The Photographer’s Lighting Handbook: Reflectance
In another post on the Photographer’s Handbook, we detailed making a white background almost black. This time we’ll go in the other direction by making that white background white not only in life but, most importantly, in the photograph.
Making a background white is a lot like the TV game show The Price Is Right. The amount of light has to be as close as possible to reading RGB:255 (Photoshop) or 100% (Lightroom) without going over. Time and again, I’ve seen photographers dump a ton of light on the background sometimes where pulling back even four f-stops in Camera Raw won’t make it the slightest bit gray. “What’s wrong with that,” you ask? “The background is white.”
There are two problems that seriously degrade the quality of the photograph. The first is edge reduction. Overbright backgrounds blow out the edges of the subject, making it slightly smaller than one shot with the background at 255 or 100%. The second issue is that over-lit backgrounds cause lens flare. Flare reduces contrast and saturation, causing the image to look washed out.
Edge reduction is most common when shooting interiors that have daylight streaming through windows. The proper exposure for the interior is several stops darker than the window light. Here, the exposure is 1 sec at f/8, ISO 80. The interior is fine while the outside light is almost completely white. Notice the breaks in the mullions that divide the windows into panes. In some places they completely disappear. This is known as edge reduction.
I combined the properly lit interior with windows shot just two stops darker (1/4 at f/8, ISO 80). The edge reduction is now gone and the mullions appear in silhouette as they do in life.
Lens flare is caused by very bright, out-of-focus highlights. Check out these two examples of a high heel shoe by photographer Katherine Schambach. Both photographs were exposed at 1/125 at f/11, ISO 100, and the only difference is the amount of light on the backgrounds. The left photo is more than four stops brighter than the one on the right. The colors are washed out and the blacks have a gray tinge. Notice the edge reduction in the top arch.
The question remains: How much more light does it take to make a background really close to white, without tripping over RGB:255 in Photoshop or 100% in Lightroom? The answer lies in understanding how reflectance works in relation to the prime variable: the exposure set on the camera.
Here, we revisit the photograph of Dana from the last issue. The exposure is 1/160 at f/8, ISO 100, read with an incident meter pointed at the source of light. A reflected reading of the white wall a few feet behind her also reads 1/160 at f/8, ISO 100. Reflective meters always give an exposure that yields a reflectance of 12.5%. The background reading made with a 1° spot meter pointed between Dana’s chin and shoulder reads exactly the same as the exposure on the camera. The background’s reflectance is 12.5%.
F-Stops and Reflectance
There’s a direct correlation between the amount of light striking an object and its reflectance. If you double the light, a one-stop increase, the reflectance also doubles. When the reflected reading of the background is a smaller aperture than the one set on the camera, the reflectance is brighter. If that reading is a larger aperture, the reflectance is darker.
Take a look at this chart. A lens set at f/8 rests on 12.5%. Doubling the amount of light hitting the background yields 25% reflectance. The reading on the meter is 1/160 at f/11. This might seem counterintuitive. Remember, reflected meters provide an exposure reading for any brightness that if set on the camera would be 12.5% tone. So, if the amount of light were doubled to maintain 12.5%, the aperture would have to be smaller.
The exposure on the camera isn’t changed. The exposure (1/160, f/8, ISO 100) is locked down on the camera. What’s happening here is measuring the difference between the exposure on the camera and the amount of light reflecting off the background.
As the amount of light is doubled again, the reading is f/16 and the reflectance is 50%. The difference in brightness is +2 stops. Doubling the amount a third time makes the reading f/22 and the reflectance also doubles to 100%—100% brightness is white. It’s impossible to record more than 100%. The difference in stops to white from middle (12.5%) gray is +3 stops.
Here’s how it works: The brightness of the background is relative to the camera’s exposure. Removing light from the background makes it darker; adding light to it makes it brighter. Two V-flats are placed behind Dana, one on either side of the background. Lights are bounced into each flat, evenly lighting the surface.
The light is increased until the reflected reading from the camera to the background reads f/22 or + 3 stops brighter. Once that threshold is reached, all that’s left is to have fun shooting.
Editor’s Note: While learning your skills behind the camera, keep your photographs organized. Here are a few tips for your Lightroom catalog. In this tutorial Matt shares 7 Tips for Organizing your Lightroom catalog.