From: "Joseph Chamberlain, DDS" Subject: Flash guide number conversion info. : Dear list members: : First I wish to say hello to all list members as I have just joined the list : and this is my first post. My name is Joseph Chamberlain and I am an Oral : and Maxillofacial surgeon. My interest in photography ranges from : professional applications in my area of expertise to a hobby I have come to : love more than any other. I hope to enjoy the group and learn from : exchanging posts with all list members. Hi Joseph :-) : My first question: : : I have contacted Canon for guide numbers on flash units I currently use for : photo shoots. The information I was actually looking for was the maximum : power output in both Watts and Lumens but (much to my surprise) Canon's : technical support department wasn't able to provide me this information. : : How can I convert a flash guide number to Watts and Lumens ? What conversion : formula can I use for this ? Lumen = The unit of luminous flux in the International System, equal to the amount of light given out through a solid angle by a source of one candela intensity radiating equally in all directions luminance = candellas/m2 lux illumination = lumens/m2 lumen, flux of light = candellas x steradian (solid angle) / m^2 guide number = distance x effective aperture, an observed light output. Effectively this is dependant on reflector design, discharge power, xenon tube diameter and length, diffuser design, flash duration and other factors , now complicated more by automation functions in modern flashes. Watts is an electrical term, the power output of the capacitor discharge and only losely compares to light output. Affected by capacitor ripple current, capacitor design and efficiency, flash tube length and width, internal resistance of the circuit etc.. As you can see, these things all loosely lie together but to derive a formula linking these units which measure different things would be quite a task anbd would be different for each and every flash unit design. k
