I have a National PE-201C flash, which has the following guide chart:
I kind of understand the chart: if I'm standing 3.5 meters away from the subject and have ISO 100 film, I should use an aperture of ƒ/5.6. However, I'm unsure when to use the different modes of the flash and how to they actually affect the exposure. It has two auto modes: green and orange and a mode labeled M (presumably manual).
Can the situations, when to use which, be interpreted from the chart above? I've noticed that in the chart the green auto's line start from 1.2 m and the orange's from 1.8; does it have a meaning or is it decoration? How the ƒ-number can be the same with the overlapping distances — how do the auto modes actually differ? When should I use the M mode and how powerful is that?
The flash being 20+ years old, I probably shouldn't rely on the auto-modes as the light meter might have had happily retired. But, in theory, when to use which?