Any flashes available with actual guide numbers?

I have tried three of the more powerful hotshoe flash units

available on the market: Sunpak 383, Vivitar 283, and Metz 54MZ.

From the the density of the images on film and from flash meter

measurements, I believe that the actual guide numbers of these

flashes are about one stop lower than guide numbers that the

manufacturers claim.

Instead of have a guide numbers of around 120, they have guide

numbers of around 80. Bill Harris in an article entitled, "Flash

Guide Numbers: Fact or Fiction?" reports a similar result with a

Vivitar 283. http://www.mindspring.com/~woharris/tech/gn.htm

One stop less actual light means that for adequate flash in manual

and automatic mode, one needs to set the ISO of the flash one stop

lower, or the f-stop setting one stop higher, or the power level one

stop higher.

Last week I forgot to make the adjustment, and a roll of film came

out less than ideal.

I would like to have a flash where one can use the ISO or f-stop

that the camera is set to. There's enough to think about in

photography already without having remember to make an adjustment to

ones flash (and in the right direction, too!).

Does anyone know of any flashes that use actual guide numbers?

The guide number depends on many factors, and are frequently hyped by the manufacturer. Does the flash head have variable focus? Are there any light modifiers in use? The GN also depends on the subject and the environment. A small, light-colored room results in an higher guide number than a reception hall. Dark clothing absorbs light that would otherwise be bounced around the room for fill. The environment also affects the performance of TTL metering.

The state of charge of the flash unit, and variability between units are other factors. The ready-light comes on at about 70% of the full charge. With TTL metering, that has little effect, since you can work off a partial charge. If you need a full pop, that's another story.

If your critical shots are wedding formals, for example, and you use guide numbers (useful!), take some meter readings and test shots under field conditions to establish the guide number you should use.

You'll also find that individual units, same make and model, will vary as well. You really do need to do a test for your individual flash. My ussual test is to put my meter on a stand 10' from the flash in a dark room, take a reading and multiply the f/stop by 10 (becasue the flash was 10 feet away from the meter) to get a working guide number for the iso the meter was set for. If you want to be even more precise, put a gray card on the stand instead of the meter, and shoot a big bracketing sequence. Which ever f/stop gives you a density of around .80 for the gray card gets multiplied by 10 to give you the guide number. The sencond way is more accurate because it tests your film, camera snd flash together, but I'm ussually too lazy, and do it with the meter.

Peter

It sounds like being able to somehow set the guide number on a flash to adjust the unit's distance scales (manual flash) and f-stop settings (automatic flash) would be a good feature. Such an adjustment would allow one to take into account manufacturer's hype, individual unit variation, and variation in where the unit is used.

Thanks for the input.

No such flashes exist. Most, if not all are about 1 stop less powerful than advertised, for the reasons others have stated above. Mostly advertising hype and differences in environment. Manufacturers test their units in optimum environments. Metz used to advertise that their 60CT-4 had a true guide number of 197. It too is about 1 stop less powerful than that in practical use.

I don't understand why it is a big problem. For auto thyristor units, just set the ISO one stop slower. On the Metz 54, there must be a switch to push the flash one stop more. Or use the camera flash compensation.

The Metz 54 does have a feature where one can adjust the output of the flash in automatic (thyristor) mode by 1/3 stops to 3 stops (I think). This adjustment stays as long as one keeps the flash in automatic mode. One can change ISO, f-stop, focal length, and turn the unit off and on and the adjustment stays in place. Change to manual or TTL mode, and the adjustment goes away, permanently.

In manual mode, the flash gives a distance readout (via LCD) in feet or meters for a given power setting, f-stop, ISO setting, and focal length. There is no permanent adjustment for output in manual mode. One has to change the ISO, f-stop, or given power setting to implement an adjustment. If one makes the adjustment to the ISO setting, then when one changes ISO setting one needs to remember to include the adjustment in the new setting.

I think that this bothers me because I am an engineer. I like things to be elegant.

Otherwise, the Metz 54 appears to be a very nice flash unit.