# External AF assist light?



## Jim Saunders (Apr 7, 2014)

There I was shooting bullriding in a cave-dark arena yesterday, and occasionally I had trouble getting AF lock on a 1Dx with a 70-200 f/2.8L II. Lo and behold Thinkgeek has flashlight-size IR lights for sale, would one of those be any use out to about 100 feet? I might shop around for something which runs on AAs and has provision for a remote switch, but only if the idea is sound. 

Jim


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## kaihp (Apr 8, 2014)

Yongnuo's YN-E3-RT has built-in focus-assist beam, and is the size of the Canon ST-E3-RT. Might just suit your bill, and give RT access to 600EX-RT's to boot.


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## neuroanatomist (Apr 8, 2014)

It always puzzles me why people refer to the AF assist lamp as 'infrared'. By definition, you can't see infrared light. If you're seeing the red light, it's *red*, not _infra_red. 

The only infrared system in use is Canon's optical triggering system for off-camera flash. The receiver for that system sees only IR light (the 'window' is black), and the ST-E2 transmitter has the same type of xenon flash tube used in Speedlites, but it's behind a filter that blocks the visible light output. Of course, regular flashes used as a master use the main tube for the optical triggering, so you do see that light.

Back to the AF assist issue, you don't want to use an IR flashlight for focus assist. Lenses have an infrared focusing mark to the right of the main index mark on the distance window/scale because IR light is focused differently. Using IR illumination to achieve focus means that the visible light – the light your sensor is going to capture – is going to be out of focus.

Your best bet might be a bright red LED flashlight, but that still might be too distracting to the riders.



kaihp said:


> Yongnuo's YN-E3-RT has built-in focus-assist beam, and is the size of the Canon ST-E3-RT. Might just suit your bill,



Alternatively and for less $$, Yongnuo has the YN12AF, which is simply a standalone AF assist lamp. But...Jim is looking for something good out to ~100'/30 m. The YN12AF is good to 12 m, the YN-E3-RT's distance isn't specified from what I can tell, but I'm sure it won't work out to 30 m.


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## Jim Saunders (Apr 8, 2014)

neuroanatomist said:


> It always puzzles me why people refer to the AF assist lamp as 'infrared'. By definition, you can't see infrared light. If you're seeing the red light, it's *red*, not _infra_red.



The entirety of your post should be a page in the book worth of education you've shared with all of us!

I had assumed the visible light from the AF lamp was just spill from imperfect filters but now that you describe it I realize I should have put all of that together myself! Adding a little bit of deep red light might not be too difficult, I could get a friend to go closer to the subject with it. I suppose the really slick approach would be to use an external shutter control to operate the camera and the remote switch for this AF light (so it isn't on too much) but one thing at a time.

Jim


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## Jim Saunders (Apr 8, 2014)

kaihp said:


> Yongnuo's YN-E3-RT has built-in focus-assist beam, and is the size of the Canon ST-E3-RT. Might just suit your bill, and give RT access to 600EX-RT's to boot.



I have the Canon one, but I might get one of these for my 5D2 so I can use both bodies with the lights in group mode.

Jim


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## surapon (Apr 9, 2014)

Jim Saunders said:


> neuroanatomist said:
> 
> 
> > It always puzzles me why people refer to the AF assist lamp as 'infrared'. By definition, you can't see infrared light. If you're seeing the red light, it's *red*, not _infra_red.
> ...



+ 100 for me too.
"The entirety of your post should be a page in the book worth of education you've shared with all of us!"
I total agree with you, Mr. neuroanatomist is one the Best Teacher , who we can learn from.
Surapon


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