I have the Canon 70-200 f4 L and love it. I have never used teleconverters and would like to double my reach to allow me more bird and nature photography.
1) What are the limitations of teleconverters?
They multiply not only your focal lenght, the also multiply the f-number. A 1.4x turns an 4/200 lens into a 5.6/280 and a 2x into a 8/400. So you need a shorter shutter speed to avoid camera shake because of the longer focal lenght but takes away the option of opening the aperture. So you have to use high ISOs or a tripod more often.
It also interferes with phase-detect AF systems for which the effective aperture is a physical limitation. The AF system of the 50D only supports lens/converter combinations up to f/5.6. Beyond that it deactivates the AF if it detects the converter because it can't uphold speed and precision. There are non-reporting converters marketed under the name of Kenko/Soligor/etc. (some models, nor all of them). Or you can tape some pins on the converter. Thus the camera does not know the true aperture and tries to focus. Depending on the combination of lens, converter and camera the results vary widely. Some can be worth the try others not.
Mounted on a tripod you can use liveview to focus manually or contrast AF (if your camera offers this) which is not physically affected by the aperture.
Also, if the camera detects an extender it slows down the AF speed to uphold precision and consistency, a little bit with a 1.4x and a lot with a 2x. When using non-reporting converters or taping the pins the speed might be faster but the hit rate will drop. I also met the problem that a non reporting converter on a f/2.8 lens left the AF sensor for f/2.8 or faster lenses active which couldn't cope with the effective f/4 aperture.
Oh, coverters often introduce some back or front focus. With reporting converters you can save different MFAs for the bare lens and the extender combinations. With non-reporting/taping you might have to adjust MFA whenever you add or remove the extender.
Canon extenders have a protruding lens element that has to fit into the rear of the lens. So you can use them only with long L lenses (your lens is compatible) and you can't stack them unless the one in front is a 2xII.
2) Who likes them and why?
I like them because I can turn my excellent 4/500L IS into a 700mm and 1000mm depending on the situation. There is no zoom lens covering this range within the same weight class. I am a little less impressed with converters on the 2.8/300L IS (maybe I have a sub-par sample) and disgusted with them on a 100-400L.
Converters 'stretch' the image and enlarge all the faults of the lens in front. If the IQ of the bare lens is excellent wide open expect great results with a 1.4x and adequate to good results with a 2x. With the latter it often helps to stop down a bit. If the lens is less than excellent, results with TCs are less positive too.
On top of the side effects of 'stretching' the image the additional lens elements degrade contrast and bokeh, enhance flare/ghosting and might introduce CA (for me one main difference between 2xII and 2xIII).
The 4.0/70-200L IS works very well with 1.4x converters, it easily beats the 70-300IS (non L). I haven't tried it with a 2x. It might do quite well in good light (AF issues aside) but I expect a dedicated 400mm lens to do better.
3) Is a Canon converter the only proper way to go?
No. With 1.4x it doesn't matter too much IQ wise. Go for Canons MKIII extenders when weather sealing is important to you. AF speed and accuracy is more difficult to assess. I litterally put my money on Canon but many others are equally satisfied with Kenkos. Converters are optimized for a certain range of focal lenghts. Many of the third party offerings do better with 200 to 300mm lenses than longer ones (might be different with the Kenko Pro series).
Tinu
