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70D's Dual Pixel AF and front/back focusing

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Thread started 27 Sep 2013 (Friday) 05:25   
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YashicaFX2
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hollis_f wrote in post #16329137external link
Really? My 7D can AF in that situation in LiveView. Not as fast, admittedly, as the 70D (I presume) but it'll do it.

OK. I got that idea from Canon's 70D promotional video. I guess I mis-interpreted it.

hollis_f wrote in post #16329139external link
The shorter answer is no.

In LV?

Post #16, Sep 27, 2013 10:11:44


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Brianbar
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Has any 70D owners found it necessary to do any MFA?

Brian

Post #17, Sep 27, 2013 10:50:34




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apersson850
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lsquare wrote in post #16328986external link
If you're an OVF user, then you won't benefit from the Dual Pixel AF technology. You do know that right?

Yes I am, and yes, I know. Live view is inappropriate for the action shooting I like most, so to me, the 70D's new AF method wouldn't make much of a difference.

Post #18, Sep 27, 2013 10:52:57


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gjl711
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Brianbar wrote in post #16329267external link
Has any 70D owners found it necessary to do any MFA?

Brian

I would think that MFA is still needed for traditional phase detect AF.

Post #19, Sep 27, 2013 10:59:25


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hollis_f
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YashicaFX2 wrote in post #16329180external link
In LV?

OK, I am guessing that the PDAF in LiveView isn't iterative - purely because the whole - Is it in focus? - No - Move the lens - is it in focus now? - No - Move the lens a little bit - Is it in focus now? - routine would make AF very slow. And the whole point of having PDAF is LV is that it isn't supposed to be slow.

Post #20, Sep 27, 2013 11:16:14


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hollis_f
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gjl711 wrote in post #16329278external link
I would think that MFA is still needed for traditional phase detect AF.

And also PDAF in LiveView - because I don't think it can compensate for lens errors, which is one of the main uses of MFA.

Post #21, Sep 27, 2013 11:17:22


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gjl711
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If you watch their sample video I'm guessing that the LV phase detect is interactive. At least it is able to sense moving objects and change focus.
http://www.youtube.com​/watch?v=SovaYO5ucUIexternal link

Post #22, Sep 27, 2013 11:34:43


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hollis_f
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gjl711 wrote in post #16329348external link
If you watch their sample video I'm guessing that the LV phase detect is interactive. At least it is able to sense moving objects and change focus.
http://www.youtube.com​/watch?v=SovaYO5ucUIexternal link

What, like AI Servo, which isn't iterative.

Post #23, Sep 27, 2013 12:51:57


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pwm2
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hollis_f wrote in post #16329153external link
That's why different lenses require different MFA values.

Another issue is use of outside AF points - the Canon cameras tries to compensate for the field curvature of the different, known, Canon lenses which makes it harder for non-Canon lenses to do as well. Only continuous focus (closed loop) evaluations will make live-view able to perfectly cancel this effect whatever lens is used.

But it isn't unlikely that the 70D live view could manage better with outside AF points even if it is not using a closed loop. But in the end, open-loop AF really is very hard as most people should remember from the old "blue dot" debacle.

Post #24, Sep 27, 2013 15:55:27


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lsquare
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hollis_f wrote in post #16329312external link
And also PDAF in LiveView - because I don't think it can compensate for lens errors, which is one of the main uses of MFA.

I think you're right. I haven't come across any information that suggest MFA is possible in LV. I didn't think it would be an issue because I was under the impression that front/back focusing problems have been eliminated.

Post #25, Sep 27, 2013 16:27:48




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lsquare
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hollis_f wrote in post #16329153external link
And it's this - the lens not moving accurately - that seems to be the major cause of front/back focusing. That's why different lenses require different MFA values. And, if the 70D's PDAF in LiveView, doesn't work iteratively then it can't help with this problem.

I call it 'Busted'.

I'm crossing my fingers and hope you're wrong! Hopefully DPR will take a hard look at the issue and address it once and for all.

Post #26, Sep 27, 2013 16:30:32




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pwm2
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hollis_f wrote in post #16329310external link
OK, I am guessing that the PDAF in LiveView isn't iterative - purely because the whole - Is it in focus? - No - Move the lens - is it in focus now? - No - Move the lens a little bit - Is it in focus now? - routine would make AF very slow. And the whole point of having PDAF is LV is that it isn't supposed to be slow.

Not necessarily. A traditional stop-and-go AF is very slow, because of the acceleration limitations of the lens, i.e. how many movements the lens can manage to perform per second.

But the 70D sensor supports partial read-outs, which should allow quite fast frame rates during AF - so it really should be possible for the camera to perform AF evaluations while the lens is busy performing a focus operation, allowing the camera to produce approximations of how muh further to move the focus. But no - I haven't spent the time to read up enough about the 70D to know if it does compute any focus while the lens is busy changing the focus. I don't even know if Canon have released such information. But partial sensor readouts can really give quite good focus times.

Post #27, Sep 27, 2013 16:49:50


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10-22 | 16-35/2.8 L II | 20-35 | 24-105 L IS | 28-135 IS | 40/2.8 | 50/1.8 II | 70-200/2.8 L IS | 100/2.8 L IS | 100-400 L IS | Sigma 18-200DC
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Varago
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Wow a whole lot of guessing going on here for something that can be checked very easy :)
My 18-35 has a MFA of 0 at the 35mm end. I set the MFA to +20 and used quick focus and the image was way out of focus, but when I used LV pdaf it was perfect, the same as with the MFA at 0.

Post #28, Sep 27, 2013 22:58:15 as a reply to pwm2's post 6 hours earlier.


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lsquare
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Varago wrote in post #16330535external link
Wow a whole lot of guessing going on here for something that can be checked very easy :)
My 18-35 has a MFA of 0 at the 35mm end. I set the MFA to +20 and used quick focus and the image was way out of focus, but when I used LV pdaf it was perfect, the same as with the MFA at 0.

Dual Pixel AF doesn't work with your Sigma lens.

Post #29, Sep 27, 2013 23:17:22




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Varago
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lsquare wrote in post #16330561external link
Dual Pixel AF doesn't work with your Sigma lens.

Site a source please. It sure seems to work fine for me :)

Post #30, Sep 27, 2013 23:34:18


Dave Lee
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