Dave

P is a slightly less auto version of auto. Green box is 100% auto.

I’m assuming P mode on your dslr is auto?

So what would you do to make the 2nd picture perfect? If opening one stop let too much light in, how do you fix that (in the camera)?

The OmniBounce will certainly diminish the light output of the flash a bit. But the ETTL-II circuitry *should* experience/perceive that reduction and cause the flash to fire a bit longer/brighter.

I’ll think about this a bit more; but off-hand I can’t explain the difference — *IF* the flash was properly operating in ETTL-II mode.

I just tried an experiment, though. I put an 18% gray card against the yellow wall, and took two pictures that were as close to identical as possible. On one, the Omni-Bounce was on the flash:

http://www.flickr.com/photos/whipartist/3959274407/in/photostream/

And the other, essentially-identical shot, was done without:

http://www.flickr.com/photos/whipartist/3959274609/in/photostream/

I’ve done a little bit of post-processing on those, but nothing that would alter the exposure. I just resized them and added Lightroom’s histogram to the bottom right corner. The EXIF data didn’t make it through, but they were both taken at f/4 and 1/60, ISO 200. Empirically, it seems that the diffuser does seem to cause some underexposure.

I would argue, however, that it’s not the StoFen diffuser that is to blame for the underexposure in your first image.

Virtually all cameras and flashes made in the last thirty years use a photo cell to read the light reflected back from the subject and to modulate the power of the flash (i.e., cut the current to the flash tube off early when enough light has returned to the camera/flash position to make what it judges to be a correct/average exposure). Nowadays this is done “through the lens” (TTL) with “dedicated” flashes; previously it was accomplished with a sensor on the flash itself. In this sense, therefore, your camera/flash circuitry *IS* aware of the diminished light output owing to the OmniBounce diffuser.

I would maintain that the underexposure is due to the predominantly light-toned background wall, and the similarly light-toned carpeting on the cat tree. As you well know, cameras aren’t telepathic. And they can’t select the “best exposure” [sic] for every scene — precisely because a camera can’t possibly know how the scene is *supposed* to look. It can’t know if you’re pointing the camera at a blonde, Caucasian bride in a white dress against a white background, or a black cat in a coal bin. So cameras are engineered to select average exposures that yield average images of average scenes. When a scene is no longer average (e.g., a snow scene or a black cat in a coal bin), the image will be under- or over-exposed respectively.

In short, I think your camera/flash was properly attempting to make the first image come out “average” — which is darker than the lighter-than-average tone that predominates in the real scene. (Plus, ALL my canon DSLRs and Speedlites — 20D, 5D, 1D Mk II, 580EX — tend to underexpose slightly when left to their own devices.)

I have some handouts from classes that I teach that I would be willing to share. Like your series, they explain exposure, white balance and other technical topics in decidedly non-technical language. Let me know if you’re interested?

Best,

Brand