mccabejc:
Has anyone figured out the guide numbers for the internal strobe on the Oly 5050, including the diffuser on the PT-015 housing? I've read (
http://www.wrotniak.net/photo/c5050/c5050-rev.html#SHUTTER) that the strobe itself has been calculated at 35, though I'm not sure what ISO that's at. Also I'm assuming that the diffuser knocks down the strobe output by one f stop. Which means that at 4 ft., the aperture should be set at f8, or f7 with the housing/diffuser. Anyone agree?
The guide number that Wrotniak states is measured in feet, not meters, air, not water, at a sensitivity setting of ISO 100, which is an unwritten industry standard.
One way to express the equation noted as the guide number formula is:
GN=A*D
Since it is an equation, it may be written various ways to determine the third fact when two others are known.
A=GN/D
D=GN/A
Also express the same concept.
if the GN=35, then any combination of aperture times distance that is similar should theoretically give an acceptable exposure (ambient light does play into this as an influence).
An aperture of f/8 at 4 feet
f/5.6 at a distance of 6 feet
f/4 at a distance of 8 feet
will all result in similar exposures with a strobe having a guide number of 35 (set at full dump).
Also I'm assuming that the diffuser knocks down the strobe output by one f stop. Which means that at 4 ft., the aperture should be set at f8, or f7 with the housing/diffuser. Anyone agree?
One aperture down from f/8 is f/5.6 (not f/7) So f/5.6 at 4 feet with a one stop rated diffuser installed is equivalent to f/8 at four feet without a diffuser.
when determining f/stop increases or decreases, the factor is 1.4. That is, multiply or divide by 1.4, which is also the square root of 2.
Dave Read
http://www.daveread.com/uw-photo/
goes into an explanation of this in detail. Another source of exposure information may be found at Peter Schulz's web site:
http://www.splashdowndivers.com/photo_gallery/underwater_photography/up_settings_exposure.htm
duncan has offered an explanation to your questions with regards to underwater applications over at digitaldiver.net:
http://www.digitaldiver.net/yabbse/index.php?board=2;action=display;threadid=12536
regards,
b
