Lenses listed in Group A, Group B, Group C... Why?

[drmikeinpdx]

http://downloads.canon.com/CDLC/EOS_1DX_AF_Guide_CDLC_Updated_June_2012.pdf   See page 37.

In the autofocus guide for the 1Dx and 5D III, Canon divides their lens lineup into groups that allow different numbers of cross type sensors to operate.  I thought this was strictly a function of aperture, but if you look at the lists, that does not seem to be the case.

Does anyone know what other factors might be involved? 

Thanks,

Mike

[neuroanatomist]

Macro lenses are 'downgraded' one or two groups, because of the light lost at macro distances.  The Group B lenses that don't activate all 5 dual crosses are all f/2.8 (not faster) all include a wide angle and most have substantial vignetting - the combination of distortion and vignetting mean poor coverage at the edges of the AF sensor, and that's where the four off-center f/2.8 crosses are located; I expect the 82mm front element of the new 24-70 II accounts for it being in group A. 

There are clearly other nuances at work, I have no idea why the 28/2.8 loses the outermost columns of f/4 crosses, or why the 800mm and 1200mm lenses start losing AF points at the edges. 

I suspect some of these assignments were not rationally predetermined, but rather were empirically determined - they mounted the lens, tested the accuracy of the AF points, and if outer ones were not reliable they were disabled in firmware by assigning the lens to a lower group.

[Mt Spokane Photography]

Since the new AF system was not around at the time the lenses were designed, I expect it was a trade off and overall performance of the AF system was given priority over a AF that was entirely compatible with every EF lens that Canon has ever made.
Its quite amazing that Canon's cameras and lenses have so much compatibility, Having had to deal with all the complex interchangability rules for Nikon lenses and bodies, its not fun.

[AvTvM]

interesting ... no wonder third party lens makers face difficulties matching the Canon lens protocol.

btw. I cannot see the EF40/2.8 pancake "STM" listed in any category - was curios, if/what limitations it has.

[drmikeinpdx]

Thanks for the interesting responses.  This forum has some very knowledgeable members!

[Stephen Melvin]

The various lenses that you'd think would make the cut due to their maximum aperture don't do so because of the location of the exit pupil.

The exit pupil is where the aperture appears to be when you look at the back of the lens.

In the case of macro lenses, the exit pupil moves as the lens focuses closer, reducing the apparent size of the exit pupil as seen from the back. A unit-focusing macro lens that's f/2.8 when wide open will be at f/5.6 by the time the lens is at 1:1.

Similarly, a lens such as the 1200mm f/5.6L has the exit pupil way up inside the tube, and becomes partially occluded from the outer AF points.

There is a logic to how lenses wind up in different categories.

[neuroanatomist]

Thanks, Stephen! 

I wonder how much is calculated, and how much is empirically-determined?  For example, the 40mm f/2.8 pancakeis down in group D (center dual cross only, no outer columns of f/4 points) - does that mean the virtual aperture is actually in front of the physical extent of the lens?

[Stephen Melvin]

Thanks, Stephen! 

I wonder how much is calculated, and how much is empirically-determined?  For example, the 40mm f/2.8 pancakeis down in group D (center dual cross only, no outer columns of f/4 points) - does that mean the virtual aperture is actually in front of the physical extent of the lens?

The 40 wasn't on the list last time I looked, because it wasn't yet out. Hrm, I can't think of why that one would be where it is. I think there was an old 35-135 (or something like that) which didn't make any sense, either. I'd have to see one of these pancakes to come up with a guess. Off the top of my head, the only thing that comes to mind is that it has something to do with it being a unit-focusing lens.

I don't think they need to figure these things out empirically in the year 2012. I'm pretty sure they know in the design stage.

[neuroanatomist]

Makes sense.  No, the 40/2.8 isn't on any lists...but since the 1D X shows you which AF points are cross type with a given lens mounted, I can see it's in Group D when I select an AF point. 

[Stephen Melvin]

Think about it in terms of angles. An f/2.8 AF sensor has a 20 degree spread, pointing to opposite edges of the exit pupil. An f/5.6 AF sensor has a 10 degree spread. Anything that can block one or more edges from the view of the AF sensor will cause it to fail.

[gtog]

For you all not seeing the EF 40mm f/2.8 STM (aka "shorty forty") listed in Group D, you may want to check out this web page that is more "up to date" than the 1DX AF Guide from April 4, 2012 (or the June update in the DLC):

http://www.canon.com.hk/en/product/catalog/productItemDetails.do?prrfnbr=200344

Enjoy!

G

[Rienzphotoz]

For you all not seeing the EF 40mm f/2.8 STM (aka "shorty forty") listed in Group D, you may want to check out this web page that is more "up to date" than the 1DX AF Guide from April 4, 2012 (or the June update in the DLC):

http://www.canon.com.hk/en/product/catalog/productItemDetails.do?prrfnbr=200344

Enjoy!

G

Thanks ... that is really good info.

[BobSanderson]

Where can I find the categorization of the lens into groups and the details of what that really means?

[neuroanatomist]

Where can I find the categorization of the lens into groups and the details of what that really means?

Either of the links in this thread. 

http://downloads.canon.com/CDLC/EOS_1DX_AF_Guide_CDLC_Updated_June_2012.pdf
http://www.canon.com.hk/en/product/catalog/productItemDetails.do?prrfnbr=200344

What is means is which AF points are available, and how they are active (horizontal line, f/4 cross, f/2.8 dual cross), with a given lens.  It only applies to the 1D X and 5DIII.

[AvTvM]

[quote author=BobSanderson link=topic=8829.msg161119#ms
What is means is which AF points are available, and how they are active (horizontal line, f/4 cross, f/2.8 dual cross), with a given lens.  It only applies to the 1D X and 5DIII.

I am not so sure if other/older Canon EOS bodies are not affected in very similar ways ... for all or most of the lenses in categories lower than A.