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Esteemed Contributor
Posts: 4,325
Registered: ‎02-17-2016

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...

http://www.bobatkins.com/photography/technical/focus_vs_light_source.html

 

Oh, and the flash has no distance measuring at all.

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VIP
Posts: 10,786
Registered: ‎08-13-2015

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...


@kvbarkley wrote:
[link deleted]

 

Oh, and the flash has no distance measuring at all.


Why do you persist in posting links so often?  I don't click on miscellaneous links.  It also violates the TOU agreement.

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Moderator
Posts: 1,875
Registered: ‎07-08-2013

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...

Hi!

 

To clarify, if someone uses a link to help illustrate a point and the page you land on isn't overtly trying to sell you anything, we will generally allow it to stay up.  Links that we usually take down include ones that are irrelevant to the conversation, ones that contain objectionable content, or where the site's purpose is intended to solicit sales.  This is not necessarily an exhaustive list of criteria, and each link's viability is at the discretion of the forum moderators.

 

In this case, we checked kvbarkley's link and it satisfies our criteria.  We'll allow it to stay up.

 

We hope this helps!

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New Contributor
Posts: 4
Registered: ‎02-03-2016

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...

[ Edited ]

kvbarkley, you might be surprised how complex AF assistant beam is. It is not a simple light.
Article is interesting. I wonder if there is IR filter in the front of AF-phase detection sensor?

 

Another interesting observation/speculation.
Considering IR light causing back focus. Notice that AF assistand beam has lots of IR light. 
Is there any compensation coefficient Canon using to correct AF in the case when Flash AF assistant light is on?
I guess it should if using AF-phase detection chip, and should NOT if measuring distance using flash.

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Esteemed Contributor
Posts: 4,325
Registered: ‎02-17-2016

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...

I know it is a complex pattern. Still doesn't measure distance, though. Just provides illumination for the camera's AF system.

 

As for the link, it is to a technical photography website that describes this issue exactly. It appears that IR light can move the focus point a bit.

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New Contributor
Posts: 4
Registered: ‎02-03-2016

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...

kvbarkley, light flash AF beam and check if you can see split lens or even pair. It looks similar to old style split focusing screen.
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Esteemed Contributor
Posts: 4,325
Registered: ‎02-17-2016

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...

Yes, it is a matrix pattern. But the flash has no sensor to read the light - unlike my old Vivitar 8^). It is all done by the camera. After all, disco-flash works just as well to focus without the complex pattern or even the external flash.

 

Besides the distance from the *flash* to the subject is irrelevant - you could be using off-camera flash. What counts is distance from the *camera* to the subject.

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New Contributor
Posts: 4
Registered: ‎02-03-2016

Re: I'm not shooting far at all. I shot at 30' and at 3'. Eve...

There is US patent, not sure if Canon goes this way, but their flash AF assist is not that simple as one might think.
Autofocus assist lighting used for rangefinding in very low light conditions.

http://www.google.ch/patents/US20100158495

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