images.
If sharpness is what you're after, a prime is a better choice. But primes are heavy, and you won't be shooting any bumble bees with those old manual focus K and M 300/f4s without extenders, because they don't focus any closer than 4 meters on their own.