I was browsing B And H's website looking at teleconverters and I had a question. I have a Canon EF 75-300mm f/4-5.6 III lens and I shoot (or try to) a lot of wildlife photography with it but it just does not have the reach that I would like. I was hoping to find a teleconverter to use in conjunction with this lens. I was looking at the Canon teleconverters but from what the website said, they are only supposed to work with L-series lenses. I was wondering if this was actually true or can I use one of the teleconverters with my lens?
The Canon teleconverters are not limited to L series lenses only.
Canon teleconverters have a protruding front element, so they only physically fit lenses that have a large enough diameter at the rear and a recessed rear element that allow the TC to be fitted....
It just so happens most of the Canon lenses that can be physically fitted with the Canon TCs are L's. The 135/2 is the shortest prime Canon recommends using with either of their Extenders. Among the Canon zooms, only the 70-200mm (all versions) and 100-400mm are physically able to have the Extenders mounted.
Not listed by Canon, but I happen to know that their Extenders also can be used with their 45mm and 90mm Tilt Shift lenses, as well as the 100/2.8 USM macro, none of which are L-series lenses. There may be some others... those are just some I've personally tried out and can confirm are usable with the Extenders. I'm sure there are some third party lenses it's possible to use them with, too.
Third party teleconverters don't have the protruding front lens elements and will physically allow mounting a wider selection of lenses. However there are other considerations.
Earlier responses are correct.... with a 1.4X teleconverter, you lose 1 stop of light, while a 2X costs you 2 stops. Most Canon cameras are not designed to be able to auto focus with less than an f5.6 aperture (1D series, 1V and EOS-3 film cameras, and 5D MkIII can focus to f8). So if you put a 1.4X on an f5.6 lens, making it an effective f8 lens instead, auto focus will not work. If you put a 2X on an f4 lens, that becomes and effective f8 and, again, AF won't work.
Now, there are a couple workarounds.... you can focus manually, though your viewfinder might be rather dim to work with (try using Live View with Exposure Compensation enabled, to brighten things up... you also can magnify the Live View display in to check manual focus accuracy). Another trick is to tape up a couple of the electronic contacts on the teleconverter, so that the camera doesn't know it's there and will still try to auto focus. This might work in good light, though it will likely be slower and might hunt.
Another concern is image quality. Teleconverters generally give better image quality with prime lenses instead of zooms. Plus, stronger teleconverters (2X and 3X) will degrade image quality considerably more than weaker ones (1.4X and 1.5X). There are varying quality among teleconverters, too... just as there are among lenses. There are so many different possible combinations, you might find others who have tried particular combinations.... or not. Just be very specific about which teleconverter and which lens)es) you are asking about, to be sure you get truly helpful information. Still, you almost have to roll the dice and try it for yourself, to see if the results meet your expectations.
I use the Canon 1.4X II and 2X II Extenders. I'll use the 1.4X on a number of lenses (occasionally on 135/2, 70-200/4 IS or 70-200/2.8 IS, much more frequently on 300/4 IS, 300/2.8 IS or 500/4 IS). I mostly only use the 2X extender on 300/2.8 IS, occasionally on 500/4 (focusing manually). I don't find image quality of the 2X on 70-200/2.8 IS good enough for my purposes. I have heard that the newer 70-200/2.8 IS Mark II zoom and EF 2X Mark III Extender work together much better than the versions I have.
Someone suggested saving up for a 400/5.6L.... which is a good recommendation and an excellent lens. Personally I prefer the 300/4 IS and 1.4X combo, because it has IS making for a very portable, handholdable combo. Plus I like having two focal lengths on hand (300mm, plus when the 1.4X is added, 420mm).
Other options you might consider are the Canon 100-400mm zoom, or one of the third party zooms in the 120-400mm or 150-500mm range. There are other possibilites at much, much higher price (the EF 200-400/4L IS 1.4X, for example).