1Dx / 5D3 AF system limitations, a (perhaps stupid) question

[3kramd5]

So, per the DP review description,

"The 5D Mark III loses out to the Nikon D4 and D800 when it comes to use with slower lenses or long lens/teleconverter combinations, in that its cross-type points can only be used with lenses that are F4 or brighter (and the double-crosses with F2.8 and brighter lenses). Canon says there's a trade-off to be made and that its approach allows the sensor to be more accurate with the large aperture lenses it expects its customers to use, and allows the F4 cross-type sensors to be placed further towards the edge of the frame."



So, the middle AF point (and indeed much of the center column) is double cross type. All points in the center three columns are either cross or double-cross points.

Does that mean that, if I'm using an f/5.6 lens (say 100-400 at max focal length), I would only be able to use points from columns 1, 4, 8 and 11? Or will the higher accuracy points function with less fidelity? Can I not use the center point with my 25-105?

Seems kinda foolish to force focus off center for slower lenses.

[Mt Spokane Photography]

That display on DPR is pretty confusing, there are three charts in one, and you use the buttons underneath to select them.

Go back to DPR and click on the three buttons under the chart to see which points are active as just horizontal points(white),  cross type points(Orange), or double cross type points(blue with x) with the three types of lenses.  The double cross points work with the wide primes because they need more accuracy to get accurate focus.

http://www.dpreview.com/previews/canoneos5dmarkiii/4

All 61 points work with all canon lenses f/5.6 or brighter, and 21 of them are cross type points..

1. f/2.8 or brighter



2. with lenses f/2.8-f/4



3.  With lenses f/5.6 or brighter.

[3kramd5]

A'ha.

Thanks.

That would have been a deal breaker.

[neuroanatomist]

Not at all.  Every one of the 61 points is single orientation-sensitive (horizontal lines) at f/5.6.  The middle three columns are f/5.6 crosses (with the central column of dual crosses also having a superimposed diagonal f/2.8 cross, which is what makes them dual crosses).

So, with most f/5.6 lenses, you have 21 cross-type points (middle 3 columns) and 40  lines.  With most f/4 lenses, you have 41 cross-type points (all the orange and blue ones in the diagram) and 20 lines, and with most f/2.8 lenses you have 5 f/2.8 (more accurate) crosses, plus 36 other crosses (all the orange) plus 20 lines. 

So, almost all f/5.6 and faster lenses use all 61 points, what varies is the number of those points that act as crosses vs. lines. 

All 61 points work with all canon lenses f/5.6 or brighter

Not quite.

I'm using 'most' because there are some exceptions. For example, the 24-70mm activates only the center f/2.8 cross, not all five, despite being an f/2.8 lens, the 100mm f/2.8L macro lens doesn't activate any f/2.8 crosses (but still uses all 41 regular crosses + 20 lines).  I say 'almost all use 61 point' because some very old zoom lenses, and the insanely expensive 800/5.6L IS, use only 47 total (the left/right sides drop off), and the 180L Macro only uses the central 33 of the 61 points.

Full details on which lenses activate which AF points can be found on this Google-translated page

That was probably way more than you wanted to know...   

[3kramd5]

^^

Yes, but it's interesting nonetheless. Thanks for the link.

How does the camera know? By communicating with the lens? Begs the question as to what happens when you strap on off-brand glass...

[neuroanatomist]

^^

Yes, but it's interesting nonetheless. Thanks for the link.

How does the camera know? By communicating with the lens? Begs the question as to what happens when you strap on off-brand glass...

Yes, it communicates with the lens.  That's how the lens name ends up in the EXIF data. The 1D X and 5DIII take it a step further - they read not only the lens type, but the serial number too (for AF microadjustment, so a studio with multiple copies of the same lens can have them all adjusted individually on each body).

Off-brand glass is reported in the EXIF too, at least the ones that AF. But that's led to issues in the past, since Tamron and others spoof the AF system sometimes (so their f/6.3 lenses look like f/5.6 and AF works).  To do that, they report the right name, but use a mismatched lens code corresponding to an old, rarely used Canon lens. It's resulted, for example, in some Tamron lenses failing to activate the outer AF points on the 40/50/60D as crosses, they act as lines instead.  Relevant to this discussion is that those old lens codes used by Tamron are codes for some of the lenses in the 'F group' on that linked page, the ones that only activate only 47 of the 61 points...

One nice feature of the 5DIII/1D X transmissive VF display is that it will only show you the AF points that are active with the specific lens you're using.