EF 300mm 2.8L auto-focus issue

[meanstreak]

I just picked up a 300 2.8L original non-IS version for a super-steal of a deal at an estate sale. When using it with my 40D everything is great but when I try my 6D I can get amazing fast auto-focus in full-auto mode and in the other auto's like "sports mode" etc. But I can't get auto-focus to work while in aperture or shutter priority mode or in full manual.

Can anyone shed some light on this? I've never updated the firmware on my 6D (or any camera for that matter) is that the problem?

 

[Loswr]

Does the AF-ON button work? Do you have back button focus set up? It does sound like a settings issue, so a reset all settings should fix it as a last resort.

 

[meanstreak]

Does the AF-ON button work? Do you have back button focus set up? It does sound like a settings issue, so a reset all settings should fix it as a last resort.

LOL oh man yep that's it! I was so excited to play with my new toy that was so cheap I practically stole it, I forgot I didn't turn off back-button focusing after my last shoot.......

 

[Loswr]

LOL, glad that fixed it.

 

[applecider]

How cheap?

The curious want to know.....