When to use the different modes of National PE-201C flash?

Question

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?

Answer

Pentax AF280T is a flash from similar era with similar green and red auto modes. According to its manual, these are "low output" and "high output" automatic modes.

For medium distances, you may use either; for edge cases (closer than 1.8 m or further than 5m on the PE-201C), you have to tell the flash if you need the light thrown carefully or quickly (the distances covered by green and red line should hint that).

So, when you take photos in green mode with ISO 100, set your aperture on 4 and the flash will adjust its power according to light reflecting back on its light sensor. For red mode, set your aperture on 2.8 with ISO 100.

Manual mode very likely just pops at full power each time (easy to verify if you actually have the flash). Out of distance, aperture and ISO, select two as you like and use the chart to select third value to match.

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