George Wilson: Determining the Guide Number for FlexFlash200W

Group 1 Exposures 1 590 288

When it comes to photography, I am a traditionalist. I use the modern DSLR, but mostly in manual mode (M) with an occasional foray into aperture priority (AP). For me, absolute control over my image dictates my use of a hand held analog meter; manual camera settings and manual flash settings. Yes, that’s correct, manual flash. I have only slightly ventured into the TTL options available. I myself am a firm believer that the nice people at Nikon (as well as Canon, Olympus and Sony) have not programmed the ability to think into our cameras or should I say, read minds. Only we, the photographers, know how we would like our images to appear at the printed stage. Please do not misunderstand, TTL is a wonderful tool, but understanding flash from a manual standpoint will give you better control over your use of TTL. Remember that you must do more than just record the scene in front of you. The challenge each photographer faces is making a connection with your viewers to inspire them to stop and take a closer look.

I recently acquired three Photoflex FlexFlash 200W strobes. I took them out of the boxes and mounted them right away on light stands to begin putting them through their paces. Everything worked wonderfully and as advertised. However, I needed to determine the Guide Number for these lights, before I could really put them into service. In the days before TTL, flash calculations were made based upon the manufacturer determined Guide Number.

The Guide Number (GN) is a measure established by the flash manufacturer with regard to its strength of output. For example the Nikon SB-600 has a manufacturer determined guide number of 98’ at ISO 100. That is really all you need to know about a flash to start using it. Owing to the fact that standard manufacturing techniques may not be all that standard, I always run a test on my new flashes to verify the Guide Number (GN) is accurate. Some flashes have been on the mark and some others not. You should also verify that the GN aligns with your shooting tastes. Some photographers like a slightly darker image, others slightly brighter. Testing and determining your GN will help your flash work align more closely with your own personal vision. I do this on my light meters as well (both handheld and in the camera) as the stated ISO’s sometimes do not align between manufacturers. My own practice dictates different cameras for different uses and in order to get a consistent exposure, Guide Numbers and ISOs must all align with each other. Determining the actual Guide Number for your shooting preference will ensure that you can quickly balance the lighting in any shot. On the business side of photography, time is money and your ability to quickly establish your lighting keeps your time expense low and client confidence (in your ability) high.

First, using the Guide Number is very easy. I will refer to all calculations using the Guide Number of 98 for the Nikon SB-600. For your own reference, please see the calculation circle as an easy way of getting the math correct.

To determine the aperture for a flash exposure: Guide Number / Distance of flash to the subject. If you refer to the calculation circle, pretend you are blocking the word “aperture” from the circle. You will see that the word Guide Number is above distance. This shows a divided-by relationship.

With a Guide Number of 98 and my subject 8 feet from the flash, I would use f12. (98/8 = 12.25) I rounded down to 12 as it was the nearest aperture stop increment. This would be my beginning exposure at ISO 100.

If I wanted to use a specific aperture, for example f8, I would do a similar calculation. Guide Number/selected aperture = distance between the flash and the subject. If you again refer to the calculation circle, pretend you are blocking the word “distance” from the circle. You will see that the word Guide Number is above aperture. This shows a divided by relationship here as well.

With a Guide Number of 98 and my aperture selection at f4, I would move my flash 24.5’ feet away (98/4 = 24.5). Distance creates other issues with maintaining a tight flash radius due to the inverse square law, but that is a subject for another blog post.

But what if the owner’s manual has long been lost or Google fails in searching for the Guide Number? I must now determine the Guide Number on my own based solely on my shooting preferences. I do this in my work regardless of whether or not I actually have the Guide Number. Having an accurate number allows me to quickly balance flashes, whether hot shoe mounted or flash heads on a light stand.

Step 1. Set up the flash and your subject indoors. My own preference is 10’ between the subject and the flash. This distance works the easiest with the math. In flash equations, the distance between the subject and the light source is important, not the distance between the camera and the subject.
Step 2. Set your ISO to 100, which is the baseline. If you want to calculate the Guide Number for ISO200, you would first have to know the Guide Number at 100 and then multiply by √2, which is 1.414. The Nikon SB600 has a guide number of 98 at ISO100 and 139 at ISO200 (98 x 1.414). To take the next step, say to ISO400, you would multiply the GN for ISO 200 (139) by by √2, which is 1.414 for a GN of 197. This works with each incremental full stop in ISO.
Step 3. Set your shutter speed to any number at or below the shutter sync. This is the speed at which your shutter opens and closes to allow the flash to fire when the shutter is fully open.
Step 4. Set your aperture to f5.6 and make an exposure to evaluate it for being on the light side or more towards the dark side. Then begin shooting a sequence based on the first evaluation. If the images are too light, stop down and shoot in full stop increments – f8, f11, f16, and f22. If the images are too dark, open up and shoot at f4 and f2.8.

In the Group 1 images, I fired the flash at f5.6 and noted in the LCD, that it was too bright. I stopped the lens down and made successive images at f8, f11 and f16. Each one is one full stop smaller than the previous.

I selected images between f8 and f11 as being within my range of shooting style. I like to be about 1/3 stop over exposed. That is just how I like to shoot with my Nikon D7100. It is interesting to note that my exposure compensation for that camera body in particular is set at +.3 in most conditions, when using aperture priority. (I do a similar test with exposure compensation in aperture priority and shutter priority to fine tune that component)

Now that I have established my range as f8 to f11, I made my Group 2 exposures at 1/3 stop intervals. This fine-tuned my exposure down to f10, which was the exposure closest to my shooting style.

To determine the Guide Number, please refer back to the calculation circle. I blocked the words Guide Number in the circle and noted that aperture and distance are beside each other. This denotes a multiplier relationship. Multiplying the aperture by the distance will provide the guide number for the flash (f10 x 10’ = 100). I hung a small tag on the flash handle that stated “GN=100”. This flash is now balanced for my shooting preference. I then completed the same test on the remaining two flashes and tagged them as well (each shot consistently at GN 100).

Your preferences may be different, but balancing your flash to your shooting style will help you produce more consistent images using strobe lighting.

George Wilson has more than 30 years of experience as a professional photographer, and his work has appeared in many national and international publications. Now focusing on nature and wildlife photography, George exhibits his infrared black and white landscape work and teaches photography at numerous art centers, botanical gardens and at the Walt Disney World Resort in his home state of Florida. A key element to George’s work is his dedication to traditional photography as his post processing is strictly limited to tools aligning with the traditional darkroom. To see more of George's work, visit

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