agfatronic flash

I have been doing available light photography up to now, but I would

like to start experimenting with a flash, especially fill flash to

reduce harsh shadows from sunlight.

In the photo equipment package I "inherited" from my father there is

an Agfatronic 240CB flash, without user manual. There is a shift

button on the front, with one setting labeled with a filled circle,

and the other setting with an "open" circle. Can anyone explain what

the difference is between the settings?

I made a test roll using a 135mm lens at 3m; the slider on the back of

the flash indicated f/8 as the aperture to use. All the frames where I

used the "filled circle" setting are underexposed, except with the

aperture at f/4. The frames where I used the "open circle" setting,

were better, except I need to correct 1 stop from the slider setting

(1 stop larger aperture for bounce flash, 1 stop smaller aperture for

direct flash). I seem to have to correct 1 stop more in either

direction with a shorter (50mm) lens, is this correct?