You're right, the Canon 1.4 teleconverter will not allow you to auto focus with the Canon 100-400mm L f/3.5 - 5.6 lens. That's because most Canon camera bodies, with the exception of the 1D series will only auto focus if your lens aperture is f/5.6 or faster. The Canon 1.4 teleconverter adds 1 stop (f/1.4) of light loss to your lens, making its effective aperture f/8.0 (f/5.6 + f/1.4). So your Canon body will not autofocus. There are several workarounds for this issue, including using masking tape on some of the converter's pins, but most don't work very well. The only workaround that I use and recommend is to use the Tamron 1.4 extender instead of the Canon one. You'll have to use a specific Tamron 1.4 extender for this to work, namely this one.Tamron makes several 1.4 extenders. The one you want is the black, non-pro version. This little teleconverter does not pass all of the lens data back to the camera body. Most notably, it omits the part about a teleconverter being attached, so your camera body still thinks that the lens is operating at f/3.5 to f/5.6 and so attempts autofocus. Notice I said "attempts". Whether it is successful or not, depends on the amount of light available and the amount of contrast visible on your subject. Autofocus is also slow, so this trick works for getting extra distance, but does not work very well for moving targets like birds in flight. When it works, it's great. When it doesn't (lack of light or contrast) its frustrating. Here's some tricks I've found to make this combination work better.
- This trick works better on older camera bodies, such as the Digital Rebel or the Canon 10D. It still works with newer bodies like the 20D, 30D, and 40D, but not as well.
- Use only the center AF point.
- Focus on the highest contrast portion of the subject. Sometimes you may want to focus on the edge of the subject, right at the transition between the edge of the subject and the background as this area may have more contrast.
- Use this trick to make far away birds appear larger in the frame.
- Do not use for action shots like birds in flight.
- Set your aperture to f/8 or smaller (f/10, f/11) to maintain sharpness and get the correct exposure.
- If you use this trick with a flash in E-TTL mode the image will not be correctly exposed, because the lens data sent to the flash will be wrong. Use with flash in manual mode or dial in flash exposure compensation after taking some test shots.
- Set your camera to one shot mode so you can tell if the camera locked on to the subject or is endlessly hunting due to lack of light or contrast for this combination.
I ran into someone at Green Cay on Friday. He was using a Tamron 1.4 extender that gave him auto focus with his Canon 400mm f/5.6 L lens. I think Kenko will also allow the auto focus to work. If you were to buy one, would you go with Tamron or Kenko?
You can use the Tamron 1.4x extender to get auto focus with the Canon 400 f/5.6 L lens and the Canon 100-400 f/3.5 - 5.6 IS L lens under certain conditions.
These conditions are:
- It has to be a sunny day, and your subject needs to be well lit with a lot of contrast. (i.e. Red Shouldered Hawks have lots of contrast, but Snail Kites don't).
- Autofocus will be slower than normal.
- AI Servo will "hunt" to find its target more than usual.
- Even though your camera will display that your aperture is set to f/5.6, in reality your exposure is being taken at f/8.
Although the trick works with a few teleconverters by Tamron, Kenko, and others; tests by users on internet forums like DPReview.com indicated that the Tamron performed better than the other brands. For this trick to work, you'll need to buy the inexpensive, "non-pro" version of the teleconverters.
Note you can also fool the camera when using a Canon or Pro teleconverter by taping the last three pins of the connector between the converter and the lens. However, the performance is much slower than the Tamron.
