A better Picture Styles choice = better RAW histogram?

[Daniel Browning]

I'd like to run this by you guys - here's a quote from the 40D Field Guide:
"If you're shooting RAW capture, you can set the Picture Style setting to a lower contrast and get a better overall sense of the RAW histogram and reduce the likelihood of clipping (or discarding image pixels)."
I always routinely push up against the saturation point in the HLs. So anything that can help prevent clipping is a good thing for me.
But does this work? The in-camera histogram is a .jpg conversion, one possible rendering of the RAW data. According to the Field Guide when you tweak your Picture Style to show a lower contrast version you're seeing a better histogram. What are they basing this conclusion on?

I don't know what the author is basing it on, but he is at least partially correct.

Do the Picture Styles actually change the capture? If I specify more saturation do the pixel wells, the receptors, really fill up faster?

No, and I don't think that's what the author meant in your book. I think he meant that it gives you a more accurate histogram, so that you can change the exposure and get a more optimal exposure for raw.

I always thought the RAW data is what it is, based on the exposure only, and it remains that way no matter what combinations of in-camera sharpness, contrast, saturation, hue (the Picture Styles) you use. The Picture Styles show different renderings, that's all. They affect the .jpg output but not the RAW data. I thought that was the whole point of RAW capture.

That's all correct.

If I'm right, then choosing a low contrast render may actually be a less accurate version of the RAW data.

As you said, the in-camera histogram is based on the preview jpeg, which is often quite different than the actual raw histogram. If you want to try to make the histogram more similar to the raw file, there are many settings you must change:
Picture profile (tone curve, saturation, contrast, etc.)
White balance (this is a *big* one, research "UniWB")
Color space
Auto lighting optimizer
Highlight Tone Priority
Exactly what the settings need to be depends on each camera model. Some are more accurate with sRGB (like your 40D), others with AdobeRGB. If you have the contrast too high, it will make the histogram clip even when the raw file isn't clipping. If you have the contrast too low, it will make the histogram show no clipping, even when the raw file is clipping. What you want to do is find the settings that most closely approximate the raw histogram. Usually this means a picture profile with no tone curve and contrast set to 0 (not -4 or +4.
Hope that helps