Photos, one right the other not so much

Ok So last night was trying out my Expodisc 2. Had the camera set to Aperture priority F/8, ISO-320. I had three 45watt CFL lights, two with 30" silver interior umbrellas and one with a 40" gold interior umbrella. If need be I can you paint to show were the lights were. What I'm wondering is why is one photo ok and the other is too dark. Was this because I forgot and to make sure I had the camera set to center weight?

Looks like it just metered the scene differently, which can happen with indoor lighting pretty easily. It thought the first scene was darker than the second and metered it for a 1/2 second exposure. The other it thought was lighter and metered it for a 1/6 sec exposure.

Best way to avoid that is to shoot manual unfortunately. Aperture priority it can mess up like that pretty easily.

I can't believe I didn't check to see what metering I had it set for. I broke out the cam and set it to center metering, going to retry again this weekend. I normally shoot in manual so no prob there, just thought I would give AP a try. Just weird. The lights, model and I stayed in the same place. I think next time around I might tether the cam to the laptop.

"In Studio" you're better off setting everything manual, IMO. The expodisk will set your white balance, and that's all. If you were on a tripod, camera in the same spot, metering on the same spot, same aperture, no lighting changes, etc., it shouldn't change. It doesn't look like that is the case though. No other changes to light in the entire room? Shooting a gray backround like that, if one of those photos is before and one after setting white balance, it is possible that one would appear darker than the other. It looks to me that it's just a matter of the camera was moved, and metered differently based on what it saw. That's going to be more pronounced with spot metering than any other mode.

First pic was I took that night was with the expodisc to set the WB. Both pics were after the WB was set. The camera was on a tripod and never moved, only zoomed in or out. Here is a set up of how everything was.

Green was the model, red was the cam, silver were the sliver umbrellas and yellow was the gold umbrellas.