amfoto1 wrote:
You CANNOT use any of the Canon teleconverters (they call them "extenders") with that lens. The Canon TCs all have a protruding front element that has to fit inside the rear barrel of any lens. The 70-300mm lenses do not have large enough diameter to allow those TCs to be fitted. There are compatibility charts all over the place.
You MIGHT be able to use a third party TC. Among those, one of the sharpest I'm aware of is the older Kenko 1.4X MC-4 DGX. It is a little soft in the corners, but using it on an APS-C camera such as 7DII crops away those corners and leaves just the sharpest center! There was a more expensive Kenko "Pro 300" version 1.4X too, which seems very good too, better in the corners, but not quite as sharp in the center. It might be a better choice for a full frame camera. I really don't know how these compare to the more current Kenko "HD" teleconverters (I do know these are among the very few or only ones that accommodate EF-S lenses... the Canon TCs don't... but there really aren't many EF-S lenses that it would make sense to use with a TC, anyway... maybe the EF-S 55-250mm).
Problem with ANY TC is that you "lose light" to the additional optics. Effectively, the maximum aperture of the lens is made smaller. With a 1.4X TC you lose one stop. With a 2X you lose two stops. With a 1.4X your 70-300mm will "become a 98-420mm f/6.3-f/8" combo. This is okay on 7DII because it is "f/8 capable". Not all Canon cameras are able to autofocus a lens with less than f/5.6 max aperture. Your camera can... BUT it will be limited to the center AF point ONLY. And autofocus will be likely to be slower and struggle more in less than ideal lighting conditions.
It varies from lens to lens and TC to TC, but there is usually some loss of image quality using a TC on a lens. How much really depends upon the specific combo. I use Canon 1.4X II and 2X II TCs on certain lenses and on most see a slight loss of IQ with the 1.4X and a bit more with the 2X. I use the 1.4X on more lenses than the 2X, partly because of this and partly because of lack of autofocus with the 2X on many more lenses. So I basically just use the 2X with a couple premium prime lenses.
Image quality really takes a hit in some cases. In others it's fine. For example, I occasionally put my 1.4X on Canon EF 100-400mm "II" and there is little to no noticeable loss of IQ... which is unusual for a zoom. But I try to avoid that same TC on 70-200mm, where the combo makes images that are only marginally usable IMO. It's even worse with the 2X... I simply don't use it on any zooms (I haven't tried it with the 100-400 II, because there would be no autofocus and even the viewfinder would be dim to try to manually focus).
I've also often used the 1.4X II on Canon EF 300mm f/4L IS lens, where it works very well as a "420mm f/5.6" combo. It isn't quite as sharp as the EF 400mm f/5.6L lens... But that lens lacks IS, while the 300mm f/4L has it and it's still reasonably effective even with the TC. Since that (and the 100-400) is a lens I like to shoot hand held a lot, I really appreciate any help I can get from the IS.
I can't say that the Kenko 1.4X mentioned above will or won't work as well with your particular lens. It might be fine... or it might produce poor image quality. Or maybe something in between. Maybe you can find someone using the specific teleconverter you're considering with the same lens you want to use it upon, and can get a report from them. Of course, even that can be marginally helpful, because it will just be their opinion and depends upon their specific uses and how demanding they are with their gear. "Professional" reviewers like Bryan Carnathan at The-Digital-Picture.com won't be any help, because they don't test lenses like yours, that "aren't compatible" with TCs, according to Canon. He does test shots with many lens/TC combos... but none with that lens because it's "not designed for use with a TC".
You might have to just give it a try and see for yourself how it works. Maybe make sure you buy from a place that allows returns? It's a pretty good bet that a longer lens without any TC - like the EF 400mm f/5.6L or 100-400mm II - would give you better results than any 1.4X you might fit to your 70-300mm. But the real question is if the smaller, lighter and cheaper option of a TC might be up to your particular requirements.
The image below was shot with EF 100-400mm II and Canon EF 1.4X II teleconverter combo (effective 140-560mm f/6.3-f/8) at 560mm and f/8 (wide open) on 7D Mark II. Be sure to click through to the larger size, but if you further click to the maximum magnification, the image may look a little "too sharp" because this was prepared at 300 ppi for printing purposes and may "over enlarge" on a computer monitor. I didn't bother sizing it down for online display.
You CANNOT use any of the Canon teleconverters (th... (
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I appreciate your detailed response, Alan. My options appear limited and I am inclined to just get a new/used lens. At the moment, I am considering the EF 100-400mm II.