Paula and I went off to see the annular solar eclipse three weeks ago. By the time the moon was covering barely 25% of the sun, she, I, and the friends we were hanging out with all noticed that the landscape had gotten distinctly darker. That's at less than a half stop down from full sunlight.
Our eyes are usually very insensitive to gradual changes in illuminance; they have what's called "brightness constancy." We have very adaptable visual systems that allow us to see over a brightness range of greater than a billion to one. Apparently, though, we have a reference point wired into our brains for how bright normal sunlight should be. I hadn't noticed that before. Interesting! Also, it led me directly to this week's little lesson.
Visual constancy isn't perfect. Our ability to distinguish different tones diminishes under very bright or very dim light. There's a sweet spot around several hundred footcandles—the equivalent of a heavily overcast day or very bright indoor lighting—where we have a maximum ability to distinguish tones. In brighter light than that, we lose the ability to separate tones near white. In the other direction, we lose the ability to separate tones in the shadows.
The total number of tonal steps we can see a surprisingly small, about 650 for the average eye over that entire billion-to-one brightness range. Under optimal viewing conditions, in a typical photographic print, we can distinguish about 250 tones. If the light is significantly brighter or dimmer than that, we not only lose tonal discrimination but we lose it disproportionately at one end of the brightness scale or the other. That's why it's so easy to see "into the shadows" in a print when it's in direct sunlight and so hard under dim indoor light. More broadly, it's why we try to make our prints under conditions vaguely like the way we expect an audience to be viewing them. If our viewing lights are significantly brighter or dimmer than our audience's, they will perceive the prints as being too dark or too light (compared to what we intended them to see).
Third contact in the annular eclipse, as seen in H-alpha light, photographed through my Coronado PST. A few prominences are licking around the edge of the sun, even past the disk of the moon at the bottom of the photograph.
Our vision also has what's called "color constancy." You can vary the color temperature of the illumination over a considerable range, and colors appear approximately the same. We take all of this for granted; it only comes to our attention because our cameras and films aren't this adaptable. If they were, we'd hardly ever have to make an adjustment for exposure or color balance.
"Approximate," though, does not mean "exact." I'm not talking about metamerism, the apparent color shifts that some dyes undergo when viewed under different kinds of illumination. That's a different problem. I'm talking about the color interpretation by our eyes. A nice silver gelatin black-and-white print, which has a pretty flat spectral reflectance and exhibits very little metamerism, still tends to look cool-toned under skylight (10,000 kelvins) and warm-toned under a 60W incandescent bulb (2,600 kelvins). It's not a big shift in apparent hue, but fussy photographers and printers are well aware of it. Of course, this affects color prints as well, metamerism or no. It's doubly important to view your color prints under illumination similar to what your audience will be using, if you have any say or knowledge in the matter.
There's another deviation from color constancy: below 4000 kelvins or so the total range of colors we can see, our visual gamut, starts to shrink. Even with illumination of sufficient brightness, we start to lose parts of the spectrum, especially towards the cool end. By the time you get down to normal incandescent lighting, our visual gamut is substantially diminished. This is why even a slight improvement in color temperature, as when going from 100W incandescent bulbs (2800 kelvins) to quartz-halogen bulbs (3200–3400 kelvins) makes the colors look so much more vivid in a photograph, even if the brightness level of the light isn't any greater.
There's yet another deviation from color constancy. It takes our eyes time to adapt to changes in the illumination's color temperature. It used to be thought that this happened fairly quickly, in a matter of minutes. Half an hour at the most. More careful experiments and research showed that along the yellow-blue axis, in particular, it can take several hours for the eye to fully adapt! During that time, our perception of the overall color balance of the photograph is slightly but perceptably shifting.
This is just the way our eyes work. There's nothing you can do about it except to be aware of it and pay attention to it, so that it doesn't compromise you too badly.
Ctein's weekly column appears with approximate constancy on Wednesdays.
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Original contents copyright 2012 by Michael C. Johnston and/or the bylined author. All Rights Reserved.
Featured Comment by Yves Papillon: "My left eye is dominant. I recently compared looking trough the viewfinder of a rangefinder camera with my left and then right eye. I noticed a clear difference in image tint. One eye showed a cooler image and the other a noticeable yellow one. My two eyes are inconstant."