Understanding the Light

In a landscape photograph, particularly a black & white landscape, composition is the essence of the image; but it is usually the photographer’s use of light that brings the drama.

Emitted Light, Incident Light, and Reflected Light

The direct light from a light source, be it the sun, a lighthouse beacon, a campfire, is emitted light.

Emitted light that falls on the surface of a subject becomes that subject’s incident light.

When we look at a subject, what we see is its incident light reflected, i.e., its reflected light.

A spot meter or the light meter in a camera is a reflected light meter. It measures from a distance the light reflected by the subject. A mirror finish can reflect more than 95% of a subject’s incident light. An area in deep shade, or covered by a black, heavy weave fabric might reflect less than 5% of its incident light.

Alternatively, some photographers (e.g., portrait photographers) often hold an incident light meter next to the subject to measure the light falling on the subject (as opposed to the light reflected by the subject).

Sometimes, light that falls on a subject is indirect incident light. It is not coming directly from the sun or other light source. It is sunlight or other light that first falls as incident light on the molecules of moisture, smoke and dust in the atmosphere from which it is reflected and then falls on the subject. Especially on a cloudy or hazy day, a lot of the light falling on a subject is probably indirect incident light. As Ansel Adams pointed out in his seminal book, The Negative: “The clearer the air, the more intense the light from the sun and the less intense the light from the sky and therefore the greater the difference between sunlit and shadow areas.” 

The Law of Reflection

A surface in a subject reflects its incident light at an angle equal to the angle at which the light strikes it. That is, the surface reflects the light away at an angle equal to the light’s incoming angle. In physics, this is known as the Law of Reflection: the angle of incidence is equal to the angle of reflection.

Ambushing the Light

It is often to the landscape photographer’s advantage to predict the light on his or her intended subject to determine when to be there to snap the shutter. Ansel Adams used to call this “ambushing the light.” With the use of one of the powerful “apps” that are available for smart phones (see "Apps Make Ambushing the Light Easy", below), predicting the light today is much easier and much more accurate than in Ansel’s day.

In any case, to ambush the light, the photographer must be in place, set up and ready to press the shutter release at the moment when he or she thinks the light is optimal.

Planning an ambush can be rudimentary, based only on an awareness of approximately what time sunset will occur and what its approximate position on the horizon will be relative to the approximate alignment of the subject. For example, ambushing the light on a landscape subject that has a northwest/southeast axis can be as casual as picking a day when you know the sun will be setting as far south as it will all year, thus having its incoming rays as close to perpendicular as possible to the horizontal axis of the subject. To execute the ambush, then, just be at your pre-determined spot early enough to be set up and be ready to click the shutter as the sun approaches the horizon.

Planning an ambush also can be as precise as determining the day, the hour and the several minutes within that hour when the sun’s azimuth will be as congruent with the subject’s axis or as perpendicular to it – depending on the photographer’s artistic vision – as the sun’s seasonal transit will allow.

Many great images are captured without laying an ambush at all. Driving by an old country church in Hernandez, New Mexico, Ansel Adams serendipitously captured one of his most famous images: Moonrise. He noticed the moon was rising over snow-capped mountains in the east behind the church just as the setting sun’s rays struck the metal cross on the church’s roof so as to seemingly set it ablaze. He slammed on the brakes and raced to set up his 8x10 camera. He couldn’t find his light meter and so he set the aperture and shutter speed based on the one luminance value in the scene that he happened to know. the moon. He pressed the shutter and then hastened to reverse the film holder to make a second exposure. In the few seconds that took, clouds partially obscured the sunlight and the light on the cross dimmed before he could get to the shutter release.

However meticulous one wants to be in planning an ambush of the light, the essentials are knowing beforehand the precise spot on the ground on which the camera will be set up, and where the light (the sun or the moon) will be relative to that spot.

Where Will the Sun Be?

Assuming clear skies, one can predict sunlight conditions on a given subject on the day and at the time one wants to photograph that subject. It is a matter of forecasting the sun’s position in the sky (and having a clear sky between the sun and the subject).

At any moment on any day, the sun’s position in the sky can be described in terms of its altitude above the horizon and its azimuth on the horizon.

Altitude

The sun’s altitude is the angle of the sun above the horizon, as shown in the diagram to the right. The sun’s altitude is the angle (i.e., the angle of incidence) that its rays fall on any part of a subject that has a vertical or partially vertical aspect. The lower the sun’s angle, i.e., the closer it is to the horizon, the more perpendicular are its rays to a perfectly vertical surface in the subject. The sun’s altitude changes continuously throughout the day, rising to its zenith near mid-day.

On April 15, 2020, at 9:11 AM, the sun’s altitude was 30°, as seen from Sausalito, California, and as illustrated in the diagram to the right. At 6:36 AM (three minutes after sunrise) it had been 0.1°, or virtually perpendicular to a vertical surface in the subject. Thus, at 9:11 AM, at an altitude of 30°, relative to a perfectly vertical surface, the angle of incidence was 30°. Of course, not all vertical surfaces are perfectly vertical, i.e., 90°, so the angle of incidence on some surfaces might have been more, or less, than 30°.

The sun’s altitude is different at the same time of day (e.g., 8:00 AM) on each day between the Summer Solstice (June 21st) and the Winter Solstice (December 21st). That is because the higher the latitude above the Equator on which the photographer is standing, or the lower it is below the Equator, the shorter and lower the sun’s arc across the sky appears day by day between the Summer Solstice (June 21st) and the Winter Solstice (December 21st). Correspondingly, the sun’s arc appears progressively longer and appears higher in the sky each day between the Winter Solstice and the Summer Solstice.

Azimuth

The sun’s azimuth is the compass bearing from the photographer’s location to the point on the horizon directly below the sun.

The sun’s azimuth on any given day changes literally minute by minute as the sun moves across the sky.

As the dark blue line segments show on the azimuth diagram below, in 2020 on the Winter Solstice, the sun rose on an approximate azimuth of 106° as seen from in Sausalito, California (center of circle). It set on an azimuth of 227°.

On the day of the Summer Solstice, as the red line segments show in the diagram, the sun rose on an azimuth of approximately 45° and it set at 287°.

On both the Equinoxes, March 21st and September 21st, as the lavender line segments show, the sun rose on an azimuth of at approximately 75° and set at approximately 257°. The Equinox sunrises and sunsets are halfway between the Solstice sunrises and sunsets.

The azimuth diagram also implies that the sun’s arc across the sky, relative to Sausalito, California, is the farthest north on the Summer Solstice (the longest day of the year), and the farthest south on the Winter Solstice (the shortest day of the year). See the first two Sun Surveyor screens below.

Indeed, on the Summer Solstice in Sausalito in 2020, there were 14 hours and 47 minutes that the sun was visible above the horizon. On the Winter Solstice, the sun was visible for 8 hours and 22 minutes.  On both Equinoxes it was visible for 12 hours and 10 minutes.

It is the sun’s azimuth that determines the angle of incidence on the horizontally aligned surfaces(s) of the subject.

The diagram to the left illustrates a 90° vertical surface in a subject that is horizontally (across the camera’s viewfinder) due north and south. It shows the sun’s rays’ angle of incidence on that surface of 240° which would have occurred at 4:35 PM on April 15, 2020 as seen from Sausalito, California.

The sunset azimuth on that day was due west: 270°. Given the 360°/180° alignment of the subject’s surface, the sun’s rays were exactly perpendicular (i.e., the most direct light) to that surface at sunset.

The sunrise and sunset azimuths decrease each day between the Summer Solstice and the Winter Solstice, and they increase each day over the following six months between the Winter Solstice and the Summer Solstice (see "Seasonal Sunrise and Sunset Azimuths" diagram above).

The higher the latitude above the equator, and the lower the latitude below the equator, the greater the average daily azimuth differential. At the Equator, latitude 0.0°, the average daily azimuth change is about a quarter of a degree, i.e., a range between the Solstices of 47° over each six-month period. In Sausalito, California, latitude 37.86°, the inter-Solstice average daily azimuth change is about a third of a degree, a range of about 60° as shown on the "Seasonal Sunrise and Sunset Azimuth" diagram above. In Pelican, Alaska, latitude 57.96°, the average daily change is just over a half-degree each day, a range of 97°. 

In reversing its six-month transit on the day of each Solstice (north to south on the Summer Solstice and south to north on the Winter Solstice) the sun rises on the same azimuth twice a year: once southbound and once northbound. On the day that is the same number of days past the Winter Solstice that a given day was that number of days before the Winter Solstice, the sunrise azimuth is the same (within about a half-degree). The same is true for the sunset azimuth.

Thus, if the sunrise azimuth is optimal for the photographer’s vision of a particular subject on May 5th (135 days past the Winter Solstice), it will be on that same azimuth again on August 8th (135 days before the Winter Solstice).

Apps Make Ambushing the Light Easy

Today there are several “apps” for smart phones that, for any location on any day and for any time of day, give the photographer analog details and graphic plots of the sun’s and moon’s altitude and azimuth. As shown at the end of this article, they also provide ephemeris for the sun, the moon and the milky way. Among the best of these “apps” are:

• Sun Surveyor; and
• The Photographer’s Ephemeris (or “TPE”); and
• The Photographer’s Assistant.

Shown below are two screens from Sun Surveyor’s phone “app.” On the left is the screen for December 21, 2021, the Winter Solstice. On the right is the screen for June 21, 2021, the Summer Solstice. These plots are relative to Sausalito, California.