How do you calculate using guide numbers?

Question

When a flash states that it has a guide number of 91' how does that help me calculate something at say 30'?

I am not poking any fun at people.

GN divided by distance = f/number

GN 91 divided by 30' = Approx f/3.0

So at 91 feet [GN 91 divided by 91 = 1] then I would need an aperture of 1.0

At 10 feet [91 / 10 = 9.1] f-9.1 or there abouts

Doesnt ISO have something to do with it too?

Answer 1

GN divided by distance = f/number

Be sure you are not mixing feet and meters.

GN 91 divided by 30' = Approx f/3.0

Accept f/2.8 or f/3.2, unless you have a manual lens that you can leave in between clicks.

Guide numbers are ALWAYS expressed in terms of ISO, which you have intuitively figured out. Unless it is otherwise specified, you can assume that the published GN os for ISO 100. The GN at ISO 200 is higher that it is at ISO 100, because the camera detects light easier. I don't know if this exactly follows the inverse square law or not, because a flash is a focused beam and not a point source of light, but the more sensitive your film or sensor, the farther your subject can be. This is also somewhat intuitive. If you can make an image with less light at ISO 200 than you can at ISO 100, it would mean that the SAME amount of light leaving your camera "goes farther" if the sensor or film is more sensitive. (Is there an echo in here?) It qould require less light reflecting from your subject coming back to the camera to make an image.

[NOTE: I took the GN of 27 from B&H. They said the "effective flash range" is 27 feet at ISO 100. I would have to re-do these calculations, but since it works out so close to matching up with apertures, let's just say my example is a hypothetical example and not for the D300, which would be about one stop faster.]

Here's an example. My D300's pop-up flash has a GN of 27' at ISO 100. With a maximum aperture of f/2.8 on the 17-55 lens, this would mean I could only shoot from 9.6' away wide open! Let's say that you gain one stop of sensitivity for each time you double the ISO. Well, you do, so that's a safe statement. Can we back it up and still do the math? With my pop-up, let's round it up to ten feet from 9.6 feet. Examine the equation from another direction.

GN = f/stop x distance.
GN27' = f/2.8 x 10' @ ISO 100.
Let's fudge a little more and switch to 28 as the GN.
GN28' = f/2.8 x 10' @ ISO 100.
That would work out perfectly.

Suppose, we go to ISO 200. Without any math, we know that an ISO of 200 would let me close down one stop to f/4.0. So we now know that at ISO 200, the same flash has a higher guide number.
GN = f/4.0 x 10' @ ISO 200, so GN = 40 @ ISO 200.

Did you notice anything convenient in the numbers? IN MY EXAMPLE ONLY - you can multiply the aperture needed for 10' by ten and come up with the guide number.

Let's see if that works.

GN 28' = f/2.8 x 10' @ ISO 100
GN 40' = f/4.0 x 10' @ ISO 200
GN 56' = f/5.6 x 10' @ ISO 400
GN 80' = f/8.0 x 10' @ ISO 800
GN 110' = f/11.0 x 10' @ ISO 1600
GN 160' = f/16.0 x 10' @ ISO 3200
GN 220' = f/22.0 x 10' @ ISO 6400

See: http://www.flickr.com/photos/samfeinstein/2146106566/

I estimated that I was 100' from the house. Of course, there is some light from the decorations, but see http://www.flickr.com/photos/samfeinstein/2146107904/ for a view without flash at ISO 1600. Also, check out the cupola on top of the garage in both images. It's not lit at all by the decorations.

Okay, back to fun and games.

My numbers above say that the GN @ ISO 6400 would be 220.
Guide number divided by distance = f/stop.
GN220 / 100' = f/2.8.

Hmm. Something's wrong here. My camera chose f/8.0 for me. Wait a minute. GO back to the ISO 1600 shot and see just how much light is already on the subject. Maybe my flash was just providing some fill to an already lit scene. I would need to try shooting at ISO 6400 without any flash and also at ISO 6400 with flash but without any Christmas lights.

If my logic is flawed here, would a pro please step in and save me? It's been over 30 years since I used GN calculations! I think the numbers work out right, but my example is not a good example, because the flash is not the only source of light.

Well, it's been fun anyway and I enjoyed making fun of people who hate math. Besides that, all I did was pop-up my flash and let the camera figure it out. I was just pretty sure that enough light would reach the house at ISO 6400 to make a difference.

~~~~~~

Hmm. B&H said the GN is 27 at ISO 100. PopPhoto says it's 39 @ ISO 100. This is close to a full top better (2.8 vs. 4.0), so all the charts above would be shifted by one stop. Nikon USA uses metric, oddly enough. It says the GN is 12 or 13. (YES - two values. How confusing. 12 for auto and 13 for manual.) At 3.28 feet per meter, that converts to GN 39-43. Maybe I'll get back when the cobweb clears and the sun rises to see how these numbers work out.

Answer 2

are you making fun of people who can;t do math? I hate math!

Answer 3

very confusing situation. check out using google or bing. that will will help!