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How to Determine Which Aperture to Use

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October 27, 2010

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October 28, 2010

Now both film and digital lack the dynamic range of the human eye. Stated another way, we can see detail in the shadows but our camera image reproduces the shadows as too dark. Now dark shadows are void of detail and the resulting image is said to be too conatrasty. The countermeasure is to use a reflector or a fill light. The idea is to add extra illumination to the shadows. In a studio setting we utilize a second lamp know as a fill. Now this lamp plays on the subject from the camera's position thus it softened the shadows from the camera's viewpoint. This preserves the illusion of only one light source. Thus the best placement for the fill is somewhere along an imaginary line drawn camera-to-subject at lens height.

To keep the illusion of one light source, the fill is adjusted so that it is subordinate to the main. Now the contrast of the finished picture is highly dependent on the difference in brightness between these two lamps. As a rule, we desire the fill set to about 50% as compared to the main. This is best accomplished using a light meter. Now 50% means 1/2 power so if the fill lamp has a power adjustment we set both the main and the fill at the same distance from the subject with the main set to full power and the fill set to half power. That assumes both fixtures are equal in all respects. Again, such adjustments in brilliance at the subject plane is best achieved by using a meter.

To accomplish we measure the subject's forehead or cheek or better, a gray placard. We take this reading with only the main operating. Then we turn off the main and read again this time with only the fill operating. Say the main only reading is f/4 we establish the fill at one f/stop subordinate so it's reading is f/2.8.

If we cannot use a meter to establish this ratio (difference), we can use lamp-to-subject distance. Assuming both fixtures are equal in brilliance and size, we measure main-to-subject distance. Now we multiply this distance by 1.4. The math calculates a revised fill-to-subject distance that achieves a 50% subornation. If the main and the fill set as described, produce a ratio called 3:1. This is known as the bread and butter ratio because it generally brings about the highest portrait sales.

A more mescaline setup that will yield more contrast is a 5:1 ratio. This is achieved if the fill is set two f/stops subordinate. A reading might be, main alone f/4, fill alone f/2. By distance adjustment, the 5:1 is set using a multiplier of 2. In other words, both fixtures equal in power, the fill is set twice as far from the subject as the main.
An even more contrasty setup is achieved if the fill is three stops subordinate. With a meter, the main reads f/4, the fill reads, f/2. If you use the distance method, the main-to-subject distance is multiplied by 2.8. The results of this math tells you where to set the fill to establish this highly theatrical (very contrasty) 9:1 ratio.

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November 01, 2010

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November 03, 2010

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November 04, 2010