Drew. Sunlight with Fill Flash. Photo © Jim Doty Jr.

Sunlit Portrait with Fill Flash. Photo © Jim Doty Jr.

Today’s question is brought to you by Powdermilk Biscuits.

Wait – wrong show. My apologies to Prairie Home Companion. (The delightful movie was on TV recently.)

Today’s Q&A is for all of you that learned to use Guide Numbers and manual flash exposure. If you are a little rusty in that department, or if you were weaned on TTL flash, you can brush up on how to use the guide number (GN) for your flash to determine a manual flash exposure by reading the Flash Basics article at my photography web site.  If you are a little rusty with f-stops and exposure, go here.

Now for the Q&A:


I have in the past established GNs for various flash units by firing them at night outside at a measured distance as you have suggested. My tests always show the manufacturer’s advertised GNs to be 2/3 to 1 1/3 stops overly ambitious. In real picture taking tests though I find my measured GN to be a bit on the conservative side possibly because I don’t usually take pictures outside in the dead of night. Am I missing something or is common to have to make some adjustments to a GN established in this manner?

Another question regarding Sunny 16 fill flash when it is being used in the dedicated TTL (aperture priority) mode.  If there is a big discrepancy between the advertised and tested GN should this be taken into consideration and added in as flash compensation? Would I achieve more predictable results using the flash and camera in manual mode?

Thank you for any answer you may be able to provide.



The guide number (GN) for a flash is a way of saying “This is the most light this flash can put out at full manual power at a given ISO.”   Once the flash has pumped out all of the light it can deliver, it can’t deliver any more light. The GN is a way to express that limit.


When I am using manual flash, I use the GN to determine the aperture.  I divide the flash-to-subject distance into the GN to get the f-stop). If the GN is 110 and the subject is 10 feet away from the flash, the aperture is f/11.  If the subject is 5 feet away, the aperture is f/22.


When I am using TTL (automatic through-the-lens metering), the GN doesn’t matter at all provided the subject-to-flash distance doesn’t exceed the power of the flash. Why? The  through-the-lens meter will shut off the flash when enough light hits the film or digital sensor.

The only thing I use the GN for when doing TTL flash metering is to make the sure subject-to-flash distance and f-stop I am using don’t exceed the power of the flash.

Using my hypothetical GN of 110, if the subject is 10 feet away, the aperture (using manual flash) would be f/11.  But I am using TTL metering,  I can use f/11 or any wider aperture and the TTL meter will take care of the rest.  When enough light hits the subject, the meter will turn off the flash.  Simple.  One quick division problem and I know the smallest aperture I can safely use.


You are right about manufacturer’s GN claims.  As one professional photographer once wrote, a lot of manufacturers are very “optimistic” with their published guide numbers.  He went on to write that it is not unusual for the true GN of a flash to be a full stop (more or less) than the manufacturer claims. I find that to be true in my own testing. When most people shot negative film (and automatic flash) any occasional underexposure was corrected when the negative was printed.  Those of us who shoot slide film are more aware of the difference between the stated GN and the real GN.  In the happy world of digital photography, underexposure is corrected in the computer (not withstanding the noise problems that occur in underexposed files).

Doing a guide number (GN) test outside at night is a very good way of determining the REAL power output (and GN) of your flash, since the flash is the only source of light.

Keep in mind that a guide number is somewhat personal and subject to interpretation. One photographer’s normal exposure is another photographer’s overexposure. We have different tastes when it comes to exposure and skin tonality. Some have a preference for lighter skin tones while others prefer “richer” skin tones. Even if you have a flash meter (accurate to 1/10 of a stop, no less!), the “correct exposure” according to the flash meter can be set to a specific scientific point of reference, but that still doesn’t mean it matches your personal preferences. A flash meter is a guide, not a dictator (and I love my flash meter).


If I am using manual flash outside at night, I go by the GN that I determined from my own testing.

Inside, you have more flash power in small rooms  (with white or light colored walls.  Why? The walls (and ceiling if it is low enough) bounce scattered light from the flash back at the subject.   When doing manual flash exposures inside small rooms with light walls, I make an allowance for the extra bounced light by using an aperture about 1/2 stop smaller than the GN would indicate.

Some flashes “zoom” to match the focal length of the lens. Focusing the light into a narrower beam increases the amount of light on your subject (and the GN). If I am using manual flash, I keep the flash at it’s widest zoom setting (no matter what focal length I am using) so my GN doesn’t change.

Nature photographers will use the “Better Beamer” and similar products to focus the flash into a very narrow beam of light for more power in photographing distant subjects with telephoto lenses. The catch with using a Better Beamer is that the light beam is so narrow that you need to use a lens with a focal length of 300mm or longer, or the beam of light won’t fill the frame. This is not a problem for wildlife photographer’s shooting with telephoto lenses. If you are doing manual flash with a Better Beamer or a similar accessory, you need to do some testing to find the GN when using that accessory.


When I shoot TTL, it is a different ball game.  I use the GN as an indicator of the true maximum distance my flash can work at. The back of my flash gives me a distance chart for the aperture and focal length I am using. With one of my flash units I have learned that the true maximum effective distance is  less than what the flash tells me. The flash output is one stop less than the GN the manufacturer publishes.

If the flash tells me that, for a given focal length and f-stop, the effective distance range is from 3 feet to 22 feet, just to be on the safe side I assume it can only be effective out to 16 feet (not 22).  If it says 2 feet to 16 feet, I assume it can only be effective to 11 feet.  I my mind I convert the distance settings on the back of my flash to the same numbers in feet as the apertures series (i.e. 4 feet, 5.6 feet, 8 feet, 11 feet, 16 feet and so on).  Since the flash puts out less power than the distance chart on the flash indicates,  I reduce the distance reading in feet on the back of my flash by one f-stop number.  If the flash says 22 feet, I assume it is really only effective to 16 feet.  If the flash says 16 feet, I assume it is really only effective to 11 feet.  If the flash says 8 feet, I assume it is effective to 5.6 feet.

An example:

My flash to subject distance is 10 feet, and I want to shoot at f/16 (the manufacturer’s listed GN = 160 at ISO 100). My flash is set to TTL and the flash tells me that at ISO 100 and f/16 I can shoot from 2 feet to 10 feet.  All should be well and good, BUT my tested GN is 110 so I assume the flash can only be effective at f/16 to 7 feet (110/16 = 6.875 feet). To insure enough light, I have three choices (or a combination thereof): (1) I use a wider f-stop, like f/11, or (2) I change my ISO from 100 to 200 (if I am shooting digital), or (3) I move to within 7 feet of my subject. All three options will give me one more stop of light output.


On to the Sunny f16 question. (You can read about the Sunny f16 rule, basic daylight exposure, about 3/4 of the way down this page .)

If you are shooting in the full sun, and using a Sunny f16 exposure and using your flash for fill light on the shady side of your subject, you want the flash to provide one to two stops LESS light than your Sunny f16 exposure for the sunlit side of your subject. If your flash puts out the same amount of light as the sun, your photo will look odd. The shady side of the subject needs to be one to two stops darker.  This is actually good news since you need less flash power than you would inside or (at night) when the flash is the main light source.


If you are using manual flash, you want to use an f-stop that is one to two stops smaller than the GN would indicate. To keep the math simple, lets assume the real GN for the flash is 80 at ISO 100. If your subject is 10 feet away, the GN divided by 10 feet equals an aperture of f/8. So you shoot at f/16, which makes your flash exposure two stops underexposed, just what you want for fill light. Using the Sunny f16 rule, at ISO 100 and f/16, the shutter speed would be 1/100 (or 1/125) second. Perfect!

Another example.  Let’s make the real GN 121 at ISO 100, still using manual flash.  Your subject is 20 feet away. Divide the GN of 121 by 20 feet to get an aperture of f/6, close enough to half way between f/5.6 and f/8, just to keep things simple.  To underexpose the flash fill light by 1 1/2 stops, use an aperture of f/11. To underexpose by two stops, use an aperture half way between f/11 and f/16.   To keep things simple let’s use f/11. Using the Sunny f16 rule, for f/11 you will need to use a shutter speed of 1/250 second.  Perfect, so long as your camera has a flash sync shutter speed of 1/250.

But, just to complicate things,  let’s assume the maximum flash sync shutter speed for your camera is 1/125 second. That means you need to use f/16 as your aperture to get a Sunny f16 exposure for the sunlit side of your subject.  But now your manual flash exposure for the shady side of your subject is 2 1/2 stops underexposed (the difference between f/6 and f/16). What do we do? Instead of shooting at 20 feet, we move up to 15 feet from the subject and now we have a usable flash exposure for the shady side of our subject (and we will need to use a wider focal length to frame our main subject). Simple.  Of course a wider focal length at 15 feet will keep the subject the same size, but include more of the background behind our subject, changing the perspective, but photography is all about compromise.  There is another option. Instead of moving the camera from 20 fee to 15 feet, leave the camera at 20 feet and move the flash so it is only 15 feet from the subject. You will need a way to remotely fire the flash, like a flash cable, or some kind of remote flash trigger or slave.


Is TTL fill flash with the Sunny f16 rule simpler. YES!

Set your sunlit exposure by the Sunny f16 rule. Turn on the flash and set it to TTL. Check the maximum distance setting on the back of the flash and compare it to the real maximum effective distance as determined by your testing.  IF the flash says 22 feet and you know the real effectivce distance is 16 feet, you are good to go provided your subject is 16 feet feet from the flash, or closer. Before you shoot, set the flash exposure compensation dial to somewhere between -1 to -2 stops so the flash fill light on the shady side of your subject is one to two stops less than the sun exposure on the sunlit side.

What if the maximum effective flash distance is less than the subject distance?  Three choices: (1) Move closer to the subject, or (2) use a wider aperture and faster shutter speed (so long as you don’t exceed the flash sync shutter speed), or (3) use a higher ISO, the same aperture and a faster shutter speed to compensate for the higher ISO (once again, so long as you don’t exceed the flash sync shutter speed).


TTL flash metering has become so accurate in recent models of flash units and camera bodies that I use most of the time. I trust TTL in many average situations. I use flash compensation for light or dark subjects. But there is an occasional situation that is to complicated for the flash to handle. When that happens I switch to manual flash and use the guide number to determine the exposure (or I use a flash meter if I have one with me).