Exposure Control and Depth of Light
By Christopher Grey
Published by SekonicLighting expert Chris Grey explains how metering your light to create a consistent level of illumination on high-contrast subjects allows you to retain detail, get proper exposure, and save yourself from time-consuming Photoshop adjustments.
Photographer Chris Grey is an expert on lighting. He's written seven books on the subject, teaches classes and workshops, and is a strong advocate for precise control in the studio. To see more of his work, or to find his books and DVDs, visit his Web site.
I was recently involved in a conversation between photographers who believed that digital cameras simply could not record an adequate contrast range. Horror stories flew about the difficulties of shooting black-suited executives with white shirts, tuxedoed grooms and white-gowned brides, and interracial couples. The problem, as they saw it, was that one side of the contrast range had to be slighted in favor of the other. The ultimate dilemma, of course, is deciding which side will take the hit. Most decided it would be best to shoot for the bright side and fix the rest in Photoshop.
While that approach works in theory, it is misguided in reality because it assumes that exposure deficiencies can always be adjusted to "normal" by using Levels or Curves adjustments. They cannot. After a certain point the adjustments will not look "normal."
The second and most notable flaw in this approach is the tremendous amount of work it requires a photographer to do just to make presentable proofs for clients. With this method, you would have to do major, time-consuming retouching on each and every image before making a single dime on prints. Such an investment of time is simply not acceptable in a successful digital workflow environment.
In truth, a little more planning on the front end can mean no work at all on the back end. No masking, no adjusting—just send your files to the printer for perfect prints (cosmetic retouching optional). We can accomplish this task simply by controlling the strength and direction of the light to ensure that subjects showing extremes of dark and light are evenly illuminated.
My models for the following images are friends of mine who were engaged to be married at the time of the shoot. He is dark; she
is light. To make this exercise more difficult, I requested that he wear white clothing and she wear black. Because I wanted substantial modeling of the planes of their faces, I used only one light for my key, along with two kickers and a background light.
As you know, the laws of physics dictate that light "falls off," or loses strength, as it travels. The inverse-square law says that whenever light doubles its distance from point A to point B, it will be one fourth as strong. This is because as light travels it expands and loses power. There is a corollary rule that says the further a light source is from its object the greater the distance over which its strength will be even. This is called "depth of light," and will determine how far I set my key light from my subjects.
Before anyone arrived, I placed two tape marks on the floor for the couple's positions and roughed in my lighting scenario. My key, a Profoto Acute D4 strobe in a Profoto 3x4 softbox, was placed 13 feet from the marks. To get the correct shadow angle, the key was moved 9 feet to the right of the lens axis. At this distance the key metered at f/8.7 on the camera-left mark and f/8.8 on the camera-right mark. By angling the light toward the camera and away from the tape marks, I feathered it until it read a perfect f/8.7 on both marks. Then I set the camera’s aperture to the corresponding f-number, f/10. Once my friends got to the studio my scenario needed nothing more than a final check with the flash meter.
Most pro digital cameras can set aperture in either half-stops or third-stops. If you're not working in thirds and can change your camera’s aperture-measuring preference, I suggest you do so right away. The added control is worth the inconvenience of learning new reference numbers.
To kick the subjects off the background and add visual interest, I added two strobes in smaller softboxes, one on each side behind the couple. Under normal circumstances I would power these lights a bit brighter than the key. Since I did not want to overexpose anything, especially his white shirt, I powered the kickers to f8.3, 1/3 stop less than the key. I knew I would still have a highlight along the edges because the angle of incidence of those two lights bounced straight down the lens axis (the angle of incidence equals the angle of reflection) and I would be photographing a specular reflection, not just a splash of light.
To add depth, I mounted a strobe with an 18-inch reflector and 40-degree grid (a favorite combo) on a boom arm. From directly above the couple, the light was aimed behind and slightly below shoulder height. It was powered to f/11, 1/3 stop brighter than the key, and metered at the hotspot.
We began the set with the man on camera right. If he was going to be overexposed, it would most likely be from this side, as he is closest to the key. Note in the enlargement that there is detail in even the brightest areas. Note also the detail in her black dress.
Because the key light is so controlled, there is no discernible difference in exposure values when they switch positions.
As we were wrapping up the shoot, it occurred to me that this light would be very romantic if the key weren’t so bright. I knew I would get a wonderful shot if I turned the key down two stops and left everything else as it was. I moved in tight to give the shot the sense of intimacy that matched my friends’ emotions. Notice the fill her face gets from the camera-left kicker light reflecting off his shirt.
Aside from a little cropping, absolutely no work was done on these images. They weren’t shot raw, but as high-quality JPEGs. You’re seeing exactly what I got off the memory card: beautiful photographs, perfectly controlled. You can get results just as wonderful. The secret is how tightly your light meter is calibrated to your camera. Front-end planning means no back-end work.
Text and images © Christopher Grey.
Category: How-ToClick for more articles of: Christopher Grey