I also have the 18-55 and though it is not as good as the LTDs, I find it more useful than the 20-40 actually. It's lighter and a better zoom range and about the same size in practical use. Cheaper too. You are compromising overall IQ, but it is a great WR alternative for when you don't want to lug the primes around or switch lenses.
I like the 20-40. It is bigger than a prime ltd, but it's sharp and WR. Though I do tend to take the 15/35M/70 more often with me. And for shooting weddings it's mostly the 21 and 35. So I don't really see the 20-40 as a prime replacement but as a smaller zoom replacement alternative. If that makes any sense. i.e. I take the 20-40 rather than the 17-50 Tamron for example.
Hi! No I never had that lens, I use a Vivitar 28mm 2.8 (very different glass). Though, I know someone who talks great things about the Sigma, but really can't give you in-depth details. Cheers!
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