I have been using a Canon EF f4-5.6 Mk III 70-300mm zoom. Not the best lens in the world, but for the price not to bad really. Obviously I am on the lookout for something better. And to that end I have been saving towards getting the EF 70 - 300 f4 - f5.6 IS USM. Now here is the question.... Is it worth hanging on to get the DO version ...? It's a lot more money but if it's a lot more lens I would be willing to wait. Is the IS USM model THAT much better than the Mk III ? and is the DO IS USM that much better than the IS USM? All comments greatfully recieved.