A photographer sent me a tech question (via the Photofocus podcast I do with Scott Bourne) about the ambient metering systems in Canon cameras when a Canon Speedlite is used.
For the answer, I turned to my pal Rudy Winston at Canon USA. Thanks Rudy! You da man!
Here goes . . .
Assuming that a photographer is using a Canon EOS Speedlite: With most of our cameras, the camera CHANGES ITS METHOD OF AMBIENT LIGHT METERING as soon as it detects a flash ready light.
Regardless of whether you're set for Evaluative, Center-weighted, Partial, or Spot metering, it instantly shifts to a full-area system – but it's NOT pure Evaluative or Center-weighted. Rather, it gives essentially even coverage throughout the frame, to assess the overall ambient brightness in the scene.
Normal Evaluative metering would tend to try to compensate for things like back-lighting, but, instead, this modified "ambient metering when a flash is being used" system tries to make sure the overall scene brightness is coped with – presuming that any foreground shadows on back-lit subjects will be filled-in with flash.
NOT ALL Canon EOS BODIES work this way – there are a select few, like some of the Rebels that WILL preserve the user's choice of ambient metering pattern when the flash is active and in use.
When photographers ask about this, rather than fuss about which camera model they have, and whether it's one of the few that lets the user preserve his or her ambient metering choice, I tell them to expect the ambient metering mode to function in the form that I mentioned above.
Thanks again, Rudy.
The pictures in this post, which I took of my son, Marco, show the effectiveness of daylight fill-in flash photography. Mo’ on that to come. . . if anyone is interested. Lemme know.