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AF Assist Malfunction - 430 EX II


I just got a new 430 EX II.  I have the flash mounted on my T3i and strangely, when I'm trying to focus with my telephoto zoom the camera is front focusing; I know it is not the lens's problem, because it works without the flash mounted. I also found out that when the 430's af assist  is on, the camera front focuses and when off it works fine. The AF assist also makes auto focus slower for some reason. I've heard that the 600 EX-RT also has the same issue.


 Is their any way to fix this problem? or is Canon just notorious for this problem...because I'm not happy!


I'm an advanced amateur, am not the average point-and-shoot photographer out there, and am willing to take any critique or advice that will improve my photography.


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

@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.

"The right mouse button is your friend."



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!

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.

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.

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.

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.

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.

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