I noticed a problem with my Pentax K-R video where the film is very grainy - I assume this is because it shoots with a high ISO setting which is set automatically. So, I want to manually set the ISO with an auto exposure lock in P mode, then film in video mode.

I've tried this with an automatic and a manual lens (though I really only care about results for the manual lens).

Using the manual lens (1.7/50mm SMC), I lock the exposure in P mode. I then switch to video mode and the lock remains (asterisk is showing). The preview looks fine, just like in P mode. I begin filming and immediately, the live view drops to almost completely black (I can very vaguely see it is filming but it is so dark that it is virtually black). It seems like there's some sort of auto EV adjustment that kills the exposure? Typically, I lock it at 1/60 shutter speed and 200 ISO (though I've tried it with 1600 - same result).

Using the auto lens, I lock the exposure in P mode. I then switch to video mode but here, the lock does not remain. So, I reset the exposure lock simply while in video mode, then begin filming. The exposure does not dip to black.

I think this is an auto vs. manual lens issue. Any ideas on how to fix this EV adjustment? Thanks! I can post a video if necessary of what happens

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