Guide numbers

[craigh]

It's been so long since I worried about guide numbers, I forgot.  I want to use my Olympus 35EC with a Minolta flast guide number 37 at asa 25.  What guide numer would I use with asa 200.

Thank you,

Craig

[Mike Kovacs]

I figure GN110 @200ASA

At 10 feet 25ASA --> f/3.7 approx f/4
At 10 feet 200ASA --> f/11 (3 stops down)

11 x 10 --> GN110

[craigh]

Thank you Mike

[Mike Kovacs]

PS - you might try overexposing by 1/2 - full stop.  The guide numbers are often optimistic, especially if you are not indoors.

[craigh]

I can't.  It's a phd camera.  No shutter speed or aperture controls.

Craig