EOS 5D Mark III Assist-beam not firing in very low light despite using "one shot"

[Airyka]

Hi there,

I have two 5D Mark 3 camera bodies which I use to shoot weddings.  At night's end, the lights are OUT and my camera cannot auto focus in such darkness or very low light, so I need the assist beam to help me out and turn on.  My cameras are shooting in 'one shot' mode, and under the menu section AF, the AF assist-beam is enabled and 'on' yet still the red beam of light will not go on. What is happening here? The problem is doubled when I am trying to fire a flash either on or off camera, and because the auto-focus can't find anything to focus on (with no red assist beam to help) in such low light, the shutter will not work. I can override this but I'm still shooting in such low light, and my beam isn't going on to help. I haven't been able to find this exact issue on the message board--or folks say it resolved for them when they got out of AI servo and put their camera on One Shot mode, which I've already done. Thanks for help!!

[MikeSowsun]

Which flash are you using in the hotshoe? What are you using to trigger off camera flash? 

[Airyka]

Hi Mike, I am using a Yongnuo Digital speedlite YN600EX-RT flash. Is it a problem with my flash? I guess I am confused about where the red assist beam is emitted and controlled from--is it actually coming from my flash but controlled by my camera? Or emitted from the flash and controlled both by settings on my camera AND the flash?

thanks!

[deebatman316]

There is a setting in the speedlite to turn ON or OFF the AF Assist Beam and one in the camera. It is likely the one on the speedlite is turned OFF. The speedlite settings override the camera body's setting.

[MikeSowsun]

Yongnuo sometimes has compatibility issues with certain features on many newer cameras. I bet if you were to try a Canon flash the AF Assist would work just fine on your two 5D Mk III cameras. 

How old is your flash? A quick Google search shows that Yongnuo flashes manufactured before Aug 2016 have problems with AF assist. 

[deebatman316]

Could it also be an AF Point selection issue. I know older speedlites can't cover the 61 AF Point system that the 5D Mark III & IV use. When an older speedlite is used with an AF point the AF Assist Beam can't cover. The "IR" AF Assist IS NOT emitted by the speedlite. This limitation doesn't exist for speedlites that use Intermittent Flash Firing (Disco Flash).

[deebatman316]

Is the AF Assist Beam enabled on the speedlite. If the speedlite is set to "Disable AF Assist Beam" the speedlite won't fire an AF Assist even if the camera is set to "Enable AF Assist Beam Firing". Also what speedlite are you using. Note older speedlites often don't support the newer AF patterns that a newer camera has. Using an AF point that isn't covered by the speedlite's AF Assist Beam. Will result in the speedlite NOT emitting an "IR" AF Assist Beam. What off camera speedlite setup are you using.

[Airyka]

Hi Demetrius,

I am using a Yongnuo Speedlite YN600-EX-RT that is at least 5 years old, maybe more (not sure) and I'm not sure how to enable the AF assist beam on the flash itself.  Do you enable that through the flash directly, somehow, or via the camera it's attached to? Do you know what AF point I can't use to make sure the speedlite assist beam goes off? 

[deebatman316]

This 3rd Party speedlite claims to work with Canon's Speedlite Control Menu. ( I don't know if it actually works). Speedlite Custom Functions can be changed from the camera's menu screen. Or put the camera in Full Auto (Green Square) Mode this overrides ALL Custom Function. To see if the AF Assist Beam works there. If it works there it must be a camera setting then.