Originally posted by panoguy So the Fuji X100 optical viewfinder is TTL?
No, but it is not a rangefinder viewfinder either. And the EVF part (since it is a hybrid VF) is TTL.
A rangefinder mechanism superimposes a smaller image obtained from a separate visor over the center image you see from the main optical visor. You confirm focus when the image in the smaller view becomes identical to the center of the larger one. The way to recognize this mechanism is by the two windows on the front of the camera - if you only have one, it cannot be a rangefinder. The Fuji cameras only copy the look of rangefinders, but they are not such cameras.
This is how a rangefinder looks like:
The rectangular window on the right is the main visor. The round one on the left is the rangefinder visor. When you focus the lens, a flange extends from it and activates a lever, which in turn changes the angle of the rangefinder mirror, panning the small image left or right, so you can try to overlap it over the main one. In practice, it is pretty fast to determine focus - much easier than doing it manually on a DSLR. But precision falls apart at very close distances or at very long ones. Check the images of the
Leica M9 and you'll see the rangefinder window to the left of the red dot.
The
wikipedia article on rangefinders has
a nice image showing an unfocused rangefinder view next to a focused one.