Why is the 5D3's AF system so sensitive to the lens you use?

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Yakim Peled
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p.1 #1 · Why is the 5D3's AF system so sensitive to the lens you use?

This AF system separates all EF lenses to 8 groups and only when you use lenses in group A can you exploit its full potential. Unlike all other AF systems which changed behavior only when the max aperture changed this AF system is sensitive to particular lenses (6 pages in the manual). It's not a matter of price, max aperture, discontinued or still in production, AF motor type or what have you.

Does any one have an idea why Canon designed this AF system this way?

Happy shooting,
Yakim.

Jun 15, 2012 at 09:37 PM

bboule
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p.1 #2 · Why is the 5D3's AF system so sensitive to the lens you use?

It's a little bit strange from the documentation but for a lot of these groups there is not much difference in actual functionality...

Pretty much anything f/4 or faster rocks on this camera. The super high precision stuff in "Group A" doesn't make much of a difference unless you're shooting at like f/1.2-f/2.5 so you're not missing much if you've got an f/4 or f/5.6 lens mounted.

Jun 15, 2012 at 09:51 PM

AGeoJO
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p.1 #3 · Why is the 5D3's AF system so sensitive to the lens you use?

bboule wrote:
It's a little bit strange from the documentation but for a lot of these groups there is not much difference in actual functionality...

Pretty much anything f/4 or faster rocks on this camera. The super high precision stuff in "Group A" doesn't make much of a difference unless you're shooting at like f/1.2-f/2.5 so you're not missing much if you've got an f/4 or f/5.6 lens mounted.

My sentiment exactly, Ben. There is no difference in functionaly that I can tell among various lenses I have in my lineup. The AF in that camera is zippy and very responsive. Maybe they just want to confuse us

Jun 15, 2012 at 09:58 PM

Yakim Peled
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p.1 #4 · Why is the 5D3's AF system so sensitive to the lens you use?

1. From my experience the difference between a cross-type AF point and a single-axis one is significant in low EV.

2. The question was why do you think Canon made the AF system lens sensitive in the first place.

Happy shooting,
Yakim.

Jun 15, 2012 at 10:23 PM

bboule
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p.1 #5 · Why is the 5D3's AF system so sensitive to the lens you use?

You need to get your hands on a 5D3 and try it for yourself.

There are 41 cross points. You rarely need to use the single-axis points.

There are also a bunch of "quad-axis" points.

When you go into AF selection mode the single-axis points flash. You can also tell the camera to completely disable them as well.

So you basically pick a cross point, and the camera focuses. And it gets it right an astonishingly high percentage of the time.

The only time I've ever even used the single-axis points is when I'm shooting action and I am telling the camera to use 5 points at once. If the single-axis point can't lock on the camera just uses one of the other points instead.

It works really, really well. It's also hard to see how it is "less sensitive" unless you're trying to use an f/8 lens without taping contacts.

Unless you're coming from a 1D or maybe a 7D the AF in this camera is "OMG" amazing.

Jun 15, 2012 at 10:29 PM

bboule
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p.1 #6 · Why is the 5D3's AF system so sensitive to the lens you use?

Whoops.. I think I misinterpreted you.

My guess is the reason some of the sensors require different apertures is just how much light the sensor is blocking..

The "quad axis" sensor (I forget what they call it, maybe it's "dual cross") is going to block a lot more light then the other sensors.

My understanding of AF is rudimentary but I think the more sensor strips you have in a given area the more light it's blocking, and so it needs a certain amount of light to get enough contrast to work?

Jun 15, 2012 at 10:41 PM

Yakim Peled
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p.1 #7 · Why is the 5D3's AF system so sensitive to the lens you use?

I agree that the AF system is great. What I don't understand is why it is so lens sensitive.

Happy shooting,
Yakim.

Jun 15, 2012 at 10:53 PM

skibum5
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p.1 #8 · Why is the 5D3's AF system so sensitive to the lens you use?

Yakim Peled wrote:
I agree that the AF system is great. What I don't understand is why it is so lens sensitive.

Happy shooting,
Yakim.

maybe because of widest point spread ever and great base-line for all the x-types?

Jun 15, 2012 at 11:12 PM

Pixel Perfect
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p.1 #9 · Why is the 5D3's AF system so sensitive to the lens you use?

skibum5 wrote:

maybe because of widest point spread ever and great base-line for all the x-types?

Yeah, I think that is the probable reason. This is the largest AF sensor ever, something like 19 x 8 mm. Slowers lens would not produce a reliable phase difference for the outer AF points.

It doesn't seem to me you lose too much though by not being in Grp A, unless you are in Grp H and really who's using either of those lenses on a 5D III anyway?

Jun 16, 2012 at 11:45 AM

Yakim Peled
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p.1 #10 · Why is the 5D3's AF system so sensitive to the lens you use?

Pixel Perfect wrote:
It doesn't seem to me you lose too much though by not being in Grp A, unless you are in Grp H and really who's using either of those lenses on a 5D III anyway?

Yeah, but I guess owners of the 800/5.6 would not be too happy that their lens is in group F, alongside a host of cheap kit lenses.

Happy shooting,
Yakim.

Jun 16, 2012 at 07:20 PM

speedmaster20d
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p.1 #11 · Why is the 5D3's AF system so sensitive to the lens you use?

In the new design Canon has been very conservative, given the AF coverage is very wide (the widest so far in any EOS camera) they have decided to disable the outer AF sensors for certain lenses due to light fall-off (vignetting) to ensure consistent performance in low-light conditions and given that camera is rated for -2EV.

Jun 16, 2012 at 07:36 PM

RobertLynn
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p.1 #12 · Why is the 5D3's AF system so sensitive to the lens you use?

Jun 16, 2012 at 11:01 PM

senzazn12
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p.1 #13 · Why is the 5D3's AF system so sensitive to the lens you use?

-2 is pretty impresive. Man oh man.

Nov 26, 2012 at 12:54 AM

timbop
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p.1 #14 · Why is the 5D3's AF system so sensitive to the lens you use?

speedmaster20d wrote:
In the new design Canon has been very conservative, given the AF coverage is very wide (the widest so far in any EOS camera) they have decided to disable the outer AF sensors for certain lenses due to light fall-off (vignetting) to ensure consistent performance in low-light conditions and given that camera is rated for -2EV.

What confuses me is the setting for the 100 USM macro - which is not known for vignetting or poor edge performance. Only the very center cluster is cross enabled, which is very strange. I have my cam set for "select cross only" so I can only use the center group with the f/2.8 100 macro, which is really disappointing considering the f/4 17-40 can use the other sensors.

Nov 26, 2012 at 04:40 AM

Yohan Pamudji
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p.1 #15 · Why is the 5D3's AF system so sensitive to the lens you use?

timbop wrote:
speedmaster20d wrote:
In the new design Canon has been very conservative, given the AF coverage is very wide (the widest so far in any EOS camera) they have decided to disable the outer AF sensors for certain lenses due to light fall-off (vignetting) to ensure consistent performance in low-light conditions and given that camera is rated for -2EV.

What confuses me is the setting for the 100 USM macro - which is not known for vignetting or poor edge performance. Only the very center cluster is cross enabled, which is very strange. I have my cam set for "select cross only" so I can only use the center group with the f/2.8 100 macro, which is really disappointing considering the f/4 17-40 can use the other sensors.

The 35L also vignettes quite a bit wide open yet it's still in Group A with all AF points go. I think the explanations so far have been plausible in theory but not when compared to the list of lenses in each group. It'd be nice to get an explanation straight from the horse's mouth. Thankfully all my lenses are either in Group A or are lenses where critical focusing isn't as important (17-40L), so it would just be to satisfy my curiosity

Nov 26, 2012 at 05:59 PM

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p.1 #16 · Why is the 5D3's AF system so sensitive to the lens you use?

My guess would be to sell more group A lenses or get people to upgrade. Hopefully the ML (Magic Lantern) patches will fix this problem so you can get all the points enabled on all lenses, or at least let us decide if we want to use them. If they really don't work, it will become quickly obvious but if they work fine, then it will be proof there is something else going on here...

Nov 26, 2012 at 06:45 PM

dwweiche
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p.1 #17 · Why is the 5D3's AF system so sensitive to the lens you use?

I'm no optical expert or lens designer, so I can only hypothesize. I would guess vignetting is one of the variables (it only makes sense that if the outer AF points of a lens see two stops less due to vignetting, your AF could be impacted), but there's got to be more to it than just that. Maybe the various optical aberrations that exist out near edges and corners impact the quality of the light hitting the AF sensor and that excludes some lenses from the lower groups...

Nov 26, 2012 at 07:12 PM

jorkata
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p.1 #18 · Why is the 5D3's AF system so sensitive to the lens you use?

The AF system identifies focus based on contrast.

I'm guessing that for certain lenses the 1DX/5DIII outer AF points fall into the zone where these lenses are not sharp enough - and thus there's not enough contrast for the outer AF points to identify focus.
Hence, the outer AF points are turned off for these lenses.

Overall, it appears that the outer AF points are turned on/off based on both lens aperture and MTF

Nov 26, 2012 at 07:42 PM

Pixel Perfect
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p.1 #19 · Why is the 5D3's AF system so sensitive to the lens you use?

Yakim Peled wrote:
This AF system separates all EF lenses to 8 groups and only when you use lenses in group A can you exploit its full potential. Unlike all other AF systems which changed behavior only when the max aperture changed this AF system is sensitive to particular lenses (6 pages in the manual). It's not a matter of price, max aperture, discontinued or still in production, AF motor type or what have you.

Does any one have an idea why Canon designed this AF system this way?

Happy shooting,
Yakim.

Well ignoring a few groups that have either junk lenses that no one on earth would use with their 5D III or a single lens, the main difference between the groups is that of f/2.8 lenses and f/4 or slower lenses. Of course you only get the maximum potential with the fast glass, but even f/5.6 lenses work dam well on the 5D III and you still get 21 x-type points.

Nov 26, 2012 at 09:18 PM

timbop
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p.1 #20 · Why is the 5D3's AF system so sensitive to the lens you use?

The 100/2.8 macro has to be an aberration - perhaps related to the fact that near focusing with the outer points is an issue. Otherwise, every theory (other than it being a scam) above falls apart given that the 100/2.8 macro only gets a small cluster of cross af points in the center and the 17-40 f/4 gets the outer points. The 100 macro is renowned for being sharp across the plane of focus even wide open and the 17-40 is weak at max aperture

Nov 27, 2012 at 12:17 AM

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