Originally posted by RKKS08 With all my Pentax DSLRs the result of a green button measurement in M mode will stay valid. If you take several pictures in a row with unchanged light conditions you need to press the green button only prior to the first shot.
And if the light changes a bit and you have the feel (and some experience), you can slightly modify either exposure time or aperture with the wheels. Again no green button measurement needed.
In many situations this behaviour can speed up your workflow considerably.
What they said ^ ^ ^
While much is made of real-time exposure control, the need is overstated for most subjects. There is not much call to change the settings once an appropriate exposure is determined. Back in the old days when stop-down metering was the norm for many SLRs and even now when using a hand-held meter, the flow was/is to determine the exposure and to shoot freely with those settings until either the subject or light changed.
When I do green button exposure in M mode, I set once per subject.
Steve