Is the Flash GN calculation or formula explained with bare strobe or softbox?

Question

I just re-read the title of "What is the quantitative relation between flash guide number and ISO?" and was wondering:

Is the GN formula stated referring to bare light or with softbox?

Answer 1

A published “Guide Number” is just an approximation. In most cases they work however you can do some fine-tuning.

It’s easy, your revision will take into account any modifiers such as room size, ceiling height, wall/ceiling reflectivity, diffuser type/size and or reflector size/type.

Setup a typical situation, perhaps has a friend model for you. Let’s say it’s a portrait in your living room, subject distance 10 feet. The published Guide Number is 160. Calculate the aperture setting thus 160 ÷ 10 feet = 16. Use f/16 as the basis for this test.

Now shoot a series bracketing this value. A full stop series would be f/22 – f/16 – f/11 – f8. If you want, you can further refine the test using series based on 1/2 f-stop or even 1/3 f-stop..

Now evaluate each image, say the f/11 shot was best. Calculate your revised guide number which will be based on your situation (room/equipment). That will be the f-number used multiplied by subject distance, in this case, 10 feet X f-11. The math is 10 X 11 = 110. This value, 110 is your revised guide number.

Answer 2

B&H's article "Understanding Guide Numbers" explains:

Guide numbers are based on a simple mathematical equation that states: the light output of an electronic flash is equal to the distance of the flash unit from the subject multiplied by the lens aperture, or f/stop. As a method of standardizing the process, manufacturers use ISO 100 and a flash-to-subject distance of 10' as fixed reference points when calibrating guide numbers.

[...]

In the case of studio lights, manufacturers go one step further by using a standard reflector on the flash head as a third fixed parameter.

Meaning that for speedlights, whatever gives the highest guide number (GN) is used for measuring¹. This usually means that the bare head is used, as soft boxes and diffusors reduce the light output. ^{You can try this: stack 20 diffusors and see how much light is left.}

¹ _{I think they do not even measure it at all - they just look up the Ws of the flash tube and then calculate the GN with that.}

With studio lights, it seems that they "fixed" this shortcoming of the measurement.

---

There is not even an agreement on which "zoom" (as in: area that the flash light covers, given in focal length-equivalent values [mm]) setting to use: Theoretically, the Speedlite 600EX should have an edge of 17 GN in comparison to the Speedlite 430EX, which it only has at its highest zoom level, thus concentrating the light in a tighter frame than the 430EX can (the 600EX has 200mm max. zoom, the 430EX "only" 105mm). Compare them at a wider angle, and the difference is far less than 17 GN.

Answer 3

Is the GN formula stated referring to bare light or with softbox?

It's based on the flash at full power with no external modifier involved using ISO 100 (unless ISO is otherwise designated). Internal modifiers, such as a zoom head on speedlights, are taken into account. The same flash at the same power level may not have the same GN at different zoom head settings. Manufacturers tend to use the measurement that gives them the highest number. This is normally at the narrowest zoom setting.

Each one-stop reduction in power also results (more or less) in the guide number being reduced by the square root of two. If your flash's GN is 65 at full power, it would be GN65 ÷ √2 = GN46 at 1/2 power, GN65 ÷ (√2)² = GN33 at 1/4 power, GN65 ÷ (√2)³ = GN23 at 1/8 power, and so on.

Using very narrow apertures to reduce overall exposure will result in image softening diffraction. Reducing power allows one to keep the light the same angular size in relation to the subject. Backing up will reduce the angular size of the light in relation to the subject and make it 'harder' light. Sometimes you might want 'harder' light, at other times you may not. For the softest light, the flash and modifier need to be as close as the subject as possible.

If full power isn't enough light for the aperture you desire, increasing the ISO setting one stop will increase the brightness of the image by one stop. This allows you to brighten the image without decreasing the depth of field as would be the case if you opened up the aperture to make the image brighter. Increasing the ISO by one stop also will increase the effective distance of the flash by the square root of two. So if you want harder light but you're already maxed out on flash power, increasing the ISO one stop will allow you to move the flash back by a factor of 1.414X and get the same overall exposure.

Increasing the exposure time won't affect the power of the short duration flash, but will affect the strength of the constantly-on ambient light. This is often undesirable as it alters the balance between the ambient light and the light from the flash.

Different modifiers absorb/deflect varying amounts of light. They also reflect or diffuse light in different ways. There's no precise way to "rate" how much light a modifier "eats". Some people, when writing reviews for a particular modifier, will meter the flash without the modifier and then meter it again with the modifier and report the number of stops difference. But there's no guarantee that the same flash with the same modifier will lose the same amount of light at one distance as it will at another, or that the difference will be the same when using full power with and without the modifier versus reduced power with and without the modifier.