Thank you Karl for your detailed and very useful answer. I realize that converting between the three units is not a simple process that can be derived from a simple equation or formula. I was hoping to get an approximate conversion that would help me in selecting studio lighting required to replace my flash units. In case you know a away to do this conversion although it may not be precise or accurate, all I really need is to have an idea of the luminosity provided by the flash so that I can replace it with other light sources. Thank you again for your detailed post and help. Best regards, Joseph Chamberlain, D.D.S. Oral and Maxillofacial Surgery ---------------------------------------------------------------------------- On 4/21/05 5:49 PM, "karl shah-jenner" <shahjen@xxxxxxxxxxxx> wrote: > > 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
