A CVS staff member took several passport photos of me against a white background. In some photos my face looked very dark, while in other photos my face doesn't look dark at all and it actually is not. He explained it was because:
He used flash in those photos, and flash can make objects look darker against a white background, and
The white background can attract light away from the objects in front of it.
Is what he said correct, and why?
1) If the flash is falling on both the subject (in this case: you) and the background,
2) If the subject is sufficiently close enough to the background that there's not a lot of light falloff between the subject and the background, and
3) If the background is more reflective than the subject
then using a camera set to an automatic exposure mode could conceivably underexpose the subject in order to prevent overexposing the background. Since most cameras meter in monochrome (B&W) and assume everything should be exposed to come out medium gray (halfway between bright white and dark black), using a white background will usually fool the camera into underexposing the photo.
Is what he said correct, and why?
No. What he told you is utter horse hockey. Light travels in straight lines; regardless of its color, no background can "attract light away" from a subject.
There are a number of explanations for what happened given what you've told us. Some possibilities:
CVS guy didn't allow enough time between shots for the flash to recharge, so the flash failed to fire on some shots.
The camera was set to a spot metering mode, and CVS guy wasn't consistent in where the camera was pointed when he locked in the exposure. The AE system was basically getting different information for each shot.
Depending on the camera and flash, it's possible that the flash wasn't aimed in the same direction for all shots.
Something changed in the environment that caused more or less of the flash to bounce toward you.
The camera was set to manual exposure mode, and CVS guy inadvertently (maybe unknowingly) changed a setting such as aperture or shutter speed.
No matter what the specific reason for the problem, you should have gotten a set of passport photos that you're happy with. For a US passport you really only need one photo. If you didn't get what you want, go back and ask them to try again, hopefully with a more competent photographer.
The option C is: That guy does not have a clue of what he is doing.
The first explanation is just a cheap excuse. The second one is plain dumb.
flash can make objects look darker against a white background.
The lack of control is the one that causes objects look darker.
When you fire an automatic flash, it fires a small pre-flash, this is a flash fired that the camera uses to measure the exposition it will need to take the actual photo.
If you have a lot of white, yes, the cameera could think "Oh. The bounced is too bright, I will use less power". But that is why you need to control the exposure.
If the camera can not be manually controled, you can simply sit further away from the white background... Easy.
The white background can attract light away from the objects in front of it.
That sounds like a supervillian power!
