>>4038754This must be an autothyristor flash, right?
Forget about shutter speed, it doesn't affect flash exposure. You set your shutter speed at the sync speed of your camera or lower. Doesn't affect flash exposure, and isn't taken into account in the flash metering.
Usually thyristor flashes need your aperture and ISO. Basic ones don't have a full range of apertures to input, only a couple choices.
I'm guessing your flash has two choices, the red and blue one (which change when setting the ISO), there must be a switch to choose which one of the two you'll be using.
So once you input you're ISO, you'll have a "blue" aperture and a "red" aperture. For instance in your pic, for ISO200 you have "blue" aperture of f/5.6, and "red" aperture of f/2.8. For a different ISO, you'll have a different pair.
The blue and red lines cover up to which distance the flash can cover, i.e. in the "worst" case that it'll have to fire at full power, up to which distance you'll get proper exposure.
You see the brighter by 2 stops aperture (4times more light), you'll get coverage up to twice the distance. This is because of the square law, 2x the distance needs 2^2=4x the light.
You don't have to calculate any distances, those are worst case scenarios, just making sure you stay within them.
So say, at 7ft, you see it's covered by both the "blue" aperture (f/5.6) and the red one (f/2.8), so you can choose either, set it on the camera, then set the switch to the corresponding colour on the flash, and the flash's metering cell will calculate correct exposure and cut the flash automatically when it's reached.
But if you wanted, say, to shoot something at 30ft, you'd only be able to use the red aperture (f/2.8) on the camera, and choose the red switch on the flash.
That is, the red and blue lines are only to make sure that your subject remains within the distances indicated by the 2 apertures the flash is able to meter (and that you should set on the camera).
Makes sense?
