flash query

[warwick_de_winter]

I use a vivitar 283 flashgun reflected into a brolly. The guide no.

is 36 at 100 ISO(m) If I were to double up, i.e. TWO 283 guns

reflected parallel into the same brolly would that give me a guide no

of 72 or what have I got? Help please. Thanks in anticipation.

Warwi

[jack_congson3]

Whenever using flashes in the studio or on location and especially when reflecting against an umbrella or "brolly", I prefer to use the concept of watt seconds or joules which is a measure of the amount of energy output by the flash unit. The F-stop of light output by the flash/brolly combination on a subject that is say 6 ft away should be measured with a flash meter right in front of the subject's face and facing the light source.

This amount of light is dependent on the flash, the reflecting surface of the particular "brolly" and the distance that the flash is mounted from the "brolly".

The 283 is a 100 watt second flash and mounting two flashes in parallel at the umbrella should essentially double the light output. I have seen an ad in Shutterbug magazine on a tool that allows you to mount 3 vivitar 283 on one umbrella, essentially tripling the amount of light available.

The concept of guide number is just a rough convenient way of determining flash distance and camera aperture when shooting in manual mode. The use of the flash meter is still the sensible way to determine accurate exposure.

[larry_albert]

Warwick,

 

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Your new guide number for two flash units is 51.

 

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You have a guide numberof 36 when using one flash unit.

To get a guide number of 72 would require FOUR flash units.

 

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Detailed explanation:

One unit f4 @ 9 feet Guide number 36

Four units f4 @ 18 feet Guide number 72

 

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18 feet is twice as far as 9 feet,

but light falls off at the square law rate.

Double the distance and you have one fourth of the original light level.

Triple the distance and you have one ninth of the original light level.

 

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To double the guide number would require four times the inital light level.

 

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Finally HOW to calculate the answer to your question;

Two of you units would give you a new guide number of 51.

That is your original number (36) times the square root of 2 (1.414) Gives the answer of 51.

 

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General case formula:

(Original guide number) times (square root of(number of flash units))

 

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Disclaimer:

This answer and formula are only valid when using flash units of identical outputs.

[larry_albert]

Warwick,

 

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Thinking more about your question a second method to get the same answer came to mind. The second method is less complex and IMHO more

insightful.

 

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Your guide number for one flash unit is 36 - f4 at 9 feet.

 

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Using two light units will put twice as much light on the subject at 9 feet. Twice as much light means that you will need to close the iris one stop.

 

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You will now be using f5.6 at 9 feet.

 

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New guide number (5.6 x 9) is then 50.

 

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This second derivation/solution may be easier to explain to your assistant. It uses less math and more insight.

 

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Good Luck

 

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Larry

[stephen_poe1]

Dear Warwick;

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I am a higher-math impaired photographer who bought a flashmeter a few years ago. I got a Sekonic L-308b for arouns $200.00 US. It is easy to use. I have used more expensive meters that have more functions, etc., for the price the Sekonic seems a good deal.

<p>

One of the reasons I reccomend the flash meter is that it will also compensate for gels or diffussion I might use on the flash heads as wel as other variables. I have been using strobe gear as an assistant here in Saint Louis for about three years or so; there is sometimes some variation (usually no more than 1/3rd a stop) in output from unit to unit. More from Speedotron, less from Broncolor. Probably has to do with how well the pack has been maintained or how old the flash tube is, I guess. I reccomend the flash meter because it will warn you of these discrepencies.

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good luck

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stefan

[andrew l. booth]

One problem you may get if you calculate all your distances/f stops is the reliability of the manufacturers guide numbers. I have a 283, and I know that the manual says something about the guide number varying depending on the room/location (I think they add in a little reflection from surrounding walls when they work out the number). A flashmeter is a reasonably cheap purchase, they remove all the math and guesswork, and the Sekonics are a pleasure to use. I second Stefan's recommendation of the 308 - since the 508 was released the price of the 308 has dropped and it's now around 100 pounds (before tax) in the UK.

[warwick_de_winter]

This is not strictly a question but being new to the net, I have not yet worked out the correct way to say thanks for the the help on my flash query (QV).

I've got the hang now but the consensus seems to be an investment in a flash meter. I'm going shopping! Thanks again, Warwick de Wint

[scott_eaton]

Warwick,

The law of squares puts the guide number at about 50, so we all get A's on photo-math 101. But.....

 

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Computing flash guide numbers may work as a last resort, but Stefan and Andrew have the best advice in that an addition of a flash-meter would be the best route and a good investment. With such a low guide number room reflectivity could really factor in variables, plus, your probably going to be forced to run both units at maximum power and this tends to lead to more unpredictable out-put - especially if your using battery packs.

 

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//scott

[frank_kolwicz]

To flesh-out the responses, I'd like to add that once you depart from the conditions that the flash is rated for, Guide Number becomes meaningless. The GN is based on direct flash with the camera at an adult's eye-level in a room with a white ceiling about 8 feet above the floor. You can't use it in any other way and expect to get accurate exposures (negative film may seem to work OK, but you won't get critical exposures and transparencies will be off.) For example, if you used the flash outdoors, you'd have to compensate by about 1 stop in your direct-flash exposures to make up for the lack of a ceiling.

 

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A flash meter is the only really good way to get accurate exposures for any kind of light manipulation without doing lots of test film on specific set-ups, which, under these circumstances, I consider a waste of film, time and money.

 

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Frank