2 x extender autofocus

[BryanShaw1]

I have just purchased a used Canon 2 x extender II.

When fitted to my 7D, the Q screen automatically changes to MF MANUAL FOCUS.

Is the extender faulty?

[BryanShaw1]

would the combination work with a EOS R?

[ebiggs1]

It will not even mount with out an adapter. 

[ebiggs1]

The bottom line is it is not a good idea with such a slow lens. Plus a lens that doesn't have the IQ, in the first place, to give up to the tel-con even if you could get it to work.

[Waddizzle]

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@BryanShaw1 wrote:

would the combination work with a EOS R?

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It might, but just not very well.  Mirrorless cameras behave similar to Live View would in a DSLR.

Let''s take a step back to your 7D for a moment, though.  The 7D User Guide should include a section about "Lens Groups", which is where the information is located where I based my previoious comments regarding camera/lens compatibility.

When you add an extender, this reduces the amount of light reaching the image sensor, and more importantly the amount of light that is reaching the AF sensor assembly.  The User Guide explains and describes AF points in terms of their sensitivity to various aperture settings, which is way of describing the minimum amount ot light needed for an AF point to function most accurately.

When you add the 2x teleconverter to slower lenses, you will be significantly reducing the amount of light that will be reaching the AF sensor assembly.  Can the AF sensor assembly function with reduced light?  At a guess, I would think it that it could, but it just won't anywhere near as accurate as without it.  

And so, the camera's firmware disables the automatic AF system when the maximum lens aperture reduces to a certain level.  But, if you switch the lens to MF, and use One Shot AF mode in the camera, don't be surprised if the camera does not beep an AF lock as you try to manually focus something as you hold the shutter half pressed.  I have focused on the Moon with my T5 many times this way as I focused in Live View.

So, what would happen with a mirrorless camera like the EOS R?  You should expect a reduction in the number of available AF points, just as what would happen with a DSLR.  

Canon has released 600mm and 800mm lenses with fixed f/11 apertures. What they don't tell you is that the cameras will only have active AF points in the center area of the viewfinder, giving you AF point coverage in the EVF similar to what you might have in a DSLR OVF.  I suspect this reduction in "edge to edge" AF point coverage could be due to vignetting in the lenses around the edges of the frame, which naturally means reduced amounts of light reaching the AF system.

[BryanShaw1]

Grateful for that comprehensive reply.

A couple of points:

When I switch the lens to manual focus, the camera still remains at 'MF' in the Q menu.

Years ago I bought a cheap 2 x from Jessops so I thought I'd try it. Interestingly, the camera now allows autofocus settings.

[jrhoffman75]

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@BryanShaw1 wrote:

Grateful for that comprehensive reply.

A couple of points:

When I switch the lens to manual focus, the camera still remains at 'MF' in the Q menu.

Years ago I bought a cheap 2 x from Jessops so I thought I'd try it. Interestingly, the camera now allows autofocus settings.

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Yes, some third party tele-extenders do not report back the "adjusted" f/stop, so the camera does not disable AF.

But, the reason Canon disables the AF feature beyond either f/5.6 or f/8 (depending on the camera) is that the reduced light can really slow down the AF speed/capability. Canon wants to maintain the user experience.

[kvbarkley]

"It might, but just not very well. Mirrorless cameras behave similar to Live View would in a DSLR."

Actually, it works much better. The R's have phase detection sites on the sensor which work much better than the contrast detection that DSLR's normally use.

That is why the RF 800 and 600 mm F/11 lenses can autofocus.

Ken Rockwell hooked up an EOS R to an EF100-400, a 1.4X teleconvertor, a 2 X teleconverter and an EOS to RF adapter to his R and it autofocused fine.

https://www.kenrockwell.com/canon/eos-r/r.htm

[BryanShaw1]

Wow, that changes things! - It was just about to go on ebay.

[Waddizzle]

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@kvbarkley wrote:

"It might, but just not very well. Mirrorless cameras behave similar to Live View would in a DSLR."

Actually, it works much better. The R's have phase detection sites on the sensor which work much better than the contrast detection that DSLR's normally use.

That is why the RF 800 and 600 mm F/11 lenses can autofocus.

Ken Rockwell hooked up an EOS R to an EF100-400, a 1.4X teleconvertor, a 2 X teleconverter and an EOS to RF adapter to his R and it autofocused fine.

https://www.kenrockwell.com/canon/eos-r/r.htm

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Dual Pixel AF uses phase detection.  It always has.

https://www.usa.canon.com/internet/portal/us/home/learn/education/topics/article/2018/July/Intro-to-... 

[CaliforniaDream]

I have an R5 and with my 600mm F11 with a 2X extender and good light it auto focuses quite well and very fast.  That same combination except with my 100-500  and 2x snaps into focus  fast enough to capture BIF's.  Amaizing camera and lens.