@MSGT-USAF-Ret wrote:
what is the second button from the bottom ((o)) looks like a radio wave button.
I do not own this lens, but I do own the EF 300mm f/2.8L IS USM which has similar (though not identical switches.)
On my lens, there is a black rubberized ring near the front of the lens and within that ring are a number of black buttons (spaced periodically around the ring). All of these buttons perform as the "AF-Stop" button.
However... there is also a ring which resembles a gear and it's behind the black ring with the AF-Stop buttons. That ring can be twisted left or right but is actually of kind of button. They call this the "Playback ring". More on this in a moment.
The panel near the back of the camera has a set of PRESET controls and on your lens it's labeled "Off", "Focus" and "Set". These work with the "Playback" ring and, if my notion about the switch with the (((o))) is correct, that switch is part of it.
Here's how it works...
Suppose you've been watching for wildlife and your lens is trained on a nest where you are waiting for a bird to return to feed her chicks. But as the mother is away, there's not much happening in the nest right now, so you're looking for other interesting targets. But when the mother does eventually return, you want to be able to quickly focus and shoot.
You can establish a memory "preset" for focus.
You do this by pointing at the target of your focus and focusing the camera as you normally would. Once focused, you slide the switch labeld "Off", "Focus" and "Set" all the way to the "Set" position (the switch only "clicks" on the "off" and "focus" positions and if you press it to the "Set" position it's spring-loaded and returns to the "Focus" position when released.
You've just established a focus preset to that specific distance.
NOW you can focus on other things, but when the event you've been waiting for is about to occur, you can gently twist the playback ring (it rocks left or right when you twist and when released it snaps back it's center position - it's not like a focus ring where you can just keep rotating it.) This causes the lens to INSTANTLY return to your focus preset distance.
And now that "(((o)))" switch.
This is a little different than the arrangement on my lens (and all long Canon white lenses). The focus preset swtich is now labeled "Off", "On", and a speaker icon. Preset is, of course, disabled in the "off" position and enabled in the "on" position, but in the speaker position the lens will make a beep when the lens has locked focus at the preset position. Our "set" is no longer on the switch but is instead it's own button next to the switch.
I suspect the "(((o)))" switch is probably to enable the beep -- it would only beep after returning to the focus preset position (when you rock the playback ring the lens will quickly snap the focus back to your preset and then "beep" for you.)
Give it a try and see if that works.
I may be wrong on that function, but most current long focal length L series lenses can beep on a focus-playback so I'm guessing yours can too.
Tim Campbell
5D III, 5D IV, 60Da
