One-button change of EV?
There is no way to save the metering on USER mode (as it changes with a lever) and absolutely no way to change between the USER modes faster than changing the exposure compensation on regular mode.
The fastest way to change the exposure is to use the M mode and change the sensitivity in full steps (you'll have just 2 clicks of the rear dial instead of minimum 4 if use the compensation in 1/2 steps or 6 if you use 1/3 steps). Of course you have to set the base exposure of correctly before, but it doesn't change that fast...
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