I use it with an EOS 600D and Soligor telephoto 100-300mm and I have no problem brightness, zoom in so that macroscopically. Once it's on, you can not set the aperture, but it's not that bad!
In any case I'm really happy with this purchase! It suits me perfectly!
I recommend if you have old goals you do not want to part (a telephoto lens Anyway it works!)