third party lenses and Canon AF lens groups

jd7

EOS R
CR Pro
Feb 3, 2013
877
242
Hi all

I recently acquired a 6DII, which as I understand it makes AF points flash in the viewfinder if a lens is using the AF point as a line AF sensor rather than a cross-type AF sensor (p138 of the manual). To my surprise, my Sigma 35 Art and Sigma 50 Art lenses cause all AF points to flash (red). I say to my surprise because I had jumped to the conclusion that a fairly modern wide aperture prime would be in Canon's Group A, even if it is a third party lens (and therefore not officially listed in Group A, of course).

I haven't tested my Sigma 85 EX yet, but I assume it will be the same as the Art lenses.

If the camera is using only a line sensor with the Sigma lenses, that would seem to be a potential explanation for Sigma lenses missing focus sometimes, particularly with off-centre AF points (on the assumption the centre point would still be at least a cross type with a Sigma lens - although maybe that's not right, as even if the centre AF point seems to be flashing on my 6DII). If the centre AF point is working as a cross type (and maybe it's not) but all other AF points are working as line sensors, perhaps it explains why the Sigma lenses seemed to get a reputation for focusing better on a 6D than an 5DIII, ie 6D users probably stuck to the centre point much of the time because the peripheral AF points were line sensors anyway).

Is what I'm seeing normal? Does anyone know what AF group the 35 Art, 50 Art and 85 EX fall into? Has everyone else but me known about this for ages?!? :)

EDIT - It seems I may misunderstand what is going on. Just did a quick test with the 35 Art and I only get the blinking AF points if I try to focus in low light (especially if focusing on a dark coloured object). For example, focusing on a light coloured wall with sun coming through the window I saw no blinking AF points, but when I pointed the camera at a black camera bag sitting on a dark coloured shelf in corner of the room, I get all AF points blinking as soon as I activate AF.

What am I missing?! I will have to investigate further.
 

jd7

EOS R
CR Pro
Feb 3, 2013
877
242
OK, I still haven't got it worked out fully, but I think the answer must relate to my settings for illuminating AF points. Will play around some more and see if I can get to the bottom of it.

If anyone can explain though, please do let me know!

Also, does anyone know what lens AF group the Sigma 35 Art and 50 Art, and the Sigma 85 EX, lenses are in? Am I correct in assuming they are effectively Group A lenses?
 
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