I have a Nikon D40. I recently purchased a flash for it, the flash is made by image? See image below It works fine, voltage is fine, I've check all that. The thing I'm finding strange is, from time to time, like when a pic is last minute, wasn't planning on taking it, but was a spur of the moment, I throw it on auto, the strange thing is, when i half hit the button for focus, instead of registering that i'm using an external flash, the built in tries to pop up, BUT then the external flash is still what flashes... is this normal????