BetterPhoto Q&A
Category: New Answers

OPUS Lighting

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June 04, 2006

Aperture:
Correct exposure is everything. How to achieve is allusive. You should own a light meter that works with electronic flash. However there are options.
1. Guide number. A published number that assists you in the setting of aperture. Look in your lighting kit manual, you will find guide numbers for various film speeds (ISO) and power setting. Let’s assume you find the guide number to be 80. Measure the distance, lamp to subject. In this example assume 7 feet. Divide the guide number by distance. Thus 80/7=11.4. Round to the closest aperture setting. In this case f/11. Set camera and shoot a test. Take a series of 9 test shots in ½ stops increments centered on f/11. You use this technique to refine the guide number for your conditions.
2. Trial and error distance method. Set aperture at f/5.6 (portraiture traditionally uses a large lens opening to minimize depth-of-field. Set lamp to subject distance at 4 feet. Set power to 100%. Take a test shot. Multiply lamp to subject distance by 1.4. Place lamp at this revised distance. Shoot test shot. Repeat again and again until you run out of room. Set power to 50%. Do it all again. You will eventually discover where to place the lamp and what aperture to use. You can even deduce your very own guide number. You can refine the guide number using the 9 test shot bracket described above.

Lamp placement:
One lamp is place high and off to the side. This lamp, known as the main, simulates the sun shinning from above. Placement is an art based of subject’s facial defects. Long nose, adjust to cast short nose shadow. Short nose - long shadow. Narrow face- light more from the front. Round face light more from the side. No one can teach this technique of facial defect countermeasure. Practice makes perfect. Second lamp is always subordinate in power to the main. This lamp known as the fill is initially set to arrive at the subject plane at ½ power. With both set to the same power, set fill further away by multiplying main to subject distance times 1.4. This revised distance causes the fill to arrive the subject plane at ½ power. Alternatively, set both at the same distance with fill lamp set to ½ power. Fill is place at lens height as close to the camera as possible. You are using it to fill shadows from the cameras prospective. Take care to place the fill to avoid it casting a shadow on the subject. OK to move fill away from the camera to avoid these shadows. If you have a third lamp, place behind subject pointing at the background. This lamp neutralizes shadows cast on the background. Its power controls the lightness or darkness of the background.

The 1.4 multiplier is important. This is the magic number that allows control over light brilliance at the subject plain. This value allows adjustment in one stop increments. Once stop = 2x change in light intensity. Multiply original distance by 1.4, place further back you get 2x drop-off. Divide by 1.4 – place lamp closer – you get 2x light increase. (Factor was deduced by behavior of light following law of the inverse square). Umbrella lighting won’t follow the law exactly as law is based on point source illuminant not broad source.

If, and only if the fill lamp is delivering ½ the illuminate at the subject plane, the light ratio will be 3:1. This ratio is considered best for portraiture. Under these conditions, exposure is based exclusively on the fill. Use the fill’s subject distance. Divide this distance into guide number for camera aperture settings. Stated another way, ignore the main for determining exposure when a 3:1 ratio. Applies only to fair complexion. Other skin tones require test by bracket method.

You can experiment with reduced fill intensity to change lighting ratio. Each test should be made only in 2x increments using the 1.4 factor. Ratio possibilities are 2:1 flat – 3:1 normal - 5:1 contrasty then 9:1 theatrical. Again, exposure based on the fill alone only applicable only at 3:1 ratio. At 3:1 ratio this exposure method places skin tone at zone 6.

I know this was more than you wanted. I known I am long winded, can’t help myself, it’s the teacher in me.

Best of luck,

Alan Marcus
ammarcus@earthlink.net

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June 08, 2006