6D and AF with TCs @ f/8?

So if the center af point on the 6D really goes down to -3 EV, shouldn't it work better autofocusing with a TC at f/8 compared to other Canon cameras?

Has anyone played around with TCs yet? I'm curious about the stated af sensitivites and performance compared to a 5D III or 1D series.

I don't know if the 6D can auto focus at f/8 with a TC. If it can one would think it should do better.

Let me get out my 70-300L and my 2x TC and I'll find out. It worked at 70mm and f4 (or 140mm f8), barely, on the 7D, but wouldn't AF at anything past that point. I haven't tried it on the 6D yet. Stand by.

Holy crap, it works!! This was shot with a 6D and a 70-300L at 300mm plus a Kenko 2x TC for 600mm and f11 and it auto-focused. I'm amazed. Now I can't wait to shoot some bird pics in sunlight. EXIF is intact if you have an EXIF reader plugin for your browser.

Wow very cool.

Yeah, it could be. I took some other shots and they all AF'd, but it's so dark and shooting at f11 I needed to get to 12800ISO, which screws up the clarity anyway. I'll do some more testing, though.

Ok, test #2. I found that if you just zoom all the way out to 600mm and f11 (with the Kenko 2x) and try to AF, it won't. But if you focus first at 140mm f8 and then zoom out, it will fine tune the focus at 600mm f11. You sort of have to get the focus in the neighborhood at f8 first, I guess, and then zoom and refocus. Still could be useful for bird and such. More testing tomorrow in sunlight.

Focus point was the pull chain.

Just tried it with my 400mm f5.6 + 1.4x TC and it DOES NOT focus at f8. Doesn't even give me the option of One Shot or AI Servo it just reverts to manual focus only.

SVTmaniac wrote in post #15464080
Just tried it with my 400mm f5.6 + 1.4x TC and it DOES NOT focus at f8. Doesn't even give me the option of One Shot or AI Servo it just reverts to manual focus only.

Canon TC? If so, then no, it won't AF because the Canon TC "tattles" on you and tells the camera not to AF. The Kenko doesn't tell the camera not to AF. My 24-105 f4L will AF all day long at f8 with the Kenko.

M_Six wrote in post #15464113
Canon TC? If so, then no, it won't AF because the Canon TC "tattles" on you an tells the camera not to AF. The Kenko doesn't tell the camera not to AF. My 24-105 f4L will AF all day long at f8 with the Kenko.

Yeah it's a Canon and I realized after I posted that you were using a non reporting TC. I got my hopes up thinking it had f8 autofocus. Oh well, I'd rather put the 400 on my 7D and still get more reach than using a TC on the 6D

It seems that serious contrast is key here. I tried to AF on low contrast areas at f11 and it just hunts. But it will lock onto high contrast targets even without "pre-focusing" as I mentioned earlier. The clock below is way in my kitchen in the dark and the camera locked onto it, albeit it slowly. The light fixture is a pretty serious crop just to show the level of sharpness at 600mm f11. Not surprisingly it isn't tack sharp.

M_Six wrote in post #15463912
Holy crap, it works!! This was shot with a 6D and a 70-300L at 300mm plus a Kenko 2x TC for 600mm and f11 and it auto-focused. I'm amazed. Now I can't wait to shoot some bird pics in sunlight. EXIF is intact if you have an EXIF reader plugin for your browser.

Interesting indeed! Thanks for testing it out, looks like the 6D might be a good option for wildlife photography.

Xcelx wrote in post #15462826
So if the center af point on the 6D really goes down to -3 EV, shouldn't it work better autofocusing with a TC at f/8 compared to other Canon cameras?

These are two things that aren't related. The ability to focus in low light is related to the sensitivity and signal to noise ratio of the AF sensor. The ability to focus at f/8 is related to the lenght of the baseline for the sensor, or in other words, the sensor has to have low enough accuracy to work with light entering a small hole in the lens.
You can make a sensor that can AF in reasonable daylight only, due to lack of sensitivity, but also can focus at f/11, due to sufficient lack of precision. You can also make one that can focus at EV -5 (perhaps), and still at f/11, if this high sensitivity is paired with low enough precision.
However, even if such a sensor does work reliably for focusing at f/11, it's probably useless already at, say, f/4, since its precision will be too low to ensure that focus is set within depth of field.

This doesn't say anything about the actual characteristics of the 6D, or any other camera, but just about the principle and physical characteristics of AF sensors.

To continue the discussion from last night, here are some shots I took today.

Here's what I've found after more testing. Using just the center AF point and trying to AF with the lens set at 300mm f5.6 (so 600mm f11 with the Kenko 2x TC), it would lock AF *if* the subject had high contrast. The first four images below show what I mean. All have definite contrasting edges. Then I switched to using all AF points and let the camera decide. I was able to lock onto anything regardless of contrast, although not always my exact intended focus point. For instance, in the pic of the window dormer on the brown roof, I was trying to get the dormer itself and the camera chose the higher contrast brown and white stripes of the building, so again, contrast plays a role. But in short, it will AF at f11 given enough contrast.

Of note here, none of these images is tack sharp, which is a consequence of using a 2x TC in general. They also have not been massaged in ACR or PS. Basically SOOC.