Sledneck7 wrote:hey bob_r thanks for your advice. Also do you know much about lens extenders? I have found some online and was wondering if that might be a better way to go with my 75-300mm lens? Generally I am anywhere between 50 to 250 yards away from the animals and just want a decent lens to help me get good pictures. I am lucky enough to have elk in my backyard year round and want to take advantage of it. Thanks
Canon teleconverters will not work with that lens, but some of the third party lenses will (not Sigma's). Canon and Sigma TCs can not be used because the glass elements protrude too far to allow them to be connected. Another problem is that the 75-300 is not a very good lens to start with and then when you add a TC, you're going to reduce the image quality even more. The results will probably not be satisfactory. Most lenses will suffer considerable image degradation with a 2X TC, but some work well with a 1.4X TC. When you add a 1.4X you lose one f/ stop and when you add a 2X, you lose 2 f/ stops. Once your f/ stop gets smaller that f/5.6, your camera will no longer auto-focus unless you're using a 1 series pro camera (there are some 3rd party TCs that don't report their existence to the camera that will AF). If you don't mind manually focusing, you can still use TCs, but you won't get as many keepers shooting wildlife because your camera's shutter speed will be considerably slower.
Here's a shot I took with a Canon 30D, a 100-400 and a Sigma 1.4x TC (I had to use manual focus). I also used a flash and a Better Beamer flash extender to increase the light.
1/250s f/8.0 at 560.0mm iso400
This is a shot with the same setup, except with a Sigma 2X TC.
1/250s f/11.0 at 800.0mm iso200