In my previous blog Bird Photography Equipment Lens and filters I compared two telephotos which I use regularly, the 300 f/4 IS USM which is an excellent prime lens, and the 100-400 f/4.5-5.6 IS USM is the best zoom lens commonly used for bird photography. I also tried the Canon Extender EF 1.4X II and found that the 300 f/4 worked well but the image quality was never superior that the 300 f/4 by itself.
Recently I tried the Kenko C-AF 2X Teleplus Pro 300 DGX with my Canon 7D and the 300 f/4 lens. To my utter surprised it worked! I had read that this was not possible since the 2X raised the f stop by two-stops making it a 300 f/8. In this particular case this combination raised the f stop to 8, as expected, but the camera was able to focus. I tested this combination extensively and found that the 7D would focus but only under good light conditions. But… after some more testing , I found that the 7D focusing system has more sensitive focusing points on the right or left sides of the 19 point AF system. After multiple test I found that the 7D will focus under all normal conditions with the Kenko 2X by using the Zone AF set to the right or the left sides as show below:
So now I have a 600 mm f/8 lens with autofocus and Image Stabilization. How good is the image quality? Here are two images (cropped) taken at the same distance and you can be the judge:
Dont try this with a Canon Extender, the camera will not focus!
Note: I thought this would be an interesting post since Canon has now priced the new 600 mm f/4 at $13,599.
To top it all, I am now a proud owner of the the new 5D Mark III and it too works with the 300 mm f/4 + 2X Kenko. It focuses perfectly well even at reasonably low light conditions at f/8! Again Canon says this is not possible..
Here is a shot with the new Canon 5D Mark III at an amazing ISO 8000 and 1/400 sec with Kenko 2X + 300 mm f/4 = 600 mm f/8 AF and IS:
Sword-billed Hummingbird at my garden taken with Canon 5D Mark III with a 300 mm f/4 L IS USM telephoto and a Kenko C-AF 2X Teleplus Pro 300 DGX. 1/400 sec, f/8, ISO 8000 Focal lenght = 600 mm