"I assume this will lower saturation as well as effect contrast."
Yup. I just posted a couple images to the tech gallery that shows the difference. Same shot, but one with partial bleach bypass. A couple things to note:
- Fresh bleach doesn't take more than a few seconds to do it's thing. 30 seconds or longer and the results are identical to full bleaching (with my processing).
- since time in the bleach is so short for partial by-pass, it's important to agitate vigorously or you'll get mottled skys and other solid tones.
It's fun stuff.