Originally posted by alwaysjerricky Hmm true in a way. Need to research more about this. Thanks for the comment.
To me the whole thing where zooms are not as good as primes is the lazy man's approach, with a prime, you use it, you find out it's characteristics at different ƒ-stops, which can be very different, and you have an idea when you might want to use it. With zooms, you have to do that for every focal length. IN the 18-135, you need to understand 18mm 24mm, 30mm, 40mm, 50 mm, and 70-135 where it's consistently centre sharp but not as strong on the edges. It's a lot more work than getting to know a prime. My wife never got comfortable with the 18-135. Using the Tamron 17-50 simplified things for her, less range to deal with.
A lot of people just say primes are better than zooms... yet almost every zoom has it's strength. Even my much maligned Sigma 70-300 gives excellent results from 70mm to 150mm, very close to some much more expensive lenses. When compared to my Tamron 90, DA*60-250 at 90mm, my $200 Sigma (at 90mm) zoom came in third, but it was a close third. A lot closer than you'd expect based on price. The Tamron 90 won with a bit better micro contrast, but it and the 60-250 were very close to identical.
For every zoom you know, you need to know the conditions for which you should take the zoom off and put a prime on instead. There are times I prefer the look of the 21 Ltd. to the 18-135, not because it's sharper but because it handles out of focus areas better. IN low light I like my FA 50 1.8. It goes without saying nothing the 18-135 can do will match that lens in low light. Typically, with the 18-135 on the camera, I'll have the 21, 35, 40 50 and 60-250 in my bag, and I'll take the 18-135 off the camera in some conditions in favour of one of those lenses. But there are also a lot of days where the 18-135 never comes off the camera.