Monolights and guide numbers ?

[mitchfalk]

I've read all the specs on practically all models and brands of monolights

available, are these claims actually true or is it (like Paul Buff states)

dependent on the reflector and angle ? Or should I just go ahead and buy the

highest wattage model and forget about GN's ?

[ellis_vener_photography]

it is absolutely depenent on reflector and angle. More power is generally always better unless

you like working at f/2-f/1.4 - -and even then you can use neutral density gels over the

lights (easier with reflector and softboxes thann with umbrellas -- to knock down the light

output.

[savagesax]

Unless you are really in need of kicking out max power my belief is they all can vary some. Maybe a 1/4 stop at full power at 60 feet. I have the photogenics and the white lighting. Also have Norman, but never use them. They all seem really good. Both the white lightings and the photogenics rebeat within a 10th of one F-stop. Pretty amazing for sure.

 

As far as the highest wattage, sure buy what you can afford! To be honest though I hardly ever go full power. I bet it's been 5 years since I cranked one up to full power. I shoot mainly weddings and portraits so I'd rather set up 3 or 4 lights to achieve even light at much lower power.

 

So the question really goes back to you... Do you need full power for your type of photo applications? Best wishes

[garry edwards]

Basically it's the guide number that counts, and as Ellis says that is dependant on reflector and angle. It is also affected by environment - i.e. a test in a small room with white painted walls and low white ceiling will produce a higher figure than a test in an aircraft hanger.

 

In a perfect world, all manufacturers would agree on a set of testing protocols. We could then trust their figures - but they don't, so we can't, unless they also publish the conditions under which the tests were carried out.

 

This introductary article may help http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00Futx&tag=

[miguel_mellado]

I read this, may be help you (Of "Light Science & Magic" Fil Hunter and Paul Fuqua) ):

 

" Strobes are rated by three basic measurements: watt-secconds( o joules), beam candle power seconds (BCPS), and guide numbers. These are all useful...

Watt-seconds measure the power used by the power supply, not the amount of light coming from the strobe head. The light getting to the subject is also influenced by such factors as the efficiency of the reflector on the strobe head and whatever diffusion material may be in front of it.

BCPS is a true measure of light output, with all elements of the system taken into account. AS an extreme example of the distinction between the two measurements, consdider an 800ws strobe head with 10000 BCPS. If we cover that strobe head with an opaque wrapper, the power consumption (watt-seconds) is not changed, but the light output (BCPS) drops to zero.

Guide numbers are based on BCPS plus the light sensitivity of the film...

Strobe manufactures put much effort into the design of theri reflectors. A strobe head with a highly efficient may light a subject several times as brightly as another head with the same power but a less efficient reflector. These manufacturers continually feel frustrated bu photographers who pay too much attention to w-s , because tht measuremente ignores their efforts to prouce more efficient srobe heads.

Unfortunately, when we place a diffusion sheet in front of a strobe head, or put the head in a soft box, we undo much of the efficiency in the design of the strobe. This is particularly true if we decide to use one of several brands of soft boxes that require us to remove the strobe manufacturer?s metal reflector entirely in order to fit the head into the softbox....

[paco_rosso]

<p>The relation beetwen BCPS and f number readed with a photometer is this:<br />First: you have to read the f provided by the flash at a distance d with a flashmeter.<br /> Never read the flash at less than 2 meters. Distance recomended is 3.16meters.<br /> BCPS = 270*f^2 * d^2 / s<br /> Where f is the f number readed in the photometer. f^2 is f*f.<br /> d is the distance in meters from flash to photometer. d^2 is d*d.<br /> s is the sensitivity in ASA number, it is, the ASA part of the ISO number.<br /> If you use feets, then must change the constant 270 to another suitable to imperial units.<br /> Recomendations: set the photometer at 3,16meters, it is the square root of 10. Use ISO 100/21.<br /> In this conditions the equation is:<br /> BCPS = 27 * f^2</p>

<p>BCPS is a magnitude, not a unit. The unit of BCPS is candles per second: cds<br />. And is the light intensity (candles) which produce the same exposition in 1 seconds that the lamp we are measuring.<br /> <br />The relation with the guide number is when the distance is 1 meter. As we cannot measure the f number at so short distance we can change the equation in this way:<br /> BCPS : BCPS = 2.7*f^2*d^2<br /> f=GN / d<br /> So:<br /> BCPS = 2.7*GN^2<br /> GN = SQRT (BCPS / 2,7)<br /> GN = 0.608 SQRT( BCPS )</p>

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