What the Eye Cannot See

Most people know that the best opportunities for landscape photography occur around sunrise and sunset in what is often referred to as the golden hour.  I used to set my camera up and wait for the sun to appear, or pack up as soon as the sun had set, but over the past few years, I've learned to observe the ambient light and shoot longer exposures at twilight to amplify details in a scene, and reveal qualities of the light that the human eye can't perceive.

Some of the most subtle and interesting light happens before the sun even breaks the horizon, or long after it has set in what is called, "the blue hour."  The blue hour is the period of twilight during dawn each morning and dusk each evening when the sun is a significant distance below the horizon and the residual, indirect sunlight takes on a predominantly blue hue.

This effect is caused by the relative diffusibility of short blue wavelengths of light versus the longer red wavelengths.  During the "blue hour" (typically the period is about 40 minutes in length), red light passes straight into space while blue light is scattered in the atmosphere and therefore reaches the Earth's surface.

A related phenomenon occurs shortly after sunset when the Earth's shadow is cast on the atmosphere, creating a striking gradient of pink, lavender and mauve hues with a darker bluish band.  The Earth's shadow and Belt of Venus are most visible in locations with a high vantage point or an open, expansive view so that the horizon line is low.  It also doesn't last very long as the sun continues to set and the colors dissolve into the darkness of the night sky.

Understanding light is one of the most important factors in creating distinctive images, so I'm learning to use my camera to capture not only what I see, but also the details my eye cannot discern.