Is it possible to get 4x focal length by attaching 2*2x teleconverters? How about a 1.4x to a 2x, for 2.8x total? I understand that the resulting lens would be slowed down proportionally as well. I'm specifically think of creating such a setup with the Canon EF 135mm f/2.0L and the EF 200 f/2.8L II lenses.
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1I know of a few Pentax guys that stack TCs, but I can't speak for Canon. In theory, the answer is yes, but it depends on the construction of the lens. – Joanne C Nov 01 '13 at 22:48
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I don't think it would work with the first party teleconverters from Canon simply because of how they are constructed. There is a rubber ring that sticks out into the lens which helps prevent bleeding from rays that aren't being used, but it also won't fit in the backside of the teleconverter. There also isn't any correction profiles for the electronic control that a camera uses when it has a Canon TC on it.
There shouldn't be any optical reason you couldn't do it with compatible third party converters though. A TC is basically just a magnifying glass that blows up the center part of whatever image enters it by the given factor. You'd have to control it manually probably at that point though and obviously it would have a very detrimental impact on the lens speed as you already observed. You'd also probably start hitting the limit of the resolution of the optics themselves at those kinds of magnifications.
AJ Henderson
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You'd also almost certainly lose AF. Though an f/2.8 lens with a 1.4x + 2x on a 1D or 5D3 would retain AF. – Chinmay Kanchi Nov 02 '13 at 20:07
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1@ChinmayKanchi - that's assuming it could survive the electronic connection across two TCs. – AJ Henderson Nov 02 '13 at 20:34
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A prime has a pretty nice image quality to start with, making it interesting indeed to challenge that IQ by adding teleconverters. For each converter to add, you degrade the quality, so I think there will be a (magnification gain / image quality reduction) factor to take into account, where the gain from getting a longer focal length versus chromatic aberration and blur. Here you can see examples of how much 1 and 2 extenders affect the IQ:
http://www.billmajoros.com/photoalbum/categories/new/Teleconverters/index.html
You should also note that AF works best at 2.8 and fine down to 5.6, so if you start with a pretty large aperture F2.8, you will still be fine, but if you instead start with a F4, teh first teleconverter will take you down to the F5.6 but the second will take you deeper and you have to resort to manual focus.
Finally, there are physical constraints. People note that Canon mk1 converters do not stack with mk2, and mk2 x1.4 and x2 has to be attached in a certain order. Canons versions are also known to physically not fit wide angel lenses, whereas Tamron pro converters do not have this problem. So maybe Tamron's will also stack better? (Ie. test which combinations work best before buying)
Michael Nielsen
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