Geometry alone does not make a good picture

You are here: Nature Science Photography – Natural light – The moon as a motif in the picture

So much for geometry. In order to effectively include the moon in an image, a little theoretical preparation is required. Remember, the moon, when visible to us, always receives the same illumination as a landscape under midday light. Therefore, to match the foreground in terms of exposure, the moon must either be in the sky, which is not yet completely dark, or be inserted through double exposure. More about this later.

First, choose a suitable moon phase for the picture composition and determine the corresponding date. The second step involves determining the moonrise and sunset times to estimate the ambient brightness and contrast. As orientation for this may serve that the complete darkness occurs in winter about 30 minutes and in summer good 60 minutes after sunset. If you take a photo between sunrise and sunset, expose for the foreground without the moon.

However, the daytime sky seldom appears appropriate. Much more exciting is the twilight, whose sky colors give a hint of the evening darkness and still illuminate the foreground recognizably. These are the ideal conditions to bring anything, especially the monthly full moon, into the picture. Since the moon only appears in the sky at approximately the same time as sunset during a full moon, most months only have sufficient brightness between the day before and the day after the full moon to align it with the foreground in terms of exposure. Usually, natural landscapes with orange-red or blue skies benefit from a moonrise before sunset, while cityscapes benefit from a moonrise after sunset, as it allows for the illumination of house lights. In addition, the day after the full moon in the calendar often provides good conditions for making the foreground appear as a silhouette. But pay attention to exposure values, because just one day’s deviation from the nominal full moon means a reduction in its brightness by nearly a whole f/stop. During the summer months from May to August, the full moon describes its lowest orbit of the year and is at photogenic heights of a good 25° above the horizon all night long. – Time enough to look for motives and variations, because during the rest of the year the full moon stays only on a part of its orbit in this pleasing area.

Simple rule for determining exposure: If the brightness of the sky or foreground is equal to or greater than that of the moon, the exposure is also based on this dominant value and the moon can be safely disregarded. Thus, until a few minutes after sunset, the surrounding landscape is still decisive for the exposure. A polarizing filter darkens the sky a bit and makes the bright disk stand out even more. However, the darker the sky becomes, the more attention must be paid to the contrast to the foreground, which increases to the same degree, and the more the sky must be included in the exposure metering and this must be extended. During night hours, the moon becomes the sole determining factor in the black sky, allowing only a silhouette to depict the foreground. But metering is then difficult because too large a metering angle will undervalue the moon with its meager ½° diameter and overvalue the dark sky, inevitably leading to overexposure. A spot metering at a correspondingly long focal length, about 400 mm, provides more precise results. By the way, the size of the moon is always the same regardless of its position above the horizon, no matter what our visual perception would have us believe. The section about „How our visual system constructs object sizes“ explains why we sometimes get this wrong. In terms of shooting, it is crucial to consider the focal length as a critical quantity, as it not only determines the image size but also the maximum possible exposure time required to ensure a sharp image. The following formula determines the size of the moon on film:

Moon diameter in mm = Focal length in mm : 109

So that you don’t have to do the math yourself, here are a few examples:

Focal length	Moon diameter	% of the 35 mm image width in landscpae format	% of the 35 mm image width in portrait format
100 mm	0.92	2.5	3.8
200 mm	1.83	5.1	7.6
300 mm	2.75	7.6	11.5
400 mm	3.67	10.2	15.3
500 mm	4.59	12.7	19.1
600 mm	5.50	15.3	23.0
700 mm	6.42	17.8	26.7
800 mm	7.34	20.4	30.6
900 mm	8.26	23.0	34.4
1000 mm	9.17	25.5	28.2

If your chosen main subject is a distant object that requires a telephoto focal length, then the process should be straightforward. Determine what focal length will render the moon at a size that suits your imagination and position yourself at an appropriate distance from the main subject (perhaps a row of trees or a row of houses). Simply wait until the moon appears halfway behind or next to the object on its path, then release the shutter. When setting the exposure, however, be sure to use an aperture that produces enough depth of field for the foreground and background and remember the upper limits on exposure time discussed below. If you didn’t get your image in the can in the evening, there is often a second chance in the morning, a good hour before sunrise, to combine landscape and moon. You should align the exposure with the sky in both cases.

However, often, a telephoto lens isn’t ideal for achieving one’s desired image, necessitating the use of alternative techniques, such as utilizing a double exposure to capture an evening city view with an appropriately large moon. With a double exposure, you can change the focal length to compensate for the insufficient magnification of the wide-angle lens needed to capture the cityscape. As you like, you can shoot the foreground in the first shot of the double exposure, then crop the camera to a long focal length and insert the moon. Try to crop the image so it appears in a cloudless sky and matches the foreground light. A grid focussing screen is very helpful in such positioning work. It is only important that the moon for the second shot be in an otherwise completely dark sky so as not to interfere with the first image. – After all, a black sky that emits no light is irrelevant in terms of exposure.

For the full moon standing high in the sky, illuminated similar to a landscape in midday light, 1/125 sec at f/11 and ASA 100 is the guiding exposure. To be on the safe side, spot metering should be used and the exposure varied by +/- ½ f/stops because the brightness of the moon changes depending on the time of year (it shines more strongly in winter, when the earth-moon system is several million kilometers closer to the sun and the moon is reached by 7% more light) and the height above the horizon. This is because the closer the moon is to the horizon, the greater the absorption of the light reflected from it in the earth’s atmosphere. The moon’s brightness decreases with increasing altitude of the observer above sea level and generally depends on the amount of water vapor or suspended particles in the atmosphere. However, the moon’s phases also alter the amount of light it reflects, as the table below illustrates.

Earth glow	Narrow crescent moon	Wide crescecent moon	Half moon	Full moon
40 – 80 sec	1/8 sec	1/15 sec	1/30 sec	1/125 sec
Valid for ASA 100 and f/11

But as the focal length used increases, the exposure time that allows a sharp image of the moon decreases. This is because the longer the focal length and the greater the associated magnification of the optics, the larger the part of the image that the moon traverses at the same time. – Ultimately, the moon’s movement speed, which is one moon diameter per 150 seconds, remains constant. As a general guideline, the exposure time should not exceed five to ten times the ratio of the normal focal length to the used focal length, with 50 mm being considered the normal focal length in the 35 mm range. Watch out: If you are using a digital camera, you must use the real focal length, which is determined by the ratio to the sensor size. By the way, you also have to consider the exposure time if you plan to enlarge a section of the resulting image. For a section of 1/x (e.g., ½) of the original size, you must consequently shorten the exposure time by x (e.g., a factor of 2). The following table compares focal lengths and longest possible exposure times.

Focal length		Exposure times
 100 mm		     2,5 - 5 sec
 200 mm		     1,25 - 2,5 sec
 300 mm		     0,8 – 1,6 sec
 400 mm		     0,6 – 1,25 sec
 500 mm		     0,5 – 1,0 sec
 600 mm		     0,4 – 0,8  sec
 700 mm		     0,3 – 0,7 sec
 800 mm		     0,3 – 0,6 sec
 900 mm		     0,3 – 0,5 sec
1000 mm		     0,25 – 0,5 sec

The particularly red moon of the summer months, by the way, comes from the atmosphere charged with a especially large amount of haze, in which the moon describes the lowest orbit of the entire year and spends a large part of its orbit in the thick layer of air near the horizon, which strongly filters the light. In contrast, at the time of the winter solstice, the full moons shine from their highest points in the sky through particularly dry and clean air and thus appear a little whiter and brighter than usual. But you will never catch him with a “different face”, because he seems to lack a rotation of his own. This is because its rotation is synchronous with its revolution around the earth. So, it takes the same amount of time for a full revolution around itself as it does for a complete revolution around the earth. Another little thing: Try not to concentrate only on the full moon, because as the sunlight falls exactly vertically on its surface during this phase, there are hardly any shadows, just like on Earth at midday. If the moon is in its first or last quarter, you will see a more vivid image of its face, with shadows highlighting the edges of the mountains and craters. The longer the focal length used, the more obvious this difference becomes.

Next Equipment and shooting technique

Main Natural light

Previous Phases of an orbit

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