Establish Outdoor Guide Numbers

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I now own several Strobist inspired manual-only flashes (Yongnou 560, the Lumopro 180 Quadsync, and the new Adorama Flashpoint Li-On) which I use more frequently now that my Fuji mirrorless cameras have become my "go to" cameras in the field. I keep a Fuji EF-42 flash when I need the convenience of TTL exposure automation, and can accept the unbearably slow recycle time. The Yongnou 560, when used with an accessory (Canon compatible) battery pack, gives me a fast recycle time and has pretty much become my indoor flash flash of choice.

Like most modern manual flashes, the light output can be adjusted down from full power in full stop, or third stop increments. Coupled with the modern digital cameras' access to extremely high ISO sensitivity levels, manual flashes are seldom used at full-power. This said, matching the output to the subject to flash distance will require some quick mental calculations. These usually involve Guide Numbers, a simple mathematical trick concerning the aperture and the distance for given ISO value. To put it another way:

Guide Number = Aperture * Distance

Backstory:Back in my film days, I did most of my best work with a twin lens reflex Mamayaflex which had a leaf shutter to control exposure. It suffices to say that leaf shutters couldn't be used for exposures of less than 1/500 of a second, but could synchronize an electronic flash at any exposure setting. In those days I was a Vivitar 180 flash, which had a guide number of 56 when used at half-power, a necessary setting if I was to shoot more than 40 shots for the rechargeable, built-in nickel-cadmium batteries. I committed to memory three critical distances; 5', 7', and 10', and three corresponding apertures; F 11, F 8.0, and F 5.6, as the appropriate pairings for each distance. Since I used Plus-X film with a nominal ASA/ISO rating of 100, the appropriate exposure time, based on the Sunny 16 Rule, would have been 1/100 of a second, 1/200 of a second, and 1/400 of a second.

I made an informal test while vacationing in Oroville, using a D70 and a single SB-800 flash to make a series of selfies from which a subjective guide number was determined. In a later post I calculated the light loss associated with Nikon's High Speed Synchronization using a Photovision White Balance Target. I liked the results so much that I decided to repeat the guide number experiment using the test target, a speedlight, a tripod, and a suitable indoor location (Photo #1, below)

A Simple Test: 

• I shot the test shots in our "Great Room", which has a very high ceiling and a maroon carpet covering the floor. For all intents and purposes, in non-reflective environment.
• I positioned a 24" Digital Calibration Target from PhotoVision exactly 10 feet from the speedlight I was testing.
• I mounted a 135mm F 2.0 Nikkor lens on D70 body, and set it to manual, ISO 200.
• Each of the three tested flashes would take their turns mounted in the light stand next to the camera. They would be triggered with a Nikon SC-17 cable.
• With each flash starting at full power output, I would shoot, check the histogram, and adjust the aperture until the histogram's midpoint was positioned exactly at the half-way marker.
• Since the flash to subject distance had already been determined, I calculated the Guide Number by multiplying the final aperture by 10.
• Once the guide number had been determined at full power, the flash output was cut in half, and the process repeated.

Photo #1

Here you can see the three histogram "peaks" representing the black point, mid point, and white point (Photo #2). A centered midpoint can be interpreted as a correct exposure for a typical subject with an 18% reflectivity, just what you might expect if an incident light meter reading had been made.

Photo #2

The calculated guide numbers were as follows:

All Flashes set to 35mm Beam Angle, 10’ from Calibration Target, ISO 200
Manual Power Setting	Yongnuo 560	LumoPro 180 Quad Sync	Adorama Flashpoint Li-On (manual)
Full Power	110	130	130
½ power	80	100	90
¼ power	56	71	63
1/8 power	40	50	45
1/16 power	28	35	32
1/32 power	20	28	22

Once I had compiled all of the is data, a thought occurred to me: If I selected a specific aperture, I could the select the output as dictated by the subject-to-flash distance.

Nikon Coolpix A, 1/500 of a second, F 11.0 ISO 200

Fast Forward to 2016. Due to the output limitations of the Fuji EF-42 Flash, I've resigned myself to shifting my flash output manually. In doing so, I've re-acquainted myself with the fundamentals of guide numbers. All of the tested speedlights can be set at full stop increments from full to 1/128 power. And two of them, the FlashPoint and the LomoPro, support 1/3 stop increments.

My photographic aesthetic has changed, too. Now, my background often includes large amounts of sky. In an effort to maintain a rich cyan color, I've started basing my exposure based on which aperture/shutter combination that gives me the best looking sky and letting my flash provide the illumination for the foreground.In this sample photo, you can see that I have both a blue sky and a hint of clouds at the given exposure settings. The foreground (the walls of my stairwell) is now dependent on supplementary lighting.  This same exposure setting could have been applied to a Nikon D70 which, like the Coolpix, can sync a flash at 1/500 of a second.

I chose the Yongnuo because the tested guide numbers aligned with existing apertures. F11 was chosen because this coincides with the aperture of choice when working with an ISO of 200 and a maximum synchronization speed of 1/500 of a second. It will also accept an accessory battery pack that will give faster recycle time.

Reverse Engineering: From the data gathered from my Yongnuo exposure experiments, I took each calculated guide number (Middle Column) and divided it by 11, my preferred aperture.The appropriate distance for the aperture could be seen at the right.

Yongnuo 560 (F11, ISO 200)
Power Setting	Guide Number	Optimal Distance
Full Power	110	10.0 feet
½ power	80	7.3 feet
¼ power	56	5.1 feet
1/8 power	40	3.6 feet
1/16 power	28	2.5 feet
1/32 power	20	1.8 feet

What Does It All Mean? Based on the table calculations, I would pre-set my camera to an aperture of F11, a shutter speed (exposure duration) of 1/500 of a second, and an ISO setting of 200.  I can then position myself so my flash-to-subject distance is 2.5 feet. Based on the table, I should select an output of 1/16 power and achieve a "proper" exposure. If wanted the flash to serve as a  fill, I could increase the distance by stepping backward, or if the flash is hand held, merely increasing the distance.

I'll be photographing an outdoor Lunar New Year celebration, so we'll see how well this system works.