I have a canon 5d MK II as some know. The 24-105 is a very nice lens but isnt ideal for really wide shots (for weddings, clubs, etc.). I think my next lens investment is going to be something wider than 24mm. I really would want a Sigma 20-40 f2.8 but it is discontinued and scarce online from what i have found. It seems there is no 3rd party or more inexpensive option to the 17-40 f4L from Canon. Any suggestions or insight?