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Author Topic: 6D autofocus capabilites: let's bust some myths about it  (Read 194520 times)

Sporgon

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #285 on: April 21, 2014, 06:05:24 AM »
This really improves af w/ fast lenses? I don't have lenses faster than f2.8, but reading the specs the outer points only have f5.6 precision and are so near the f2.8 center point that my guess was that focus and recompose still is better than using the outer points? This is the reason I wouldn't buy a fast prime for the 6d with the intent to af, but I'm happy to stand corrected.

Yes indeed. I think f5.6 precision is misleading - it's requiring f5.6 to function. It will then not be as accurate ( in theory) as the f2.8 'precision' sensor.

Here are four consecutive shots taken with the 85 f1.8 at 1.8, three of which using some of the outer points. The contrast and lighting is good but it has got it bang on each time. As a comparison the last frame is with live view. These are 50% enlargement of a medium jpeg.

When using the single orientation outer points you must consider the detail orientation of the target. If this is done then I find the outer points very good in suitable conditions.

Don't let the outer points of the 6D stop you from getting a faster lens !

Just an addition: the distance here was 1 metre, so at f1.8 the dof was 10mm. At f5.6 the dof would have been 40mm, and f2.8 would have given 20mm. So you can see that this is concrete proof that the '5.6 precision' points give much greater precision than f5.6.
« Last Edit: April 21, 2014, 06:51:42 AM by Sporgon »

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #285 on: April 21, 2014, 06:05:24 AM »

Sporgon

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #286 on: April 21, 2014, 06:12:16 AM »
If you go into the 6D's custom settings (Magic Lantern not required), you can set it up so you don't have to press the AF-point selection activation button (or whatever it's called - I assume you mean the button near the top right corner of the camera when you have you eye to the viewfinder) to select an AF point.  You can set it so you simply press the 8 way switch/dial to move the AF-point around.  It's still a very sad second best to having a joystick (like on a 7D/5D series/etc), but it's an awful lot better than having to press the AF-point selection button first every time you want to move an AF point.

Aaargh ! Why was this hidden in the 'Custom Button' function and not 'Custom AF' !?

Thanks jd7, that's a big improvement. I might have time to start complaining about the outer points now  ;)

As someone said' "CR is the manual" !

3kramd5

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #287 on: April 21, 2014, 09:56:01 AM »
and I suspect if being able to AF at -3EV gave a significant practical benefit in many circumstances, it would have found its way into the 1Dx and 5D3 (although I suppose it's just possible it wasn't ready in time for those cameras but it was ready in time for the 6D).

Maybe the sensor needs to be physically larger to be that sensitive and doesn't lend itself to a dual cross type configuration. But I think practicality probably has more to do with it. Someone spending money for a sports/bird AF unit likely won't be impressed by the capability to AF in a situations where long exposures and tripods are required - just manually focus using LV at that point.

For all that though, I suspect the 6D's AF is probably "good enough" for many people much of the time. 

No doubt. There are thousands of great images taken with 5D2 AF, or older, or manual focus cameras. The 1Dx / D4 AF capabilities are nice-to-haves, not need-to-haves. They no doubt improve hit rate, but aren't strictly necessary.

Marsu42

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #288 on: April 21, 2014, 02:47:43 PM »
Yeah, the 6D doesn't handle subjects moving towards or away well at all, especially towards, probably more noticeable due to the shallower DOF.  Side to side is OK.

Did you try all af options? "Release priority" is the certain way to doom, and in my experience "Tracking sensitivity" responsive is better for side movement while "locked on" does better for forwards/away movement.

Don't let the outer points of the 6D stop you from getting a faster lens !

Thanks for the sample, I'll keep that in mind - though in my current experience the non-cross hunting of the outer points is enough reason not to use them on anything that moves or is low-contrast.

No doubt. There are thousands of great images taken with 5D2 AF, or older, or manual focus cameras. The 1Dx / D4 AF capabilities are nice-to-haves, not need-to-haves. They no doubt improve hit rate, but aren't strictly necessary.

Nearly nothing is strictly necessary, but when having the opportunity to take one non-repeatable keeper shot (i.e. hit rate either 100% or 0%) and your af fails that's frustrating on a camera in this price category - esp. since if the 6d af happens to hit you get a taste what could be done.

If you go into the 6D's custom settings (Magic Lantern not required)

Not quite, w/o ML you cannot use the SET button to quickly switch back to the center point (like on 60d for example) - I consider this to be essential in a 1pt af camera with some outer gimmicks. Also the ML version doesn't need you to select the af point and then focus, but instantly focuses once you selected the point  - though the latter is a matter of preference.

Sporgon

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #289 on: April 21, 2014, 03:26:20 PM »
This whole subject of the 6D outer points has piqued my interest now. This afternoon I tried shooting moving and static subjects in both 'one shot' and 'servo' using just various of the outer points. Light was reasonable, EV 7 to  8, so not dark but not really bright either.

I put together a few shots, and you can see that the camera hits focus using a 50mm 1.4 at f1.6 - 1.8 very accurately with the outer points. I don't know what the engineering and physics are of the 'f5.6 sensitive points', but I can show you that these points nail focus on an appropriate target. "Appropriate' being the key word because you do have to be aware of the orientation you are using.

I would say one shot = 10/10 in reasonable light and appropriate target.  Servo on static subject = about 9/10. Servo on subject moving across your view = about 7/10. Remember this is f1.6 on a 50 mil. Servo on subject coming towards you fast = forget it.   

My point here is to show: A, that the outer points are quite useable in a number of situations, and:B, they are accurate to f1.6 on a 50mm focal length.

Skirball

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #290 on: April 21, 2014, 03:42:02 PM »
That ought to give this thread enough fuel for another 10 pages or so.

Marsu42

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #291 on: April 21, 2014, 03:49:01 PM »
My point here is to show: A, that the outer points are quite useable in a number of situations, and:B, they are accurate to f1.6 on a 50mm focal length.

Interesting, though my experience is different. Btw your shots are too downsized and compressed to make an assessment, note that if I say "in focus" this means it nailed the eye and you can see the eyelashes @100% crop. I'm usually taking pictures of wild horses with my 17-40L and 70-300L in good to bad light:

* With a standing, i.e. barely moving horse in good light the outer points work ok-ish (wild guess: 8/10) though I'm getting the occasional hunt around close to infinity. You have to be careful to focus on a contrast part though. Works for repeatable shots but is not "good enough" for me to use on non-repeatable keeper shots, the horses don't pose for me. In bad light the performance degrades noticeably and more hunting occurs which is to be expected as they only work up to +0.5LV at all.

* With a moving horse to the side it works sometimes, sometimes not (maybe 5/10). I'm using "Tracking sensitivity = responsive" as which immediately results in hunting if an outer point loses focus - but "locked on" doesn't work with thin dof on erratic horse movement. As you've written, the movement towards the 6d camera is a complete joke, it's  maybe 3/10 with outer and 6/10 with center point - *if* you're lucky, I've had much worse.

I find the real issue with the outer points is not the precision, but the non-cross characteristic which make them too unreliable for me in any important situation, so I revert to focus & recompose. I have to say I didn't try my f2.8 100L much with the outer points because the specs say they're not precise enough, maybe the a larger max. aperture does improve the performance?

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #291 on: April 21, 2014, 03:49:01 PM »

Sporgon

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #292 on: April 21, 2014, 03:57:50 PM »
I've made them fit the page but have included a magnification in each one where you can see that the very narrow dof has been achieved over the focus point.

3kramd5

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #293 on: April 21, 2014, 04:24:05 PM »
I've made them fit the page but have included a magnification in each one where you can see that the very narrow dof has been achieved over the focus point.

Not to be argumentative, but that isn't a particularly narrow dof. How far away is the dog?

Narrow to me means if you hit eyelashes, the retina is OOF.

edit: I have no idea why my post is nested in yours. Weird.
« Last Edit: April 21, 2014, 04:43:21 PM by 3kramd5 »

neuroanatomist

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #294 on: April 21, 2014, 04:41:18 PM »
I've made them fit the page but have included a magnification in each one where you can see that the very narrow dof has been achieved over the focus point.

Not to be argumentative, but that isn't a particularly narrow dof. How far away is the dog?

Narrow to me means if you hit eyelashes, the retina is OOF.

+1

@ Sporgon, your 'very narrow DoF' looks to be over 2 m deep, distant subjects aren't as taxing for an AF system.
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Sporgon

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #295 on: April 21, 2014, 04:44:26 PM »
I've made them fit the page but have included a magnification in each one where you can see that the very narrow dof has been achieved over the focus point.

Not to be argumentative, but that isn't a particularly narrow dof. How far away is the dog?

Narrow to me means if you hit eyelashes, the retina is OOF.

Narrow depth of field is relative to the distance. If you are taking a picture of a football field and have the centre line in focus with a depth of field of two metres; that's narrow.

These pictures are intended to show that even at f1.6-8 on a 50mm focal length the focusing is accurate enough to drop the depth of field over the correct point of focus.

Where the two dogs are moving I was about two metres away, so dof would be about 18 cm. Where the individual ones are further away I would say eight metres, so a dof of 2.5m. However remember that at f5.6 you would have a dof of 12 metres with the 50mm lens, so 2.5 is 'narrow' in comparison with 12. 


Skirball

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #296 on: April 21, 2014, 04:47:38 PM »
I've made them fit the page but have included a magnification in each one where you can see that the very narrow dof has been achieved over the focus point.

Not to be argumentative, but that isn't a particularly narrow dof. How far away is the dog?

Narrow to me means if you hit eyelashes, the retina is OOF.

+1

@ Sporgon, your 'very narrow DoF' looks to be over 2 m deep, distant subjects aren't as taxing for an AF system.

Even as a 6D owner I can laugh at the (unintended) sarcasm of posting pictures of “very narrow DoF” on the order of 2 meters.  See, the 6D can do f/1.6 as well!

3kramd5

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #297 on: April 21, 2014, 04:51:57 PM »
Narrow depth of field is relative to the distance. If you are taking a picture of a football field and have the centre line in focus with a depth of field of two metres; that's narrow.

Finding the exact plane of focus is the real trick. The problem with those images is it's impossible to see where that plane is because it's surrounded by so much volume of sharp focus. To me, it doesn't illustrate what you intended it to even if the camera focused dead nuts where you wanted it to.


Focus here is on his right eyelashes, and the photo would have been a reject had I been wider than f/2.8.




I love your panoramics, btw. Never had a chance to talk with you before on this forum. Kudos.
« Last Edit: April 21, 2014, 05:06:31 PM by 3kramd5 »

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #297 on: April 21, 2014, 04:51:57 PM »

Sporgon

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #298 on: April 21, 2014, 04:57:11 PM »
I've made them fit the page but have included a magnification in each one where you can see that the very narrow dof has been achieved over the focus point.

Not to be argumentative, but that isn't a particularly narrow dof. How far away is the dog?

Narrow to me means if you hit eyelashes, the retina is OOF.

+1

@ Sporgon, your 'very narrow DoF' looks to be over 2 m deep, distant subjects aren't as taxing for an AF system.

Even as a 6D owner I can laugh at the (unintended) sarcasm of posting pictures of “very narrow DoF” on the order of 2 meters.  See, the 6D can do f/1.6 as well!

If you look at the pictures of the bear at the top of the page you'll see a dof of 10mm with an outer point and it's accurate every time. ( The bear's nose is about 40mm long ). Some of the points being raised in this thread are based upon the accuracy of the 6D's outer point during movement and low light. Just how much movement and low light is anyone going to shoot with a depth of field of a few mm ?

Shooting at some distance with wide apertures requires AF accuarcy, despite the fact that real dof is much greater. Try shooting a moving target with the 135L at f2 at a distance of around fifteen metres with cameras that are not in the league of the 1D and see how you get on with a dof of 1.5m.

neuroanatomist

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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #299 on: April 21, 2014, 05:23:19 PM »
Consider that we are imaging three-dimensional space, and compare the XY dimension (FoV) to the Z dimension (DoF).  When magnification is low, the DoF is relatively large compared to the FoV.  For example, with a 50mm lens and 8 m distance, Z/X is ~0.4.  At higher mag, e.g., a close up of the dog with the 50mm lens at <2 m, Z/X is <0.1. 

Basically, misfocus is less obvious with a lower magnification.
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Re: 6D autofocus capabilites: let's bust some myths about it
« Reply #299 on: April 21, 2014, 05:23:19 PM »