1. ## Re: Flash output question

Pearl has a store at 225 Chicago Ave. They sell foamcore, and probably plexi also:

The flashmeter is held by the subject while each stobe is fired individually, then the lighting ratio is adjusted, then a reading is taken at the subject with all strobes firing.

2. ## Re: Flash output question

I suspect that the watt-second rating was originally used to disguise how weak strobes were (and are) compared to flshbulbs. Over time, guide numbers morphed into a light output relation ship to ASA 100 film. Originally, they were for different groups of film. When I worked for a newspaper, I used a #25 Westinghouse flash bulb @ f22 at 10 feet on the lens board. Super Panchro-Press film. In focus area 7 ft to 22 feet with a 6 inch lens. The first strobe I ever saw was a Honeywell Strobinar. It put out a lot of light. But it took heavy batteries...the Strobinar weighed about 35 pounds with battery. My Kalart flashgun weighed maybe a pound and a half, and the bulbs were light as a feather. I used a mechanical synchroniser until we were issued cameras with built in synchros. Mechanical synchronizer meant all voltage applied to kicking off the flash bulb~none to a solenoid synchronizer. Oh yes, #5, and 25 bulbs were supposed to be equal...but some brand bulbs were more equal than others. I used Westinghouse 25's because they were more equal than the other brands.

3. ## Re: Flash output question

Originally Posted by Norm Buchanan
I am not sure how to determine the f-stop light output. I don't have a flash meter. Is there a typical distance from the flash you would make this measurement?
It's called the Guide Number (GN). The GN divided by the distance in feet equals the f-number for ASA 100 film.

GNs are typically provided with on-camera flashes.
They usually not given for studio strobes because you're expected to use a meter with those.

The whole point of a flash meter is that studio setups typically use several strobes, perhaps of different power levels and with
different modifiers. There's no way you could accurately calculate the resulting light level at the subject by hand.

If you're serious about studio work you need to get a flash meter.

- Leigh

4. ## Re: Flash output question

I shoot outdoors at night a lot, using fairly massive flash power (I light up moving freight trains.) I have some thoughts. Those little on-camera flash such as Nikon SB-28 are rated (perhaps very optimistically!) by Guide Number. This factors in the amount of light actually delivered on the subject. Studio lights, such as the monolights I use, are rated by the amount of power their capacitators can dump. The reason they use that figure is it is an objective way to compare the potential output of the strobes. There is huge variation between the actual amount of light that reaches the subject because when you use monolights you are often using a light modifier such as umbrella, softbox, grid, etc. and those eat up some of the light. The actual GN of the monolights depends on the reflector size that you are using. I too use "brute force" with flash since I'm shooting outdoors and there are no walls to reflect the flash. Currently I am using up to 10,000ws of power with 8.5 inch reflectors. I sometimes use a grid on the lights which eats up about a stop of light depending on the grid.

Now to discuss shutter speed. It doesn't matter. The flash duration is something like 1/800s. The light pops, and 1/800 of a second it is over. Whether you are shooting 1/30s or 1/250s it just doesn't matter because the flash is SO brief. Note that most DSLR/SLR cameras have focal plane shutters and can't sync faster than 1/250s anyway. Large format shutters in Copal 1 can sync up to 1/400s, and my Rolleiflex MX can sync at 1/500s. ALL of those syncs are slower than 1/800s. While shutter speed does not matter for flash, it does matter for ambient light. Usually there isn't any for me, out in the middle of the night on the dark prairie. The thing to remember is that flash is controlled by aperture, and ambient is controlled by shutter.

Now for that 50% figure. To gain one stop of light, you have to double the number of flash. Here's how it works, assume you are using something like Nikon SB-28 flash (or whatever.) You start with one flash. To gain another stop, you will use two flash. To gain a second stop, you will use 4 flash. To gain a third stop, you will use 8 flash. Etc. There is no "50%" about it. To really nail a flash exposure, use a flashmeter such as Minolta Autometer IVf. Don't forget to factor in light fall-off if you are shooting some distance from your subject. I might be 50 yards away from the trains I photo. If I place the strobes 50 yards from the train, and then I am taking the shot from 50 yards away from the train, I meter the flash while standing on the tracks, then add one more stop of light. You have to factor in the TOTAL distance the light is traveling: 50 yards + 50 yards = 100 yards.

Now here's something for you to chew on. Using one SB-28 flash, I can get an exposure of ISO 800 & f4 with flash placed about 25 feet from the tracks. Exposure is measured by standing on the tracks using a flashmeter. If I replace with one White Lightning x3200 and shoot I get an exposure of ISO 400 and f8, or a difference of three more stops. One SB-28 is roughly equal to 70ws, and the x3200 is ~1300ws. In theory the x3200 should equal the output of 18 of the SB-28, but instead it's more like only eight of them. The question for you to answer is why this is?

Kent in SD

5. ## Re: Flash output question

"The output for any kind of photo lighting is given in W.s (watt-seconds)."

No. Watt seconds is the storage potential of the flash. The output is measured as BCPS or as a guide number. BCPS and a guide number will vary depending on the angle of the refelctor, the design of the flash tube, the length and diameter of the cables and wiring, the number and type of connectors between the flash tube and the capacitors, etc.

So a 40° reflector and a 120° reflector used on the same power pack at any given power setting will have two completely different guide numbers even though the ws ratings were identical at 100% full charge.

Watt seconds are as reliable a means of determing exposure as horse power is to how fast or quickly a car can go.

There is no formula to directly convert a WS into a F stop.

6. ## Re: Flash output question

Paul Buff has a nice table of expected light output with various modifiers. Even though the OP doesn't own Alien Bees, he can still use the table to estinmate light loss with various modifiers.

http://www.paulcbuff.com/output.php

I don't rely on power ratings. Direct measurement of light output is much better, either by using Guide Number or with a flash meter.

7. ## Re: Flash output question

Originally Posted by Bob Salomon - HP Marketing
"The output for any kind of photo lighting is given in W.s (watt-seconds)."

No. Watt seconds is the storage potential of the flash.
Watt seconds are given for both generators and heads - you are right on the former, but for heads, the W/s give the electrical throughput of one full discharge. With integral compact flashes, YMMV - Bron and Multiblitz used to give the latter figure at least on the model generations I own, but some makers prefer the more euphemistic W/s of the capacitors...

Originally Posted by Bob Salomon - HP Marketing
The output is measured as BCPS or as a guide number.
It is hard to give a meaningful figure for light output - most makers of serious flashes don't even attempt it, at least in their German language marketing. The usual Chinese amateur flash importers often will give fantastic GN figures for narrow-angle reflectors which nobody would ever point directly at a subject, to disguise the rather underwhelming low power of the devices.

Originally Posted by Bob Salomon - HP Marketing
There is no formula to directly convert a WS into a F stop.
Right. However every experienced studio pro will know how many W/s (s)he'll need for the chosen reflector/diffuser and given flash-to-subject distance - on the other hand, I have never encountered anybody calculating their flash needs by BCPS or GN.

8. ## Re: Flash output question

Originally Posted by Sevo
Watt seconds are given for both generators and heads - you are right on the former, but for heads, the W/s give the electrical throughput of one full discharge. With integral compact flashes, YMMV - Bron and Multiblitz used to give the latter figure at least on the model generations I own, but some makers prefer the more euphemistic W/s of the capacitors...

It is hard to give a meaningful figure for light output - most makers of serious flashes don't even attempt it, at least in their German language marketing. The usual Chinese amateur flash importers often will give fantastic GN figures for narrow-angle reflectors which nobody would ever point directly at a subject, to disguise the rather underwhelming low power of the devices.

Right. However every experienced studio pro will know how many W/s (s)he'll need for the chosen reflector/diffuser and given flash-to-subject distance - on the other hand, I have never encountered anybody calculating their flash needs by BCPS or GN.

Sevo,

I was the Product Manager for Rollei Studio Flash equipment (Berger) as well as the Product Manager for Multiblitz flash and was employeed by EPOI as the USA national salesman for Bron flash.

Watt sceconds is no way to rate a flash except as a sales tool.

9. ## Re: Flash output question

Originally Posted by Bob Salomon - HP Marketing
Watt sceconds is no way to rate a flash except as a sales tool.
That's like saying that you just can't tell how much light a 100W lightbulb will produce.

10. ## Re: Flash output question

You are correct. Is the 100W bulb clear or frosted? Is it halogen or tungsten? Is it the equivelent of 100W LED? What reflector is behind it? What type of shade is in front of it?

I just replaced six 40W frosted tungsten bulbs in a bathroom with six 40W equivelent LED frosted bulbs and the new ones are far brighter. So much so that I am now looking for 25W LED ones.

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