Guide numbers? Gaming????

[robert_davis2]

I thought guide numbers were

 

GN = aperture X distance [feet or metres]

 

So if you move closer the aperture gets smaller. If you move farther

the aperture gets wider. But the GN stays the same.

 

Now I've been reading the postings on flash power and I think I've

learned a few things. But now I think I'm confused on GN. The postings

mention GNs being misleading because the manufacturers pick

non-standard distances. Which would seem to mean I'm lost when it

comes to GNs.

 

So what are Guide numbers then?

[garry edwards]

Guide numbers are the most accurate expression of power. Joules or Watt seconds are less informative because, although they are related to power (eg a 5000WS flash will be more powerful than a 100WS flash) they are really about stored energy rather than actual output.<p>Problems with the interpretation of guide numbers arise in 2 ways.<br>1. With professional studio flash units, the reflectors can be changed. A highly-polished reflector will produce a much higher guide No. than one that is painted flat white, or even grey. Which reflector was used when measuring the guide No? A narrow-angle reflector will produce a much higher guide No. than a wide-angle, again which one was used when measuring the guide No?<br>2.Small, on camera flashes often have zoom settings. Which zoom setting was used when measuring the guide No? If, for example, they used a 105mm setting then the guide No. will be high - but if you are using a 35mm lens you will have to set the zoom on the flash to 35mm, where the true guide No. is very much lower.<p>What it really comes down to is the integrity of the manufacturers. Some manufacturers, Elinchrom for example, actually publish the guide numbers that can be obtained with each of the various reflectors and accessories that they sell. Others may be more concerned with marketing hype than with providing accurate information.<br>Hope this helps.

[tony_johnson1]

Robert, you are basically correct. The Guide No., which is a measure of the maximum amount of light produced by the flash (on Manual setting only, the Auto setting can reduce the light output) is a constant for each flash unit.

 

eg With a GN of 110 and an object at 10ft, an aperture of f/11 is required. If the object is moved to 5ft, this will receive four times the amount of light( remember the square law applies, halving the distance gives not twice but 2 x 2 the amount of light)therefore you need to open up by two stops to f/22. (Each f-stop doubles the amount of light through the lens).

 

You can see that both 10 x 11 and 5 x 22 equal the constant GN of 110.

 

There is also the need to use the correct GN for either feet or metres. In the above example, if you measure the distance in metres, the GN is 33. And to further complicate the issue, GN's are usually quoted for 100ASA film. If you have 200 ASA film, the GN would be 220.

To sum-up, if you are not mathematically inclined, use the chart on the back of the flash or the Auto mode!

 

Tony

[bruce_rubenstein___nyc]

"The postings mention GNs being misleading because the manufacturers pick non-standard distances. Which would seem to mean I'm lost when it comes to GNs."

 

If you read Timber's "Guides" you will get a lot of not quite right information. There are some basic ways a manufacturer can fudge GN's. The first is to use calculated, based on the flash design and not actual tested numbers. The other thing is the room the flash is measured in. If the room is small and has bright white ceilings and walls the meter will pickup the reflected light and give a higher numbers than if it were dark or large. Another common thing that they do on adjustable reflector/zoom flashes is use a zoom setting that doesn't fully cover the field of view of the indicated focal length. This gives a hot spot in the middle and light fall off in the corners. When I use my SB 28 I set it to manual zoom and set it to a wider "focal length" than the focal length of the lens that I'm using.

[peter_witkop]

You've got the basics down pretty well, and the above posts are good advice. To anwser about the manufacturers guide number, they don't use non-standard distances (and as you point out, the distance wouldn't a differance), as their testing procedures. Standard GN tests are done in 10' long narrow white hallways, which bounce tons of light around, giving higher effective guide numbers than you'd get an a more realistic testing situation. The last step for you to do is to do a guide number test. Setup your flash, and setup a gray card 10' away (10' makes the math simple is all, any distance can work), and shoot a frame using the manufacturers GN, then open up in 1/3'rd stops for about 2 full stops. Which ever frame gives the density of the gray card at 0.80, was the correct aperture, for example say F9, and using the GN forumla you've got a gn of 90. Alternatly (though a tad less accuratly) you can use a flash meter at 10' from the flash to get the correct f-stop. Hope that was helpfull :o)

 

Peter

[jbq]

Tony: actually, doubling sensitivity only multiplies the guide number by 1.4 (which doubles the surface to be lit).

[robert_davis2]

I remember awhile back some one claiming that a Canon flash was equal in power to a small monolight. When I went and checked they did have the same guide number but the flash was rated with a 105mm lens.

 

Okay I'm thinking of a monolight. The manufacturer claims 180GN but with the standard reflector they claim F/22 at 6 feet. So can I believe that it really is GN 132 then?

 

A bigger reflector would equal wider coverage? Right? So when I compare lights a set with a bigger reflector and a higher GN should be more powerfull?

[bruce_rubenstein___nyc]

"A bigger reflector would equal wider coverage? Right?"

 

Wrong. It's the geometry of the reflector that determines how wide or narrow the coverage is. Comparing the Guide Numbers of shoe flashs to monolights is almost meaningless. For instance my SB28 and Q flash have pretty similar GN ratings. When I measured their output, on axis, with a light meter (same room, lightstand, meter, etc.) there was only about a 1/2 stop difference. Move off axis and the difference is dramatic, because the Q flash has a much broader, even coverage. The total light output (radiated into a hemisphere) is greater with the Q flash. I'm sure you'll see the same sort of difference with monolights vs. shoe flashes.