Equipment and shooting technique

You are here: Nature Science Photography – Natural light – The moon as a light-giving object

We do not want to close the chapter on the moon without dealing with a special feature that emphasizes the difference between our visual perception and that of a photographic image carrier – photographs taken in the light of only the full moon.

The chapter on „The Perception of Lightness and Color“ shows that we perceive our environment at low light levels with the rod cells in our eyes, which have only limited color capability. This leads to almost black and white vision in semi-dark or dark environments. Film or electronic image carriers, however, are not subject to this limitation and, provided the exposure time is long enough in this particular shooting situation, show us a normally invisible colored world. Leaves, branches and flowers that are gray to us regain their green, yellow or red; wood shimmers brown again; stagnant black waters suddenly reflect the color of the sky; and fast-flowing water glows in the white of moonlight, which is much less yellow than sunlight. Upon initial observation, this world of motifs bears a resemblance to our familiar daylight, yet a peculiar allure emerges only upon closer scrutiny. It largely lacks shadows because the moon wanders through the long exposure time, illuminating the subjects from different angles and filling their shadows to a good extent. So be careful, because here applies: „What you see is not what you get!„

The moon must, of course, reflect a respectable amount of light for this type of picture to be possible. In most months, this occurs during the full moon or the two days immediately before or after it. Two to three hours after its rise, the moon has then reached the right height above the horizon to provide both sufficient light and at least some shadows. – A factor that is lost when our little neighbor reaches the highest point of its orbit. Now you can start working on the subjects. But be patient. Such a night shift can easily last two to three hours and yet generate no more than ten shots.

The equipment

A few words about the equipment. A stable tripod is a basic requirement in view of the long exposure times, which we will discuss in a moment. The tripod should possess some weight to withstand any wind gusts. To prevent battery failure during exposure, the camera should be as mechanical as possible. Possibly even work with two or three housing units to make better use of time. Of course, the camera should have a lockable cable release. As far as focal lengths are concerned, the faster the better. A widest aperture of 1:2.0 or 1:2.8 is ideal, as it keeps exposure times short and the nighttime viewfinder image reasonably bright. And since short focal lengths have an advantage over longer ones when it comes to depth of field at wide apertures, wide-angle lenses between 20 and 35 mm are likely to be the most common. The choice of film material or the sensitivity setting on digital cameras goes in the opposite direction. Faster is not better here. High ISO values often result in visible grain or electronic noise, which is highly undesirable under challenging lighting conditions. It is more important to choose a film with stable Schwarzschild behavior that can handle long exposures largely without color cast. For instance, Velvia and Provia have a tendency towards blue-green tones, but you can rectify this with an FL-D (fluorescence filter). Whichever you prefer, you have to decide for yourself depending on the subject. The reason I only mention reversal films, as I’m sure you’ve noticed, is that negatives lack their sharpness and clarity, and you never know what the lab will do with the difficult-to-expose prints. A handy but powerful flashlight and a stopwatch, preferably illuminated, round out the equipment. And, oh yes, in the cold season, a thermos of tea keeps the photographer warm and happy!

Technique and design

Before we delve into the much-anticipated exposure problem, we must first address some fundamental aspects of visualization and design. In principle, we can photograph almost any subject in moonlight with an appropriately long exposure time, making it appear almost as if it were in daylight. Such a picture loses almost any nocturnal impression because it appears similarly bright, flat and shadowless as under the light of the midday sun, but due to the mixture of natural moonlight and nocturnal artificial lighting (e.g., street lamps or house lights), it still conveys a certain surreal effect. The only thing to watch out for is the brightness of the artificial light with respect to the moon. The average full moon is about light value 3.5, i.e., f-stop 2 and ½ seconds.

It is best to select subjects for photos taken in full moonlight only in daylight to be able to better assess the reflection behavior.

If you are aiming for a daylight-like reproduction of the subject in accordance with your image idea, the image composition should follow at least two general guidelines. First, you should avoid having too much of the sky in the picture, because the sky – of course, it is black – does not reflect any light. Moreover, from the airplane to the star, there are many more small sources of light moving against it than you can perceive with a quick glance, and these appear as unwanted additions to the image after the necessary long exposure time. The only exception to this rule is a thin veil of clouds covering the sky. As long as it does not obstruct the moonlight too much, it a) swallows the unwanted traces of light, b) softens the moonlight just like the sunlight on an overcast day, and c) blurs into an unreal nothingness due to the long exposure. Second, to enliven the image, it is advisable to combine objects with many different surfaces and thus reflective properties in the subject. Rocks, trees, any kind of plants and water, especially flowing water, are very suitable for this purpose. The latter reflects different amounts of light depending on its flow speed. And mutual shadows cast by the objects are quite desirable.

As an exposure guideline for such an exposure, without correction for the Schwarzschild behavior (correction values for very long exposure times for different films can easily be found on the Internet) of the respective film material, we can assume 2.0 minutes at f/4 and ASA 100 if we are dealing with a moon at the following, approximately average, conditions:

a full moon, about 15° in front of or behind the zenith position
in rather pure air at an altitude of about 1,000 m
when the earth is neither in the point closest to the sun nor in the point farthest from the sun
and also the moon is neither in the point closest to the earth nor in the point farthest from the earth.

However, due to our limited ability to see detail in the dark, a detailed image that resembles daylight can be confusing. Therefore, to create a night effect that is not entirely recognizable, we need to approach the process differently and pay close attention to the reflective behavior of the objects. Arrange these in the image so that only a few well-reflecting main objects stand out against a dark background. Essentially, we can achieve this by underexposing the image by approximately 1 stop, starting with the previously specified guide value. However, since the distribution of brightness values in the image is largely random, it is preferable to search for a scene in bright sunlight that exhibits a wide contrast range between the foreground and background. We then use this information to determine the exposure value for the main object. Given that the light of the average full moon, as defined above, is approximately 400,000 times weaker than that of the sun, we extend the exposure by 13 exposure levels.

The exposure time is calculated in the following order:

Basic exposure 
+ /- correction factor for the desired image effect 
+ correction factor for atmospheric conditions
+ correction factor for the Schwarzschild effect

Light equals brightness

Regardless of the option you select, several external factors influence the moon’s brightness and exposure, necessitating accurate assessment to achieve the desired technical outcome. Let’s take a closer look at the individual items.

The moon phase exerts the greatest influence on its brightness because it decides how much of its surface is available for the reflection of the sunlight. Between the 1st quarter and full moon alone, the brightness of the moonlight varies by 3 1/2 f/stops. A moon phase angle of 0° indicates that the moon is directly opposite the earth from the sun, or in full moon position. The table displays the basic exposure times for f/4 and ISO 100, corresponding to the number of nights before and after the full moon.

1 night	2 min 50 sec
2 nights	4 min
3 nights	5 min 39 sec
4 nights	8 min
5 nights	11 min 19 sec
6 nights	16 min
7 nights	22 min 38 sec
8 nights	45 min 15 sec
9 nights	91 min

A stone viaduct with multiple arches spans a grassy field. The sky is dusky, and a trail of light on top of the viaduct suggests a passing train. Trees are visible in the background. The whole scene is lit only by the light of the full moon. — Figure 40: Landscape in the full moonlight 1 One night after the full moon, 9 min, f/4, 24 mm. The white stripe on the viaduct comes from the light of an ICE train passing through.

A brick church with large, arched stained glass windows and a cross on its gabled roof is illuminated at night. The sky is clear with faint star trails visible, indicating long exposure photography. The whole scene is lit only by the light of the full moon. — Figure 41: Landscape in the full moonlight 2. Three nights after full moon, 4 min, f/4, 24 mm

Since the intensity of light decreases in percentage with the square of the distance, the positions of the earth, sun and moon in space naturally also play a role. The distance between the earth and the sun, for example, varies between 147.1 million kilometers at the point closest to the sun (perihelion) and 152.1 million kilometers at the point farthest from the sun (aphelion). Add to this the varying distance between the earth and the moon, ranging from 356,400 to 407,000 kilometers. In the sum of both effects, the brightness of the moon varies by 6.9 % related to the distance to the sun and by 30 % related to the distance to the earth, or 1/3 exposure step. – Enough to noticeably change your image.

The third important factor is atmospheric extinction. This refers to the reduction of light in the atmosphere as a function of its state. In this context, it is important to distinguish between two key quantities. 1. The extinction value represents the extent of light reduction per unit of air. The key words here are molecular absorption, molecular scattering (Rayleigh) and scattering by particles (Mie). The extinction value is comparatively small in the case of dry, pure air and comparatively large when the atmosphere is humid, dusty and hazy. 2. the amount of air (air mass) in the light path. It varies from 1 air mass when the moon or sun is directly over our head to 38 air masses for a position near the horizon. The combined effect of both quantities, a large air mass potentiates any large extinction value, exerts an enormous influence on brightness. Even with a relatively clean atmosphere, the reduction in air mass between the horizon and the zenith alone provides an increase in brightness of 6 to 8 f/stops. For this reason, the months of January to March and October to December, when the full moon describes the highest orbits of the year and culminates with positions of 50° to 60° near the zenith, are the preferred times for photography at full moon. The table gives information about the approximate lengthening factors for the combined atmospheric effects.