Thanks for your replies.
To summarize the problem - in a studio lighting setting I set my K10 to Av mode, F 5.6, ISO to 100 the shutter speed should have been 1/8th under the studio lighting condition. But the pictures were dark.
When I set the camera to manual mode with F 5.6, ISO 100, shutter speed 1/8 under the same lighting conditions the pictures were lighter.
Could I have set something wrong while in Av mode? Why would the camera need more light in Av mode than Manual Mode?
I manually set the ISO to a specific number all the time.
Unfortunately I deleted most of the dark pictures. This first picture (3238) is a picture in Av mode and I had to change the ISO to 400. The second picture (3288) is in manual mode.
Also there were 11 other people there that did not have the same problem.
Thanks.