Guide Number vs Watts-Second

Guide numbers are mostly used on small flashes usually mounted on

cameras and watts-second is being used on studio strobes/lighting.

Does anybody know the computation from Guide Number to Watts-Second

and vice versa?

I don't think you can do it without knowing the angle of illumination.

 

The guide number simply tells you how much light you will get reflected back from your subject at a given distance. Most shoe mount strobes have an angle of illumination of about 50 degrees. If you had two strobes and both had the same guide number but one illuminated an area of 50 degrees and the other 120 the second one would be several times more powerful.

There is no easy conversion. Ratings in Watt-seconds tell you how much power is going into the strobe head. How much light comes out depends on the efficiency of the head, the bulb, and any reflectors or diffusers in use.

For a given flash unit (and reflector setting) you can relate the guide numbers at various power levels. The ratio of guide numbers is equal to the ration of the square root of the power in joules (watt-seconds). The question is largely moot, because most flash units designate power reduction in terms of f/stops anyway.

 

As others have said, there is no relationship between power and guide number because the distribution of light (power) varies widely with reflector settings and design. You will notice that many flash manufacturers specify the guide number at "105mm", because the light is more intense when focused for a longer lens.

The guide number of a flash has nothing to do with how much light is reflected back from your subject at a given distance. A white subject will reflect much more light than a black subject.

 

Guide numbers do tell you how much light from the flash is falling on your subject.

 

As David said, watt-seconds are a measure of the power going to a flash tube. The amount of light (guide number) hitting the subject depends on the design of the flash head, its reflector, any modifyers such as softboxes, scrims and umbrellas and the distance from the flash to the subject.

This question gets asked about evry three months or so. The answer is that while some

studio flash manufacturers do publish guide numbers for specific head and reflector

combinations, there cannot be a general rule or equation for converting GN to watt-

seconds or vice versa. Watt-seconds is a measure of potential energy stored in flash units

capacitors while guide numbers are derived from actual out put of the flash unit.

Additionally there are differences in how efficiently a flash head & pack or monolight

convert the potential energy stored in the capacitors to actual light, and different

reflectors further influence the intensity of light - -a wide angle or "umbrella" reflector

spreads the light over a wide area while a narrow angle reflector concentrates the enrgy

into a much narrower area of illumination. the final complication is that there is no

industry wide standard for reflector choice , distance to subject , etc. used when taking the

measurement. Even small flashes are subject to this lack of standards.

This is like converting weight of a car to miles per gallon. Or maybe calories consumed verus body weight. Or number of shoes a woman owns verus her age or weight:). Or number of knives or outboards versus a mans latitude or beers drunk daily.:) There is a trend of data; but no real exact conversion; or maybe none.<BR><BR>Small strobes are normally used indoors; and include bounce from walls and ceilings. If used in a cave; or during a night football event; the guide number is way less; for a given watt-seconds.<BR><BR>Watt-seconds is the energy in the flash capacitor. It varies with the capacitor's capacitance. It also varies with the square of the DC voltage across it. Many are unregulated when in AC adapter mode. A 5 percent drop in AC voltage will be about a 10 percent drop in watt-seconds; about a 10 percent drop in light. <BR><bR>The ready lights come on slightly below full voltage; to allow for tolerances. The units will be a tad brighter if you wait abit longer.<BR><BR>In engineering filming of machinery; some strobes and strobed flash lamps give charts for "Guide Number vs Watts-Seconds"; for BARE-BULB; and with several types of reflector . This is with the old GR strobotach units and others. <BR><BR>Even with the refector known; there is alos the efficiency of the flashlamp; what type it is; what spectral output; whether filtered for UV; and even the bulbs temperature. <BR><BR>If driven in disco mode of a alot of flashes per second; the bulb 's temperature rises during a sequence; and light's output varies somewhat during the bulbs firingBR><BR>Runs some experiments with YOUR strobes!

Thank you all of you for your feedback about this matter. The reason why I asked this is because I am trying to setup a small studio and I am looking for flashes, monolights and the likes to purchase, and one of my basis of buying these lighting equipments is the GN and/or watts-second. I am on a very tight budget and I am not skimping on equipments. I just need what I need, nothing more. I have a 15 feet x 15 feet x 12 feet high space that I can use. Can anybody suggest flash GNs and watts-seconds that I should be looking at? Thanks again!

Another warning regarding using these numbers.

 

Guide number is something that can be measured with a meter, so they are straight forward.

 

Wattsec are not easily measureable and has many different measuring method that it has become a marketing game with the strobe manufacturer.

 

The potential energy at the capacitor travels throught the trigger circuitry, socket, plugs, cables, flashhead circuitry, flashtube to generate the light. Any of these component become a factor in transferring that enery to light.

 

What you should measure is the actual light output of different brands in controlled situation. Mount the head in the speedring of a chimera softbox and measure incident light at 10 feet or incident reading at the surface of the box from center to edge.

 

I have worked a few years selling and renting this equipment in a proshop before I started shooting full time so I have time and resources to play with the equipment before plucking down my money.

Best to rent some kits with pack, head, modifiers and paly with them over a weekend and see what you actually need from different manufacturer, kind of like test driving a car. Some retailers who rents would give you a certain portion of the rental that you have paid out as a credit toward the final purchase price.

<p>Maybe you can use this review of Elinchrom D-Lite 200 (200 W/s) as a guide line:<br>

<em>To put some real figures on output power, a measurement was taken from three metres using an 18cm reflector. At full power, this gave a shooting aperture of f/11.6 at ISO100. Fitting an Elinchrom 70cm softbox reduced this to f/8. For those who like to think in guide numbers, that’s 35 and 24 (m), or 116 and 80 (ft).</em><br>

<br /> British Journal of Photography<br>

http://elinchrom.com/products/compacts/D-Lite/D-Lite_presse.pdf<em><br /> </em></p>

 

<p>If one is applying for a job use proper terms lilke watt seconds W-s ; not watts per second w/s. This error is spread all over the internet. Its sticks out like is somebody said the iso is F11; or if the shutter speed is 1/4-20. Watt seconds is energy; what is in the flash capacitor.</p>

<p>Kelly, you explination is right. Although, to be pedantic, one applying for a job should use Ws, not W-s.</p>

<p>It is not as cut and dried as you would like it to be. Reflector size and shape, efficiency of the flash tube, etc, all can vary from one flash to the other. Watt-seconds applies to the energy directed to the flash tube, guide number is the amount of light that is actually cast on the subject. A reflector that concentrates the light into a smaller angle will produce a higher guide number, with the same flashtube and power source, as one that casts a wider angle of light.</p>

<p>An old topic, but there has to be some correlation. Even if you just assume bare bulb. If I want to know how many SB800s to use to get the same power coming out of the same softbox as an Einstein 640 ws there should be some way to do it.</p>