How can I set the brightness control for my iMac's video camera manually?

When on a Zoom conference call, the brightness of my image varies greatly depending on whether my face is in the center of the frame or not, as I am in a room with a dark background. The brightness level of the iMac's video camera is trying to adjust to rapidly changing light levels. I need to be able to set the brightness level to an appropriate value, but I can't find the controls to accomplish that.

Thanks for your help!