The Canon EOS 80D offers enough performance value and “bang for the buck” that to many advanced amateur shooters, it’ll be a compelling possible purchase. One area that more and more experienced D-SLR shooters are asking about is use of telephoto lenses with Canon EF tele extenders, and the EOS 80D opens some new ground here.
The 45-point AF system in the EOS 80D adds the ability to focus at effective maximum apertures as slow as f/8 — a big departure from previous mid-range EOS cameras, where AF was possible only until maximum apertures of f/5.6 were reached. This is going to open a lot of potential to wildlife and nature photographers, as well as sports and other shooters who are looking for ways to leverage telephoto imagery. Coupled with the camera’s 7 fps shooting capability, this makes the EOS 80D a very cost-effective option for good performance along with relatively compact size.
We’ve explored this question in previous articles on Canon USA’s on-line Digital Learning Center, but it bears a quick repeat, especially since a camera like the EOS 80D will likely attract many users new to either D-SLRs in general, or new to mid-range cameras with these kinds of advanced features.
The AF systems in the vast majority of today’s digital SLRs, including all Canon EOS models to date, are designed to work with lenses of varying maximum apertures. This holds true for wide-aperture, “fast” lenses with low f-numbers (think of lenses like the EF 50mm f/1.8 STM, a high-end lens like the EF 85mm f/1.2L II USM, or a zoom like the EF 70–200mm f/2.8L IS II USM). And, with lenses having smaller maximum apertures, the AF system continues to function — until you reach lenses where f/5.6 is the widest possible aperture.
Canon’s AF system can accommodate virtually all EF or EF-S lenses in the system with maximum apertures up to f/5.6. In fact, as of early 2016, there are no lenses in the Canon EF or EF-S system with maximum apertures slower than f/5.6.
But that changes if you add a tele-extender to any compatible Canon lens. We’ve written a pair of articles that detail how Canon tele extenders operate, and about the issue of light loss when extenders are used. These articles can be found here:
- Changing the character of lenses with Canon EF Extenders: Part I
- An inside look at Canon EF and tele extenders: Part II
The key point here is that there’s an inevitable light loss of one stop in effective maximum aperture if a 1.4x extender is combined with a compatible Canon EF telephoto lens, and a two-stop light loss if a 2x tele extender is used. These are unavoidable physical losses in light transmission, and cannot be overcome optically.
In real life, this means if you have a lens with an f/4 maximum aperture (example — the EF 70–200mm f/4L IS zoom lens), if you add a 1.4x extender, its effective maximum aperture becomes one stop slower — f/5.6. And, if you add a Canon-brand EF 2x extender, it now slows down to an effective maximum aperture two stops slower — now it’s f/8.
The bottom line is that users who contemplate using tele extenders have always had to consider whether adding that extender made their lens cross that f/5.6 maximum aperture threshold, and if so, many previous cameras (such as the previous EOS 70D model) would shut down AF completely. But the new EOS 80D’s updated AF system can now provide AF if the maximum aperture is f/8 or wider.
Just to clarify: AF systems on digital SLRs are only concerned with what the maximum effective aperture of a lens, or lens + extender, is. You’re completely free to take pictures at smaller apertures, like f/16 or f/22 if you want the effects of a small opening, and this has no impact on the lens’s ability to focus! With the EOS 80D, as long as your lens + extender’s maximum effective aperture is f/8 or wider (in other words, a lower f-number), AF will function as described below, and you can take actual pictures at any final shooting aperture you want to with no further effect on AF performance.
The reason? All AF operations occur with the lens in a wide-open position, regardless of the aperture you’ve set manually, or that the camera may set automatically when actual pictures are taken. This is standard operating procedure for AF with all Canon EOS digital SLRs, when focusing using the eye-level viewfinder and the camera’s independent AF sensor.