Lens Extender Compatibility Help

[adub94]

I have a EOS Rebel T3 with 75-300mm Canon lens. I want to get a 1.4x or 2x lens extender but I'm not sure of compatibility. I'm almost certain the III will work for my camera but I'm just getting into this and do not want to shell out that kind of money right now. So i guess my question is, will the II work for my set-up?  Is this the best and somewhat most economical option availabe to me? Any help is greatly appreciated!

Thank you!

[TCampbell]

Two things:

First, a tele-extender not only changes the focal length by its multiplier factor, but ALSO the aperture by the multiplier factor. That means 300mm and f/5.6 with a 2x becomes 600mm and f/11; with a 1.4x it becomes a 420mm and f/8.

Your camera's autofocus system will not work correctly at f/8 nor f/11 -- it's not enough light.

Second, a tele-extender will moderately degrade optical quality. The 75-300 is already probably the most marginal zoom lens in the lineup. I would not want to further degrade it's optical quality even if not for the focal ratio problem.

The combination won't work.

[adub94]

I'm not so much worried about the AF. I manual focus anyway. This would be more for night time shots of say the moon and what not from a tripod.

[TCampbell]

Neither the Canon compatibility table nor the Kenko compatibility table list the 75-300 as being compatible with their respective extenders.  This usually means that either the extender wont fit (sometimes the lens wont even mount) or it wont function correctly (communication between lens and camera wont work correctly).  

Kenko will usually list if a lens that is partially compatible (e.g. it fits can be used if you focus manually but auto-focus wont work) but in this case, they don't even list the 75-300 as being partially compatible.