This is what I found out.The D40 under low ambient light will not let me set custom WB unless flash is activated. What happens is that when the flash icon inside the finder starts blinking, the only way to do a custom WB measrement is to raise the flash. But, that fires the flash and the WB is measured for flash, not ambient only. This is a serious shortcoming. Is there a way to do Custom WB measurement without raising the flash in low interior ambient light?