Perception and Color Photography

January 1, 2011

By Glenn Rand

Perceptions are not real, but they are really important. Whether you’re lighting a scene, viewing a monitor or approving a proof sheet, our perceptions affect our success with color photography. Regardless of what the real facts are, we make our decisions based on what the perceptual system presented to us is.

But if color perceptions are not necessarily facts, and a color-management system is numerical, why should we be concerned about the perception of color? Because we believe that because we have calibrated monitors, that what we see on our screens will be what we get.

The concept of color translation is important for both taking color images and managing workflow. Translations that affect the interpretation of color happen throughout the image-making process.

In color photography, metamers are different colors that can look alike under one light source, yet look different when seen with other light sources. Our role in color photography is to control the translation to get the color we desire.

Our perceptual system is one of the translators for color photography. In order to understand how our perceptions affect our color photography, let us look at a few perceptual constructs and see their effects:

Perceptual Constancy
Constancy is the mind’s ability to hold our interpretation of what we see as constant, even when they vary slightly. For example, we see a white house as white, whether it is bright and sunny or overcast outside. Our mind operates as an automatic white balance in this situation; not to actually color-correct the input, but to tell us that our assumption and expectation about the color of the house is constant: “white.” It works the same in photography.

The graphs to the right are spectral energy distributions for midday sunny light, clear sky northern light and tungsten light. The color bias is obvious from the slightly cyan of a sunny sky to the highly blue color shift of the north sky and the reddish color shift of tungsten light. These shifts will affect the way we see the scenes we are photographing, whether we see them or not. It will also become important in our managing the color at our workstation.

Viewing Flare
Flare happens when light, any light, enters an optical system. This means that when taking a photograph, even with the sun at your back, there will be some flare. Also, when looking at a monitor, because the monitor is a light source, there is a certain amount of flare created in our eyes. The effect of flare is to reduce contrast and dilute saturation. This is particularly true in highlight and shadow areas of an image.

Absolute Brightness
Another factor that affects viewing is the absolute brightness of the light. The brighter the light, the more problems with understanding color saturation and the less ability for the eyes to perceive shadow detail.

Adaptation happens in three ways: As general brightness adaptation, chromatic adaptation and/or lateral brightness adaptation. Since our optical system, eyes and mind will adapt to brightness and color, this adaptation will impact making and managing photographs.

General Brightness Adaptation
Our eyes work over a broad dynamic range of approximately 20 stops. This means that we can see black ink on white paper under starlight, and also perceive light intensities close to that of the sun. Our perceptual system adapts by switching from photopic vision (color and brighter light with the cones in the retina) to scotopic vision (using rod vision that reacts only to intensity). Cones require far more energy to be activated and thus we need more light to see color and the reverse is also the case that seeing color makes us believe that more light is present.

While our visual system can adapt quickly to variations within the brightness range, over time this adaptation becomes more pronounced and our vision under the adapted light level becomes more effective. We have all noticed going into the bright sun after being in a darkened room or vice versa and the lag time before our visual system adapts to the new light level.

Objects with high intensity are perceived as closer to the viewer. Since our eyes require more light to see color, then the greater the saturation of color the closer the object is perceived as being. This means that bright colors as well as saturated colors will tend to pull forward compared to muted colors. Also more saturation is perceived as higher in intensity.

Chromatic Adaptation
Just as our perceptual system adapts to the brightness of the light, adaptation also takes place based on the color within our visual field. If we are in a green light space, such as under normal fluorescent lights, our perceptual system will adapt to our color vision so that we do not readily perceive the green cast. Furthermore, the longer we are exposed to the color environment, the less observable the color bias. This goes beyond constancy and is actually a physiological change in how we see color.

Lateral Brightness Adaptation
This effect on tonal and color vision is also known as simultaneous contrast. In this effect an object’s apparent tone and color are affected by the color or tone of the surroundings. A uniformly toned area will be perceived to be darker if bordered by a lighter area or vice versa. This perceived tonal change is seen in the border area and happens in opposite ways on either side of the tonal border.

Lateral brightness adaptation affects the perception of color. Both the perception of hue and saturation are effected in this way. Because the eyes will scan across the boundary between tonal or color areas, the neighboring color or tone will affect how we interpret the target color.

How Perception is Used
Once we capture an image we work within a light environment as we edit, modify and color-manage our images. Here these perceptual concepts can affect our work and its ultimate look without us noticing. The more critical the color, the more attention needs to be paid to the way our perception is modified as we work with our post-captured images.

Constancy will override our normal visual system as we work with images either on a light table or on the computer monitor. Our perceptual processing of the image will try to keep the lightest part as white. When this happens we lose our visual ability to notice small color biases. This puts a premium on the use of numerical systems to measure and quantify color.

The image viewing area, light table or desktop of the computer will be the major concern of the light within the environment, the overall ambient light must be considered. While constancy will slightly affect our viewing of an image, the illumination of a light table or computer monitor will have a major effect on our ability to accurately see color. In terms of absolute brightness and flare, our eyes will not accurately see the tonality of an image. For this reason we need to reduce the amount of bright surfaces and open space on a light table and maintain a lower illumination level for the desktop area on our computer screen. It is also important to control the flare potentials from other light sources or reflective surfaces in our working environment.

As nice as it might be to look at a beautiful scene outside our workspace, this is not a good idea if you are working on color images. The bright surroundings will adapt your eyes and will create additional flare. Thus, it is best if the background for a computer is a neutral gray wall. Just as we would not want to have the desktop on our computer bright, you would not want the surrounding area to interfere with your work.

The neutrality of the light environment can be beneficial to your working with color images. Regardless of the color of the desktop on a computer, lateral brightness adaptation will take place. For this reason most advisors on this subject recommend middle to dark gray as a desktop.

While there are many other aspects of perception that interact with photography, these basic ideas are fundamentally important. In color photography these concepts allow us to change emphasis and tension depending upon the relationship of the colors within the images.

Also, realizing how the color of illumination translates the color of a scene allows us to utilize colors to accentuate or mute other colors within a lighting scheme regardless if we can see these colors. Most photographers easily understand how these aspects of perception affect color photography. It is harder to appreciate how they interact with color management.

Glenn Rand and Tim Meyer will teach a MasterClass, “Chisels and Brushes: The Tools of Fine Art Portraiture,” on Tuesday, February 22nd from 3:30 p.m. to 5:30 p.m.

Glenn Rand has worked as an artist and professionally  in photography for more than 35 years. His work is in major museum collection in the U.S., Europe and Japan. He has published and lectured extensively about photography and digital imaging. Presently Dr. Rand teaches in the graduate program at Brooks Institute of Photography in Santa Barbara, California.