I had the chance to finally try out the teleconverter, bringing the 300mm Sigma to 420mm.  The problem was distance... 250 to 300 yards(add about 1/10 if you think meters), hazy air and bright sky behind the subject.  

This is two Osprey chick in their huge nest.  The nest is about 80 feet above the ground with a perfect view of the nearby lake and inlet.  Sky was bright and normal exposure was 1/1200 sec at f/5.6.  This would leave the nest as a total silhoutte!  I moved the shutter to 1/320 sec for the shots shown.

The tower and nest are about 1/4th of the viewfinder look. Even 420mm will not do it at such distance.  Still, I am pleased with the results considering conditions. Next time, I plan to use a remote shutter release to avoid any shake.  Enjoy!  I sure did, just seeing the birds in action.

   TomDart.