I should have said the aperture dial on the rear of the Canon 5D Mark II. Turning the the dail normally adjusts the the aperture. Now it only works in the "Auto ISO" mode. The camera is primarily used to should HD video. Any ISO setting other than "Auto" will not allow the the dial to change the aperture. Any ideas? Thanks for your help.
So this is a 5D II? That's an important detail.
In "M" (Manual) mode, the default is that the front dial controls the shutter speed, and the rear dial controls the aperture IF you throw the switch in the position which enables the rear dial. The power switch has three positions... (1) "OFF", (2) "ON", and (3) a position with a line drawn connecting to the rear dial. The power switch must be in the position that points to the rear dial or it wont work to control aperture.
In P, Av, or Tv mode the rear dial will control exposure compensation (but only after you activate metering.) Again... the switch has to be in the position that points to the rear dial.
This is how Canon handles the rear dial lock function and I've known 5D II shooters who hadn't realized this nuance. On the 5D III is a bit more straight forward because the on/off switch is just an on/off switch and there's a seperate switch near the rear dial labeled "Lock" which disables the rear dial.
Some photographers want to disable the rear dial because they feel it's too easy to accidentally bump it when setting up for a shot and get unintended exposures.
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