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Portraits using natural lighting for 35mm b&w film

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May 07, 2008

We light to give depth. Additionally most believe we must preserve an illusion that just one light was used. Likely if multiple directional shadows are detected, the picture will look weird. So we light using a main light fixture set high to suggest midday sun. We position this main light in such a way to enhance features. For a long nose, the countermeasure is short nose shadow. Short nose; countermeasure is long nose shadow. Oval face; light from the front to make the face appear more rounded. Fat face; light from the side. Opposing cheek in shadow, creates the illusion of a thinner face. Positioning the main is an acquired skill.

Both film and digital lack the dynamic range of the human eye/brain combination. When taking a picture we are able to see detail in the shadows that the camera is unable to record. You are advised to fill shadows from the camera’s prospective. Thus a fill lamp is to be placed somewhere along an imaginary line drawn between camera and subject, at lens height.

Now consider a portrait set-up 2:1 (Flat Lighting)

We place the main high and off to the side causing it to shine down on the subject. Let’s say the main delivers 100 units (watts if you like) on the subject. The fill is placed near the lens axis and adjusted to deliver its light with an energy that is equal to the power of the main. Thus the main and the fill each contribute 100 units of light. Now the frontal part of the face receives light from both. Thus the total on the frontal areas of the face will be 200 units. Now some areas of the face are in shadow. These are locations where the main could not reach (we are talking about dimples and valleys and nose shadow etc.) Consider the circumstances; 200 units on the frontal areas and 100 units in shadow areas. Mathematically this can be stated as a ratio. We can reduce this ratio by dividing both sides by same number; in this case 100. The reduced ratio is written as 2:1. This is the 2:1 lighting ratio; it is flat and nearly featureless. You should know that this is the classic mass merchandising technique. It’s used by mass merchandisers because it’s a no-brainer, (plain vanilla one-size fits all) but, frankly, it does not create the most becoming portraits.

Now consider a portrait set-up 3:1 ratio (bread and butter lighting). This is the one that wins contests and this is the one that sells best.
To achieve this we reduce the fill energy at the subject to ½ power as compared to the main. We might do this with settings, a knob on the fill lamp if available or we can just move the fill fixture back further away from the subject. This method assumes both main and fill are identical. We measure main to subject distance and multiply by 1.4. The answer is a revised fill-to-subject distance. This added distance reduces the light energy playing on the subject from the fill. The reduction is 50%. The 1.4 factor is from a law in physics known as the inverse square law. The idea is to cause the main to deliver 100 units and the fill 50 units. Now visualize the frontal area of the face getting light from both fixtures. The values are 100 main + 50 fill. The frontal areas receive 150 units. Shadows receive 50. Ratio is 150:50 which reduces to 3:1.

Consider 5:1 somewhat more zippy lighting.

We reduce the fill to 1/4 power by knob on the unit if available or by setting the fill even further back. This time, multiple main-to-subject distance by 2. This calculates a revised fill-to-subject distance causing the fill light energy to be reduced to just 25% of the main’s energy. Now the frontal area receives 100 from the main and 25 from the fill for a total of 125 units. The ratio is 125:25 or 5:1. This is contrasty lighting.

Consider 9:1 -- somewhat theatrical very zippy lighting.

We reduce the fill to 1/8 power by knob on the unit or by distance. This time the multiplier is 2.8 times main-to-subject distance. Fill now delivers 12½ units. Main continues to delivers 100 units. Total on frontal area is 112.5 units, Shadows contain 12.5 units. Ratio is 112 ½ : 12 ½ = 9:1 highly contrastry. This lighting is considered theatrical.

Note 9:1 is the maximum ratio. Any more fill reduction and it will not record. In other words, after 9:1 you are into a single lamp set-up.

In summery:
2:1 flat lighting – fill same power as main.
3:1 bread-and-butter lighting – fill is 1 f/stop subordinate to main.
5:1 contrasty – filll is 2 f/stops subordinate to main.
9:1 High contrast theatrical – fill is 3 f/stops subordinate to main.

Hope this is not too confusing – others likely will do a better!

Alan Marcus (marginal technical gobbledygook)
ammarcus@earthlink.net

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May 08, 2008