I've just made the jump from the Canon 30D to a Canon 50D. Faster burst rates, twice the megapixels, bigger, brighter LCD, I couldn't wait to get out and do some shooting.
My first sets of test shots showed, me two things:
First, some of my lenses need to go in to the manufacturer for service. That's another post.
Second, and the subject of this post, was noisy shadows. Noisy, nasty shadows, even at ISO 100. The speckles of red, green and brown, were especially noticeable in areas that were supposed to go pure black.
"What the ___?" was my first thought. "Oh, crap, do I have a bad sensor?" was my second. I worried that I was going to have to send my brand-new camera in for warranty service inside the first week.
But before I completely panicked, I did some Googling, and found other folks discussing the same problem. The root cause was not a defective sensor, but rather Canon's Auto Lighting Optimizer (ALO).
This "feature" is turned on by default, even when you're shooting raw. It tries to bring up the shadow detail in photos, which introduces sensor noise there.
So, I dug through the custom functions menu and turned ALO off, ran back outside, set the flash back up, and reshot. Low and behold - the sensor noise the shadows was gone!
I'm sure there situation where ALO is a help. But if you're planning on shooting anything dramatic, with black backgrounds or deep shadows, you'll be a lot happier turning it off. Pictures like, say, this one: