The "S" in EF-S stands for "Short backfocus". The rear-most element on an EF-S lens sits farther back than it does on an EF lens... and typically proturdes just slightly beyond the lens mounting flange. The lens is designed to mount on something with a hollow cavity to accomodate the short backfocus. Teleconverters aren't unlikely to provide enough space.
But that's just the "tip of the iceberg".
When you use a teleconverter, you have to multiply both the focal length AND the focal ratio by the teleconverter factor. It's the focal ratio that can be a problem. I'll use an example to illustrate why.
Suppose you have a Canon EF-S 18-135mm f/3.5-5.6 lens and, hypothetically, you found a 2x teleconverter for it. This would convert the lens into a 36-270mm (that part makes sense) lens with an f/7-11 focal ratio range. Notice how I had to multiple the focal ratios?
The "problem" with this, is that camera generally cannot auto-focus at high focal ratios. The Canon 1D X and 5D III can both auto-focus at f/8... but that's it. No other Canon camera can do this (and it's the same for Nikons... most cannot). They're designed to auto-focus up to f/5.6 and there are 3rd party lenses where f/6.3 might be the best you can do... and AF works on them -- but you're really pressing the limits at that point. This means you might get the lens to focus at f/7... but that's only available when you're zoomed all the way "out" to the 36mm end. As soon as you zoom IN (and the whole point of the teleconverter is to let you zoom in more becuase the lens would have provided 36mm natively without using the 2x teleconverter) you're jumping up beyond f/8 where there's absolutely no way the AF system is going to work.
The focal ratio limits for the AF system aren't arbitrary. If I hand you a 35mm film camera with manual focus and a split prism focusing aid in the viewfinder, but dial the focal ratio down to f/8 and have you try to focus with it, what you'll discover is that you can get one half of the split prism "moon" to show the image but the other half will be black. Barely move your eyeball and they'll reverse... the other half is visible but the first half is now black. It's near impossible to get both halves visible at the same time to allow focus because of the way the prisms work (even manually).
Teleconverters work best on lenses that have low focal ratios to begin with... e.g. f/2.8 lenses when using a 2x teleconverter (which turns it into an f/5.6 lens) or f/4 lenses if using a 1.4x teleconvert (which also turns it into an f/5.6 lens).
But even after all of that... the teleconverter will noticeably degrade your optical quality. It's never as good as if you didn't have to use a teleconverter.
I have a Canon EF 300mm f/2.8L IS USM lens... that was a pretty expensive piece of glass (I think it was around $4k). If I put a 2x teleconverter on it (and I can -- it works)... I get optical quality out of it that looks like I bought a $200 lens.
Part of the reason this happens is teleconverters are a bit generic. They aren't tuned for one specific lens. Canon makes an $11k lens with a built-in 1.4x teleconverter and it's remarkable. But that built-in TC was optimized and tuned for that one lens only.
Tim Campbell
5D III, 5D IV, 60Da
Especially the hand held one at 1/8th. I am very interested in seeing it.
