This discussion has been going on for a while now on various forums. Multiple Canon 5Diii users are reporting a little bug with the Canon 5Diii when using an AF assist light from either a Canon 580 Ex II or a Canon 600 Ex . For whatever reason, the 5Diii really struggles in low light with the Speedlite Assist beam. The sad reality is in fact not so much that it is the AF beam or something from the Speedlite, the 5Diii struggles to focus in low light. Without the Speedlite, its even slower.

Another important note is how much better the older 5Dii does WITH the AF assist light.

I posted 2 targets, Target 1 at 20 feet (evenly exposed at 1.60, f2.8 ISO 400- I believe close to EV7), Target 2 at 4 feet underexposed 7 Stops on the same settings, and then focused back and forth 30 times with 3 different Canon bodies using a Canon 600 Ex Speedlite AF for focus assist.

Camera/Total Time through 30 focus locks (Target 1 is always faster BTW)

Without Speedlite: (No AF Assist)

5Diii- 92 Seconds
6D – 73 Seconds
5Dii- 126 Seconds

With Speedlite

Canon 5Diii – 70 seconds
Canon 6D – 52 seconds
Canon 5Dii – 45 seconds

1. Im using a 24-70 2.8 L II lens, all shots were taken at 70mm
2. The underexposed Target 2 was 4 feet away, this is where it struggles.
3. Center Focus square

Canon is aware of the glitch and a number of forum readers are anxiously awaiting a reply from them. I would like to do a few more tests on it but I am very pressed for time until the 6D Crash Course is finished.

Canon 5Diii Low Light Focus Glitch