Lighting ratios are based upon the intensity of light reflecting from the highlights of the object illuminated vs the light reflecting from the shadows. The fill light on the flash illuminates everything the camera sees. The light from the off camera key light overlaps the fill. When two flashes of equal intensity are used at the same distance the overlap produces highlights which reflect twice as much light as the shadows. This is denoted as a 2:1 highlight-to-shadow or reflected ratio. The overlap of key over fill that puts two units of light in the highlights for every one in the shadows. A 2x difference is equal to one f/stop.

If you want to stay sane and get predictable results when using manual flashes for candid work I strongly recommend using two identical flash units at the same power setting with identical modifiers. It's really not necessary to have a flash meter for setting ratios, but then again its not a bad thing to have either if you have time to take the measurements. It's also worth noting here for beginners that shutter speed doesn't affect flash exposure because the flash duration is so short. When using manual flash you want the camera in M mode with the shutter between 1/125th and 1/250th or your cameras "x-sync" speed. The aperture (f/stop) is used to control the exposure. See the Hot Shoe Primer for details.

Having both lights identical allows the lighting ratio to be controlled by the relative distance of each light to the subject. It is the only remaining variable.

Equal distances will produce a 2:1 H:S ratio. This ratio has light open shadows with considerable detail. Is well suited to portraits of women, children and animals where shadow detail is desired

Moving the key light closer than the fill will produce darker shadows. These easy to remember distances combinations will produce a 3:1 lighting ratio, with exactly one-f/spot difference in exposure between them:

camera fill / key light on stand: 16ft/11ft, 11ft/8ft, 8ft/5.6ft, 5.6ft/4ft, 4ft/2.8ft

You'll note how the distances are similar to f/stop numbers. The "key" light is always the distance equivalent of one "f/stop" closer than the on-camera fill, resulting in 2 units of key + 1 unit of fill in the highlights for every 1 unit of fill in the shadows; 3:1 reflected ratio.

The illustrations below show how two manual flashes of equal power would be configured to achieve a 3:1 highlight:shadow ratio from as shooting distance of 8 ft. for full face and oblique facial view. The 5.6 foot distance of the off camera light and its 45 degree angle to the subject's nose is maintained for both views; the key light and subject's face rotate to the right by the same amount.

Camera fill / key light on stand distances of: 16ft/11ft, 11ft/8ft, 8ft/5.6ft, 5.6ft/4ft, 4ft/2.8ft will all produce a 3:1 highlight:shadow ratio when equal powered flashes are used.


Determining Manual Exposure by Testing

Before doing the exposure test get some string and a tape measure. Tie the string to both flashes and mark or tie knots at 11, 8, 5-1/2, and 4 feet. The string simplifies measurement. Use it until you can accurately estimate distance by eye, paces, etc. A tripod also makes the test easier.

Place the camera and fill light at 8ft and the off camera "key" light at 5-1/2 feet from the subject. Use your eyes as the modeling light. Stand behind the off camera light as you move it so you see what the camera will illuminate. Place the off camera light where you see an oblique view of the face with the eye socket profiled to put a short lighting pattern on the face. It should be a foot or more above the eye line.

Now shoot a test series over a range of f/stop settings to determine the one which exposes the highlights ideally. To determine ideal exposure use a white towel over the shoulder of your subject on the key light side. Open the files in Photoshop and find the first exposure in which the texture of the towel is lost and use an f/stop 1/3 stop less. Shoot another test at that f/stop to confirm.

Change the set-up to put the camera at 11 feet and the key light at 8ft and repeat the test to find the ideal f/stop at that distance.

Repeat the test for 5-1/2 ft, camera, 4ft key light distance.

It may sound complicated at first, but with practice you'll realize that those three distance combinations of 11 and 8, 8 and 5-1/2, and 5-1/2 and 4 will cover 99% of your flash shooting situations. All you need to remember are the three f/stops obtained for those distances by testing. If you want to use a 2:1 ratio repeat the test with the camera and off camera light at equal distances. I put a chart on the back of my diffuser with a "cheat sheet" of the distance - aperture settings.