Posted by Carol Nichols on March 16, 2012 at 1:13 PM
I am seriously considering the purchase of the new Canon 5D Mark III, but have read in several places that it cannot autofocus at f8. I know this has something to do with extenders, but other than that I don't know what people mean when they make this adverse comment about the camera. (I believe it also applies to the new 1DX body.) I would greatly appreciate it if someone could explain this to me.
Posted by Keith Bauer on March 16, 2012 at 2:42 PM
Carol: What this means is that if you have lens or lens/teleconverter combination that yields f/8 as the largest possible aperture, the body will not autofocus. Example: 500mm f/4 with a 2x converter yields a 1000mm f/8 lens, or you have a 400mm f/5.6 lens with a 1.4x converter, yields a 560mm f/8 lens. Neither of those combinations will autofocus. This has been the case with Canon bodies in the past, and now, with the exception of the 1D series bodies. At f/8, they will autofocus, but only using the central autofocus sensor, and autofocus speed is diminished. The newly announced 1DX reportedly will not autofocus at f/8 either although there has been a lot of complaining and Canon has indicated it might be possible via a firmware update. I would not count on that. If you are not in the situation where the largest aperture your lens can produce is f/8 then you have nothing to worry about. I don't own any Canon bodies that currently support autofocus at f/8 and have not found it to be an issue. Hope that helps.
Posted by Carol Nichols on March 17, 2012 at 10:06 AM
Thanks for your explanation Keith. So my longest lens is a 300mm f/4 + 1.4TC which yields 420mm f/5.6, so that should work right?
Posted by Keith Bauer on March 17, 2012 at 10:17 AM
Hi Carol. Yes your 300mm f/4 will work just fine with your 1.4x converter.
Does this mean that any time I shoot at above f8 with my Canon XSi, no matter which lens, it will not autofocus? For example my 24mm-105mmL f4 with a 2x converter will not auto focus at all?
Posted by Richard Ditch on March 24, 2012 at 3:49 PM
Gee, doesn't Canon cover this sort of stuff in the user manual with every camera and lens like Nikon does?
This is from the manual for my Nikon D300 compatibility chart for auto focus (the S and C in the first column represent Single Shot and Continuous AF modes).
I've highlighted the info that indicates AF only works with lenses or lens/converter combos with a maximum aperture of f/5.6 or faster.
Posted by David Bostedo on March 24, 2012 at 9:00 PM
Does this mean that any time I shoot at above f8 with my Canon XSi, no matter which lens, it will not autofocus? For example my 24mm-105mmL f4 with a 2x converter will not auto focus at all?
Seems like there's still some confusion.... here's a little more info on how your SLR camera uses a lens with electronic aperture :
When you look through the viewfinder of your camera lens, the aperture is wide open - to the widest setting (smallest f number) no matter what you have your exposure setting on. For instance, if you have an f/4 lens, and you are shooting at f/16, when you look through the viewfinder, the lens is at f/4. (One exception - If you use a DOF preview option, then the lens will go to f/16.)
When you press the shutter button, the camera will use the auto-focus sensor to focus the camera, again at the widest possible aperture for the lens. That way the maximum amount of light will hit the sensor, allowing for better autofocus.
Once the camera is done auto-focusing, the viewfinder will blank, as the mirror will flip up out of the way of the main sensor, and the lens will stop down to your chosen shooting aperture (in this example f/16). When the photo is done, the lens will open up to maximum aperture again, and that's what you see in the viewfinder again.
So if your shooting aperture is f/16, the lens is only there momentarily, right when the photo is being taken.
Since autofocus always occurs at the lens' widest aperture, cameras generally autofocus better with wider maximum aperture lenses. And as the maximum aperture gets smaller, eventually there isn't enough light for the autofocus sensors to work well or properly.
Canon, on their "consumer" bodies, has therefore chosen to disable autofocus if the lens' maximum aperture is f/8 or smaller (f/11, f/16, etc.) Note that they don't have to do this, but they've long chosen to simply not allow users to attempt it and get awful results. (Nikon has tended to just let the camera autofocus get bad in these cases.)
On Canon pro models, the better autofocus sensors have meant that they allow the camera to autofocus with a lens with a maximum aperture of f/8. Until the 1DX, where they are supposedly going to disable it (although speculation is rampant that they'll find a firmware fix to allow it).
So to the original question, a 24-105L f/4 lens with a 2X teleconverter will have a maximum aperture of f/8 and will not autofocus on any of the "consumer" or "prosumer" Canon bodies. It WILL autofocus on the 1D series camera, but possibly not on the 1DX.
And, since the maximum aperture, and NOT the shooting aperture is what matters for autofocus, of course you can shoot with any lens at smaller apertures than f/8 - probably to f/22 which is as small an aperture as most DSLR lenses go.
Posted by Carol Nichols on March 24, 2012 at 9:45 PM
David, thanks so much your thorough, and understandable, explanation.
Discussion
7 repliesThis is from the manual for my Nikon D300 compatibility chart for auto focus (the S and C in the first column represent Single Shot and Continuous AF modes).
I've highlighted the info that indicates AF only works with lenses or lens/converter combos with a maximum aperture of f/5.6 or faster.
Seems like there's still some confusion.... here's a little more info on how your SLR camera uses a lens with electronic aperture :
When you look through the viewfinder of your camera lens, the aperture is wide open - to the widest setting (smallest f number) no matter what you have your exposure setting on. For instance, if you have an f/4 lens, and you are shooting at f/16, when you look through the viewfinder, the lens is at f/4. (One exception - If you use a DOF preview option, then the lens will go to f/16.)
When you press the shutter button, the camera will use the auto-focus sensor to focus the camera, again at the widest possible aperture for the lens. That way the maximum amount of light will hit the sensor, allowing for better autofocus.
Once the camera is done auto-focusing, the viewfinder will blank, as the mirror will flip up out of the way of the main sensor, and the lens will stop down to your chosen shooting aperture (in this example f/16). When the photo is done, the lens will open up to maximum aperture again, and that's what you see in the viewfinder again.
So if your shooting aperture is f/16, the lens is only there momentarily, right when the photo is being taken.
Since autofocus always occurs at the lens' widest aperture, cameras generally autofocus better with wider maximum aperture lenses. And as the maximum aperture gets smaller, eventually there isn't enough light for the autofocus sensors to work well or properly.
Canon, on their "consumer" bodies, has therefore chosen to disable autofocus if the lens' maximum aperture is f/8 or smaller (f/11, f/16, etc.) Note that they don't have to do this, but they've long chosen to simply not allow users to attempt it and get awful results. (Nikon has tended to just let the camera autofocus get bad in these cases.)
On Canon pro models, the better autofocus sensors have meant that they allow the camera to autofocus with a lens with a maximum aperture of f/8. Until the 1DX, where they are supposedly going to disable it (although speculation is rampant that they'll find a firmware fix to allow it).
So to the original question, a 24-105L f/4 lens with a 2X teleconverter will have a maximum aperture of f/8 and will not autofocus on any of the "consumer" or "prosumer" Canon bodies. It WILL autofocus on the 1D series camera, but possibly not on the 1DX.
And, since the maximum aperture, and NOT the shooting aperture is what matters for autofocus, of course you can shoot with any lens at smaller apertures than f/8 - probably to f/22 which is as small an aperture as most DSLR lenses go.