by Roger N. Clark
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Here is my experience with TC's:
Note with Canon EOS series cameras: all consumer SLRs (digital or film) require an f/5.6 lens or faster to enable auto focus. Pro bodies allow f/8 auto focus with the center AF sensor only. Thus, if your lens is f/5.6 and you add a 1.4x TC, then then you have f/8 and no auto focus. Some people tape over some of the contacts to defeat the f/stop reporting to enable auto focus, but you'll find that the focus will hunt and often overshoot, depending on the lens. You also run the risk of gumming up the contacts so nothing works.
When I bought my first Canon EOS series camera around 1990,I started with some canon and mostly Sigma lenses and somehow got a 3rd party 2x extender (I do not remember the brand). Because it was a 3rd party, it did not indicate to the camera that the f-stop changed. Thus the camera tried to auto focus (an EOS 650 back then), and usually found focus. My experience with several telephotos was that it made the image bigger but blurrier. When I did a test on the moon, I found a very slight improvement in image detail, but it was not worth the effort.
Then I got a Kenko Pro 300 and some better lenses. That made all the difference. The Kenko Pro line does very well with the fixed focal length telephotos. I also occasionally use them on a 28-135 IS lens if I'm not carrying any telephotos and I need the focal length.
I also stack the 1.4x and 2x TCs and still get great results. To do this, I put on the 1.4x, focus, switch to manual focus, then add the 2x in the following order: lens, 1.4x, 2x, camera body.
Here are some examples that show the quality of this setup:
Relative Lens sharpness Testing
Note that with long focal lengths plus 2x TCs, camera shake can be a problem. While I tried to minimize any camera shake, I was also trying to simulate real world field conditions, so some may be limited by camera shake (noted on some images).
Some images with a 500mm f/4 lens + 1.4x TC (use your back button to return to this page):
Sandhill Crane Takeoff.
Hawk close up.
Sandhill Crane Take-off Sequence.
Baby Great Blue Herons and Parent.
Hand held 500mm +1.4x TC: Red-Tailed Hawk.
Note the stars near the frame edge are almost as good as at center (a star test is one of the toughest tests for a lens): The Pleiades Star Cluster, M45.
Some images with stacked 1.4x + 2x TC's on a 500 mm f/4 (use your back button to return to this page):
(I've made 16x20 inch prints from this one that surprised
me in terms of quality):
This one also shows the advantage of adding multiple
images together to improve signal to noise:
The planet Saturn with a telephoto lens.
You'll notice from the above series, that I use the 1.4x TC the most, and only occasionally use higher. That is because it is difficult to get good, sharp images as the focal length gets higher. For focal lengths above 500 mm, you really need a carbon fiber tripod (carbon fiber dampens vibrations). I tried to use a Sigma 170-500mm lens on a Bogen 3001 tripod and got poor results due to camera shake. (Note my 170-500 Sigma performs very well, see the test at: Lens test Sigma 170-500 versus canon 100-400 ) Then I upgraded to a Bogen 3221 tripod with a Bogen 329 head (very heavy). Still vibrations were a problem. I stuffed wooden dowels in the bottom section of the Bogen aluminum tripod, and vibrations dropped about a factor of 2. But it still was not enough. I now use a Gitzo 1329 carbon fiber tripod with a Wimberly head. The tripod is still a limitation when I add TCs, so I will eventually get a larger carbon fiber tripod. I will also be using the 2x TC more now that I have a pro body (a canon 1D Mark II) which will auto focus at f/8.
First published August, 2004 Last updated November 24, 2009