Have you ever noticed when you are plein air painting how the colors of objects look so radically different in the very low light just before dawn or twilight? Take a red rose, for instance. We know that the flower’s petals are bright red against the green of the leaves in daylight. But, take a look at dusk and you will see that suddenly the contrast is reversed, with the red flower petals now appearing a dark red or dark warm gray, and the leaves appearing relatively bright.
This difference in contrast is called the Purkinje effect, or Purkinje shift, named after the Czech anatomist Jan Evangelista Purkyne, who discovered it on his early morning walks in 1819. It is the tendency for the peak luminance sensitivity of the human eye to shift toward the blue end of the spectrum at low illumination levels.
The effect occurs because the color-sensitive cones in the retina are most sensitive to yellow light (like when you look at the sun). The retinal rods are more light-sensitive (good for low light levels), but do not distinguish colors very well at all. They are mainly sensitive to green-blue light. This is why it is very difficult to distinguish other colors in moonlight.
The result being that we become nearly color blind under low levels of illumination. As the light dims, the rods take over from the cones, and before color disappears entirely, our color perception shifts toward the blue-green spectrum.
This brings us around to the subject of outdoor painting nocturnes again. The Purkinje effect explains why we can’t see many colors at night other than the blues and greens that our rods can sense. However, that doesn’t help us make a good night painting en plein air. After all, the painting itself is not meant to be viewed by moonlight, yet it must contain the magic of that light, which is really more appreciated by being there at the moment.
The job for the plein air artist is to somehow capture the beauty of that moment, and the secret to that is to add more than can be physically perceived at the time. We have to engage our imaginations and put back into the subject some of the color that has been lost. We also play with boosting the chroma of the colors that are present and expand the value range so that there is more depth of field and a bit of detail in the subject. This is tricky to get right and bends the rule of “never paint what isn’t there,” but with practice, it can be done well. And, when it is done well, a nocturne can be every bit as powerful as any painting executed in daylight. Try it. At least you won’t get sunburned.
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–John & Ann