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SUNNY 16 RULE

How to use the gn (guide number)?
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December 15, 2000

Sunny 16 Rule:
The sunny 16 rule is what my father used for estimating exposure outdoors for many years . . . he was an expert at it. Now, with all the internal metering that camera bodies have had since the late 1960's, it's almost a lost art. Under clear clear sky in bright sunlight a proper exposure can be made by setting the aperture to f/16 and setting the shutter speed to 1 divided by the film speed. Example: With ISO 100 film, you would theoretically set the aperture to f/16 and the shutter speed to 1/100th second. Since the closest speed on most modern cameras is 1/125th, you would use that (it's only 1/3 stop faster) and you would be close enough. You can trade aperture f-stops for shutter speed stops. In the example, if you wanted to use 1/250th for a shutter speed, you would set the aperture to f/11. For 1/500th, set the aperture to f/8. For situations other than bright sunlight, such as a cloudy day, it gets a little more complicated. You have to compensate for less light. If you read the exposure portion of a film data sheet, this tells you how to compensate under conditions other than bright sunlight in using this rule. Kodak now prints this on the inside of the film box instead of on a separate sheet (I'm giving away my age with this).

-- John

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December 15, 2000

This posting is about your GN question.

Flash Guide Numbers:
The GN indicates how much light a flash unit will output at full strength. You will often see "in feet" or "in meters" and "at ISO 100" given with a flash unit's GN. The GN is found in the specifications for the flash unit, usually at the very beginning or at the very end of the instruction book for it.

If the film speed is something other than ISO 100, you have to convert the GN for use with a different film speed. To convert a GN for ISO 100 film to any other film speed, multiply the ISO 100 GN by the square root of the film speed divided by 100:
GN = Squareroot(film speed/100) * (GN for ISO 100 film)
Example: ISO 200 film and a flash with an ISO 100 GN of 110.
GN = Sqrt(200/100) * 110
GN = 1.414 * 110
GN = 156 at ISO 200 (approximate)

In the days of flash bulbs, which had a single output level, using the GN for the flash bulb was a necessity to have a proper exposure. It is used, along with the distance between the flash and subject, to calculate the f-stop number for the lens aperture.

To use the GN for an electronic flash, you _must_ use the flash at full output in manual mode. Divide the GN by the distance between the flash and the subject for f-stop number to set the lens aperture. That is why it is specified in either feet or in meters. Be certain you use the correct one! You can estimate the distance if the focus ring on the lens shows distance and you manually focus the lens on the subject.

Set the shutter speed to the flash sync speed (X-sync) for the camera body being used. With older SLR's this is often 1/60th. With newer ones, it can be as high as 1/125th or 1/250th. Your camera instruction book will tell you what the X-sync shutter speed is.

Example 1: ISO 100 film, flash-to-subject distance of 10 feet, and flash GN of 110 in feet @ ISO 100. 110/10 = 11. Set the aperture on the lens to f/11 for a proper exposure.

Example 2: ISO 200 film, flash to subject distance of 20 feet and flash GN of 110 in feet @ ISO 100. First covert the GN for ISO 200 film. From the previous example this is 156. Now divide 156 by 20. This is 7.8, for which the closest f-stop setting on a lens is f/8. This will be close enough for a proper exposure.

To keep from having to convert flash GN ratings at ISO 100 to different film speeds all the time, I computed them once for each different flash unit I have, and for all the common film speeds. Then I wrote them on a 3x5 index card, and put that in the camera bag.

-- John

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December 15, 2000