Okay, so I am using a Canon 450D and I'm shooting wildlife.
The Tamron70-300 has given me some great macro shots of insects which a professional photographer has complimented me on, and some good shots of garden birds at the telephoto end of the scale. However, I am conscious of the fact that at the telephoto end of the scale the smaller subjects have to be not to far away for the results to be any good.
Would the addition of a 2x teleconverter to the 70-300 be a help or a hinderence I wonder?
Not sure a teleconverter would work with that lens, at least it doesn't on Nikon's version. Might be worth looking at the Canon 100-400mm f4.5-5.6 L IS USM Lens but it will cost £1,216.99 or the Sigma 50-500mm f4.5-6.3 DG OS HSM - Canon Fit at £1.178 known as the Bigma. Also, the Sigma 150-500mm f5-6.3 DG OS HSM Lens - Canon Fit at £768 is very good, as I know someone who uses that with a Sony and gets great results.
I have the Canon 100 to 400 it cost 900 second hand as is a brilliant quality lens ............I have also used the Sigma Bigma testing it for a friend .I preferred the canon but there was little in it .
It will cost you there's no doubt about it but it also retains the value .