This article is primarily written for my brother, who cannot seem to remember these rules, no matter how many times I explain them. You can read along for kicks, since there is no other systematic discussion available anywhere. This is organized by what type of camera and flash you have, so look at the headings until you see something like yours. This will not cover TTL, which is better described by the directions that came with your equipment.

1. General Principles Governing Fill Flash

The following are basic principles which limit fill flash. Don't ask why these limit you; if you do enough experimentation you will know exactly why. If I save you the trouble of working these out, I hope you excuse the fact that I am not going to explain the "whys." None of these is a show-stopper, but the more rules you obey, the broader your range of fill flash possibilities.

a. Slow film is better than fast film.

b. Fast shutter maximum synch is better than slow. 1/125 is practical minimum; 1/60 is close to useless.

c. Never set a shutter speed faster than max synch. So if your camera has a red 1/125, do not set it at 1/250.

d. Closer subject distance is better than far.

e. Leaf shutter is better than focal plane.

f. More powerful flash is better.

g. More flash auto modes are better. Or more power factor modes.

h. You cannot do it in program mode.

2. Any Camera With Manual Flash / Manual Camera With Manual Flash

In a perverse way, this is the easiest way to do fill flash, because manual flash does not care about how reflective the subject is. Learn the guide number of your flash (feet or meters is unimportant, but you have to use the same unit of measure for all steps). Focus the lens and note the distance (same unit as before). Divide the guide number by the distance. This gives you the aperture. Then meter the overall scene. Set the shutter speed that corresponds to this aperture for the overall scene. If you have to set a shutter speed above the maximum synch speed of your camera, you can't use fill flash.

Example:

a. Flash Guide Number 30 in meters.

b. Subject Distance 5 meters.

c. Aperture (based on flash) = 30/5 = ~5.6

(This far will produce accurately exposed flash pictures against a dark background)

d. Measure overall exposure and select shutter speed based on flash aperture. F/5.6 and 1/125 (other choices were f/4 and 1/250; f/8 and 1/60; f/11 and 1/30)

Leaf Shutter: It is worth noting that this technique works a lot better with cameras with leaf shutters that synchronize at up to 1/500 sec. With a leaf shutter, you can fill-flash in daylight with 100-speed film.

Half Power Fill: If the flash has a half-power setting, simply select it. If the flash does not, pick the aperture/shutter combination that has the aperture one stop below what the computation wants. For example, if the GN/distance dictates 1/125 and f/16, select 1/60 and f/22.

3. Any Camera With Automatic Flash

An automatic flash is a great aid in doing fill flash, and it greatly simplifies things. First, set exposure for the background scene, using any shutter speed at or slower than the maximum synch speed. Next, choose an automatic mode on your flash that (a) matches your distance and (b) is one of the combinations that matches the background scene.

Example:

a. Flash has automatic modes of f/2-f/16

b. Background exposure is 1/125 and f/16 (other choices were 1/60 and f/22; 1/250 and f/11; 1/500 and f/8; 1/1000 and f/5.6)

c. Set flash to f/16 mode (provided your subject is close enough)

Leaf shutter: If you have a leaf shutter camera, in the example above you could also select the aperture to f/11 (with 1/250 sec) or f/8 (with 1/500 sec) as well, selecting the appropriate auto mode on the flash. Some flashes have one auto mode, some two, some more. The Nikon SB-20 I use has 8, ranging from f/2 to f/8 with 100-speed film.

Leaf shutter with LVS/EVS: If your camera has a system which can lock the shutter speed and aperture rings together (like the Rolleiflex 3.5MX-EVS, a Kodak Retina IIC, etc.), you set the background exposure and to switch both aperture and shutter speed, you just turn one dial. This lets you cycle through the shutter speed/aperture combinations much more easily and as far as I know is the only good use for the LVS/EVS system.

Half Power Fill: pick an aperture/shutter speed that is one f-stop smaller than the flash mode you are using. So if the background exposure is 1/125 and f/16, pick the flash auto mode for f/11.

4. Aperture-Priority Camera With Automatic Flash

Aperture priority follows the same basic rules as manual, but you have to watch the shutter speed, which cannot be allowed to get higher than the maximum synch speed. Let the camera meter the scene. Rotate the aperture dial until you get a shutter speed within the synch range. Set flash to automatic mode dictated by the aperture you selected.

Example:

a. Flash has automatic modes of f/2-f/16

b. In this example, background exposure is 1/125 and f/16 (other choices were 1/60 and f/22; 1/250 and f/11; 1/500 and f/8; 1/1000 and f/5.6)

c. Set flash to f/16 automatic mode

Leaf shutter: If you have a leaf shutter camera, in the example above you could also select the aperture to f/11 (with 1/250 sec) or f/8 (with 1/500 sec) as well, selecting the appropriate auto mode on the flash. Some flashes have one auto mode, some two, some more. The Nikon SB-20 I use has 5, ranging from f/2 to f/8 with 100-speed film.

Half Power Fill: Pick a flash auto mode that is one f-stop larger than the aperture you have selected. So if the camera is set to f/16, set the flash auto mode to f/11.

5. Shutter-Priority Camera With Automatic Flash

Aha! Now this is a tough one. Set the camera to the maximum shutter synch speed (typically 1/125 on these cameras). Let the camera pick the aperture. Choose the automatic mode on the flash to match.

Example:

a. Flash has automatic modes of f/2-f/16

b. It's a sunny day. You set to 1/125 sec; the camera sets the aperture to f/16.

c. Set flash to f/16 automatic mode

Half Power Fill: pick a flash auto mode that is one stop larger than the aperture the camera has selected. So if the camera picks f/16, set the flash to the f/11 mode.

As Dirty Harry says, "A man's got to know his limitations." The best fill flash system ever made was the Konica Hexar AF, a camera which makes two exposures in P mode - one for the background measured by ambient light, and one for flash, measured by focused-subject distance. It knows how to balance these. Nifty!