Larry C:
Can anyone tell me what this statement on the description for this light means? "ISO 100 guide no. 70"
.
ISO is the sensitivity setting of the film or sensor during testing for the strobe's intensity. ISO 100 is the unwritten industry standard at which strobes are tested at. The lower the ISO number, the more light that is need to properly expose the film or sensor. For instance, ISO 50 requires twice as much light to hit the sensor as ISO 100 to achieve a similar exposure. An ISO setting of 200 would require only half as much light to hit the sensor as ISO 100, but at a potential cost of creating additional grain or noise in resulting image quality.
Guide Number is the number given to represent the intensity of light that is emitted from the strobe. The higher the number, the brighter the light.
What is missing from the information supplied is if the measuring was done in feet or meters and if the strobe was tested in air or underwater. I assume that the Guide Number of 70 implies that the strobe was tested on land, light traveling through air, and measured in feet.
To oversimplify the testing procedure, my assumption is that at a distance of ten feet from strobe to subject, the aperture chosen that resulted in the best exposure when the strobe was set to full dump was f/7.
The formula to determine the guide number is : GN=A times D, or guide number equals aperture times distance. If the Guide number was determined to be 70, then f/7 times 10 (feet) equals 70.
Larry C:
...................Would we be better off picking up one of the many YS60's, or SB102-105's on e-bay for similar money in unknown condition?
Considerations:
What type of camera do you have.
What material is the underwater housing is made of.
How will the strobe "talk" to the camera.
What shooting style do you prefer, such as full manual, auto, or TTL.
What types of lenses will you be using to determine coverage requirements.
What are your future goals.
hth,
b