Canon 5D mark III & Dual cross type sensor with Tamron 24-70

[dswatson83]

Can anyone who owns a 5DIII and the new Tamron 24-70 f/2.8 VC confirm if the 5D recognizes the Tamron as a f/2.8 lens to use the 5 dual cross type focus points? Given that even the original Canon 24-70 mk1 does not, this had me a little worried.

[Mt Spokane Photography]

Canon will not recognize a Tamron lens at all.  What happens is that Tamron sends a lens code to the Canon body with the code from another Canon lens, likely the old Canon 24-70mm L.  It could be sending the code for the new 24-70mmL, but its unlikely.

[Marsu42]

It could be sending the code for the new 24-70mmL, but its unlikely.

I've asked the same question in multiple forums and here, too - no one even owning a 5d3+Tamron24-70 has come up with an answer yet, I guess it's because you simply cannot tell if an af point is cross or double cross?

[neuroanatomist]

You can tell if you test for it, with a subject (test target) that offers a contrast in only one orientation.

[Marsu42]

You can tell if you test for it, with a subject (test target) that offers a contrast in only one orientation.

Um, that might tell me if it's horizontal or cross, but that doesn't make a distinction between cross and double-cross? For that, maybe you'd have to do some tests on very low contrast subjects and see if the suspected double-cross af point does better than a known single-cross. But even then it's more like an educated guess.

[neuroanatomist]

Good point, although I still think careful testing could determine the difference since the line pairs are at 45-degree angles.

I wonder what numerical value is reported for the Tamron 24-70/2.8 VC in the LensID field of the full EXIF readout?

[Marsu42]

I wonder what numerical value is reported for the Tamron 24-70/2.8 VC in the LensID field of the full EXIF readout?

The Tamron reports 137, seems to be the same as the Sigma 24-70 since Lightroom confuses these both. What Canon lens equivalent is that (is there any list?)?

[Mt Spokane Photography]

Code 137 is the Code for a 85mm f/1.2L, so it should use the double cross.  Apparently, all the Sigma and Tamrons also use this code.
 
Here is a link to the list of Exif Codes.
http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/Canon.html

[Marsu42]

Code 137 is the Code for a 85mm f/1.2L, so it should use the double cross.  Apparently, all the Sigma and Tamrons also use this code.

Thanks for the Link. This means that the Tamron 24-70 (and all other Sigma/Tamrons) could perform better af-wise on the 5d3 than the original Canon 24-70 (and many other Canon lenses)?!

The question now is if the disabled double-cross sensors in "group b" are (just) a marketing ploy, or if there is a real technical reason too like much distortion and/or sharpness falloff. Seeing the Tamron's reviews, it could very well be that the top and bottom two double-cross sensors are enabled, but won't be precise so it's a draw to the Canon mk1 or even worse.

It is hilarious that most 5d3 reviewers don't even seem to bother to read the manual and tend to say that only the max. open aperture is the decisive data that enables/disables the 1dx/5d3's af points. So with this even more complicated issue, it might be next to impossible to tell how 3rd party lenses (f2.8 or other) really perform with the new af system.