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Flash has long been a valuable tool for portrait photographers and it increasingly becoming a valuable tool for nature photographers. Used as a main light or fill light, an understanding of flash basics is important.

An understanding of exposure is basic to understanding flash.  Go to the exposure article at this site if you need a review.

The amount of light changes with the square of the distance.  Put another way, evey time you double the distance from the flash, you lose two stops of light.  If f8 is the right aperture for a given film and a given flash unit at 10 feet, at 20 feet it will be f4 and at 40 feet it will be f2.  When you cut the distance in half, you gain two stops of light.   If f8 gives the right exposure at 10 feet, at 5 feet it will be f16 and at 2 1/2 feet it will be f32.  This is illustrated by the exposure scale on the back of a typical manual flash unit for 50 speed slide film.



















The proper f stop is listed above the distance scale.  At 11 feet, use f/5.6,  at 22 feet use f/2.8.  With a different speed film, the scale would be different.


Whether you determine flash exposure manually or automatically, you need to know the guide number of your flash. One very fast way is if you have access to a flash meter.  Set your flash to full manual power.  Put the flash meter 11 feet away, set the flash meter at ISO 100 and fire the flash at the meter.  Read the f stop on the meter and multiply that by 11.  As an example, if the f stop reading on the meter is f/8, multiply f/8 times 11 feet and you get a guide number of 88 for that flash with that film speed.

You can also test the guide number manually.  With 100 speed slide film in your camera, a normal length lens, and your flash unit set for full manual power (check the flash manual for how to do this), go outside at night, put a friend exactly 11 feet from your camera and photograph them with every aperture on your lens.  Take notes.  When you get your slides back, decide which f stop gave you the most natural looking skin tones on your friend.  Multiply this f stop by 11, this is the Guide Number (GN) for your flash with 100 speed slide film.  GN = f stop x distance. Example: If the best f stop was 11, the Guide Number would be 121 (11x11).  If the best f stop was f8, the GN would be 88 (8x11).

Once you know the guide number for your flash and a given film, it is easy to determine the flash exposure.  Divide the distance of the flash into the GN and the answer is the f stop.  If the distance is 22 feet and the GN is 88, then the s stop is 4 (88 divided by 22).  If the distance is 44 feet, the f stop is f/2 (88 divided by 44)

Here are the three Guide Number (GN) formulas:

1)  GN = f stop times distance
2)  f stop = GN divided by distance
3)  distance = GN divided by f stop

If you know any two numbers in the formula, you can solve for the third.  If the GN is 88 and you want to use an aperture of f/11, divide 88 by f/11 to get a flash to subject distance of 11 feet.

With these formulas you can determine correct exposure time after time with full manual flash.

How do you know the subject distance?  Focus on the subject, then look at the distance scale on the front of the lens.

Remember, you must do a test or use a meter to determine the GN for your flash and the film you want to use.

Practical application.  You want to take a picture in low light of an animal forty-four feet away with a telephoto lens with a maximum aperture of f4.  Using our example GN of 88, you determine the aperture (88/44 = 2) is f/2.  You can't take a properly exposed picture of the animal at f4, it will be two stops underexposed.  Why two stops UNDER?  The flash only provides enough light for f/2 at that distance, f4 (your widest aperture) lets in two stops less light than f/2, hence the underexposure.  You have several solutions, switch to 400 speed film (two stops faster), move closer to the subject, or buy a faster telephoto lens.

How close would you need to get?  Inverting the formula looks like this:  distance = GN/aperture.  88/f4 = 22 feet.  With our "example" flash you would need to be 22 feet away to take the picture at f4.

Let's look at an example from the other extreme.  You are taking a closeup flash picture of a subject at a distance of 2 feet. The aperture would be (88/2) f/44. What if the smallest aperture on your lens is f22.  Your picture will be 2 stops overexposed.  You have two choices: switch to a slower film speed, or get the flash farther from the subject.  How far?  Distance = GN/f stop.  88/22 = 4 feet.  What if you don't want your camera that far away?  Leave the camera at 2 feet and back the flash up to 4 feet.

There is one other option. Many flash units have variable power ratio.  Instead of full power, set the flash at 1/4 power (2 stops less light), putting it within the f stop range of your lens.

Once you know one GN you can calculate the others without testing.  If 100 speed film gives you a good full flash exposure at f8 at 11 feet (GN =88), 50 speed film will give you a good full flash exposure at f/5.6 (one stop less than 100) for a GN of 62.  25 speed film would give a good full flash exposure at f/4 at 11 feet (one stop less than 50 speed film) for a GN of 44. Read through this more than once, try it out and it will begin to make sense.

This is a lot of math, too much in fact.  Here are the basics.

Double the distance, lose two stops of flash power, cut the distance in half and gain two stops.  f stop  = GN/distance.  You do need to know the maximum distance at which your flash can work at any given f stop when you are working at those limits.

The three Guide Number formulas again are:

1) GN =  f stop  x  distance in feet
2) f stop = GN divided by distance in feet
3) distance = GN divided by f stop in use

It can be unhandy to carry all this around in your head, better on a 3x5 card in your camera bag with the GN for your favorite films.


Many flash units, or the flash manual will have an f stop and distance scale like the one at the top of these notes.  These scales are based on the guide numbers of the flash.  Why not read the scale and forget the testing and the math?  (1) Flash scales are often figured for inside where walls act like reflectors and add to the light.  (2)  Most people use print film so a slight underexposure won't matter.   (3) Manufacturers are often overly optimistic in advertising the GN of their units to make them look more powerful when compared to the competition.

Go outside, do the test above, and compare to the flash scales on your flash or in your instruction manual.  If your flash scales are close, use them.  I have learned through testing that the flash scales on my flash ore off by 1/2 stop, Whenever I take a flash picture, I open up 1/2 stop from what the scale on the flash says.  Using the scales with my 1/2 stop compensation saves me from all the GN math.  It still helps to understand the theory behind flash exposure.


If your flash unit has automatic modes, and most do, why not just use them?  If the automatic mode is based on a sensor on the flash (independent of the camera metering system), and not on the light falling on the film, they can be inacurate: close enough for prints, not close enough for slides.  You still need to know the maximum distance your flash unit will work at at a given aperture.


Some of the most recent flash units work on Through-The-Lens (TTL) metering, they meter the light striking the film during the exposure and turn off the flash when a proper exposure has occured.  TTL flash is a wonderful tool.  Use it!!  Do the full manual flash test anyway so you know the distance limits of your flash.

IF the GN for your TTL flash is 88 and your subject is 11 feet away, you can take the picture at f/8 (88 divided by 11), or any wider aperture than f/8.  The TL memtering will turn off the flash at the right exposure.  You can't take the photo at f/11, f/16, or f/22 because the flash doesn't have enough power to light the subject at that distance.  Even in TTL flash, the GN number formula tells you the maximum available power, or in other words, the smallest f stop you can use at a given distance.


It is after sunset and you want to take a flash picture of a moose at 44 feet with your f/4 300mm lens.  Using our example flash you determine the f stop. 88/44 = f/2.  With your f/4 lens your picture will be underexposed by 2 stops.  What to do?  Move closer?  The moose doesn't appear friendly.  Switch to your 50mm f2 lens?  Your properly exposed moose will look too small.  There is a solution,  TELEFLASH TO THE RESCUE!

A teleflash uses a fresnel lens to take a wide beam of light and focus it into a narrower and more powerful beam of light.  A commercial unit giving you about a 4 stop increase in light will cost $150 or more.  Simpler units for $30-$35 will give you a two to 3 stop increase in flash power.  Two different, popular, and Inexpensive units are available from Arthur Morris and from Tory Lepp (at his father's web site).

You can make your own for less than $10 and have a 2 stop gain in light.

To make your own, buy a fresnel lens about 4x6 inches in size.  These are often sold as reading magnifiers.  I purchased a Rogers No. 04139 Card Lens Pocket Magnifier at Office Max for 75 cents.  Buy a two quart plastic container whose bottom end dimensions are approximately the same as the fresnel lens (mine cost less than $2.00 at a local grocery store.  Buy some hook and loop (Velcro) material to attach it to your flash unit.

Cut out the mouth end of the 2 quart container so it just fits over the head of your flash unit.  Determine the focal length of your lens (focus sun light on a flat surface and measure the distance from the lens to the surface.  Cut enough off the bottom of the 2 quart container so the distance from the bottom to the front of the flash head is about the same as the focal length of the fresnel lens.  Attach the lens to the front of the container with Velcro.  Put Velcro tabs on the mouth end of the container and the head of the flash.  Use Velcro strips to attach the container to the flash when you want to use it.

With some flash units, the teleflash unit will project an image of your flash head onto the subject your are photographing.  If this happens, shorten the container a little.

It will take a little practice to aim the teleflash unit where your lens is pointing.  The teleflash is for telephoto lenses only.  Since the beam is narrower, it will not longer cover the width of a 50mm lens.  Telelfash is only practical for 200 - 300mm or longer lenses.

You should have a 2 stop gain in flash power.  Do the Guide Number test above with your teleflash unit on your flash.  With 100 speed film, put your friend at 33 feet, put the flash on full manual power, put on your teleflash unit, and take pictures at all apertures.  Multiply the aperture of the best exposure by 33 to get your teleflash GN with 100 speed film.   If the best aperture is f/5.6, your new guide number is 185.  Now go back to our moose dilemma.  GN/distance = f stop.  185/44 feet = f/4.  You can now take your moose picture with your 300mm f/4 lens.

With TTL metering, let the metering system do the figuring, just don't exceed the aperture distance your new teleflash system allows.


You will need to do closeup tests.  Set up your camera, flash and closeup equipment the way you would for a closeup picture.  With 100 speed slide film, photograph a series of closeup pictures of a medium toned subject at several apertures.  Take notes.  Select the exposure you like best.  Use that exposure whenever you use the same closeup equipment with that film.  A light toned subject will require a little smaller f stop than your standard f stop so you don't wash it out.  A dark toned subject will require a little wider f stop so it isn't too dark.  If your best exposure with a medium toned subject was f16, you may need about f22 for a really light toned subject and about f11 for a really dark toned subject.  Experience will teach you how much to compensate.

You will need to do an exposure test for each differnt closup set up you use.  It will take awhile, but once you've got it, you've got it.  It is not as bad as it sounds.

For help with manual closeup flash exposure, read JOHN SHAW'S CLOSEUPS IN NATURE.


Set your aperture and take the picture, let your camera and flash do the rest.  BUT, remember the same problems with subject tonality and backgrounds that complicate non-flash automatic metering can also plague TTL flash metering.  Really light subjects will require more exposure than your metering system thinks.  Set the flash/camera system for about +1 for really light subjects.  Set the exposure for - 1/2 for dark subjects. Experience will teach you how much to compensate for different tonalities.  Read Larry West's HOW TO PHOTOGRAPH INSECTS AND SPIDERS for help with TTL closeup flash.

Feb 15, 2001

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