#1. "RE: Why Shutter Exp mode does this? " In response to Reply # 0
Remember that there are two meter readings in play. First there's the ambient. In your manual mode, you are set for 1/60, f/8. This is a significant underexposure, so if the flash doesn't go off, you get a very dark frame. I know that because in S mode, you set the same shutter speed and the ambient exposure is f/2.8 - three stops faster.
Why is the resulting histogram close to the same? Because the flash is being used to illuminate the subject, using TTL metering. The flash is working a LOT harder in the manual case, because without flash you're underexposing by three stops. (Or it could be more than that.) In both cases, the flash is providing illumination, but in the S case you're opened up three stops more, making the flash three stops closer to correct exposure.
If you repeat the experiment with a macro-distance subject, say a flower at 10" distance, you will find that the background is very, very different between f/2.8 and f/8.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
#2. "RE: Why Shutter Exp mode does this? " In response to Reply # 1
I do recall reading Russ's good flash blog and your comment reminded me of it. Your point about background distance hit the nail on the head. I never have used Shutter mode with flash so this was a trial and I had not thought the process through.