Manual flash of 20D

[Simon Spiers]

Hi all I wonder if anyone can answer this?
I am off to photograph some owls tonight but i will be sitting a good 30 feet away from there hole.
I have an old manual flach gun that doed fire with the Canon but have no idea of guide number or any other info.
It is a vivitar 5200 and has a sliding head to 85mm.
There is a setting for manual , auto 1/2 1/4 and 1/16.
The only thing that changes the flash intensity seems to be the last three.
If i use a zoom of 300 mm will this flash help me with any extra light at all ?
What camera setting shall i use?
Shall i sync to the first curtain ?
Thanks .

[RichardtheSane]

You will probably need to use full power if you are photographing in really low light, and then you would need to basically experiement with your aperture while viewing histogram untill you get the correct exposure.

What light levels are you expecting and what sort of owls are you photographing?

[Simon Spiers]

I am expecting dusk lighting and photographing little Owls.
Had a try this evening and it seems i only need a lowish power at dusk but full at near dark.
Thanks for the input !