Tandem teleconverters
 

Has anyone experience of using two 1.4 teleconverters (equivalent to a times2 TC).
Will this work or are there problems?

I have used stacked converters several times Vernon (1.4 + 1.4 and also 1.4 + 2x) Results will much depend on the lens that you are using as will AF.With Canon bodies you must have a non Canon converter next to the body as you cannot physically connect a Canon tc onto a Canon tc because of the protruding rubber lug. I think Nikon and Sigma tc's have similar limitations.
The easy solution is to get something like a Kenko Pro 1.4 as one of the tc's - any tc will connect to that one.
You obviously know that two 1.4's will lose 2 stops and a 1.4 + 2x will lose 3 stops of light.

Hi Vernon, I have never tried it and probably never would. To start with the introduction of a teleconverter means slowing down the lens you are using ie the largest aperture is effectively cut down - so an f3.5 becomes f5.6, add another and you are making it effectively f11 so you will need very bright conditions to take a good photograph with a reasonable shutter speed. Also you are adding at least two extra layers of glass between your lens and the camera so you will probably loose sharpness or the image will be degraded in some other way. There are effectively two further opportunities for imperfect glass to affect the image. If possible I would invest in a longer lens rather than buy two converters. I notice Roys post and agree that there are some converters you just cannot do this with due to their construction.

Here is a few examples that show that stacked tc's can make reasonable shots at a push. They were all taken with my 400mm f5.6 plus 1.4 and 2x tc (giving 1120mm focal length). You do need good light, a steady tripod and manual focusing.

Thanks for all the advice.
To be more specific, I am using a Canon 30D camera possibly with a Sigma 120-300 2.8 lens. This will still auto focus with 2times TC.
Does this information modify your comments.
Thanking you in anticipation

Teleconverters are just sophisticated magnifying lenses and as well as introducing their own faults multiply the faults in the main lens. With stacked converters you are adding more glass air interfaces so one 2x converter would be better than stacking two 1.4's. A recent magazine article I read concluded that a high quality converter was slightly better than just cropping on the computer but I imagine that stacked converters would definitely be worse. I would stick to the 1.4 and possibly buy a 2x for the extra reach as well but I would not stack them.

Strangely enough Mike I did a test a few months ago on the difference between stacked 1.4s versus a 2x fully expecting the 2x to be better. My results showed there was very little in it which did surprise me.

Something else that never fails to surprise me is how good images turn out that are shot through double glazed windows. You would have though that the extra two pieces of glass would crucify the quality but I have had some very good results.

Very impressive.

I think the key to the good results is the very high quality of the main lens, the 400 f5.6 L is one of the optically best that Canon do.

Thought I would resurrect this thread, as I've been dissapointed with the results from my 2x teleconverter.
Should I be using 2x 1.4x converters instead?