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.
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Moon Walk by John Hulsey, pastel painting.
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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.
Join us on The Artist's
Road for more interesting and informative articles. We look forward to hearing from
you!
--John & Ann