What is this stuff?
The moonlight brightness predictions provided in the accompanying tables are the result of computer calculations made by LunarLight Photography's NightLandscape program. This program calculates the precise position of the moon and sun in the sky at a given time. The program then calculates the lunar phase angle, and uses established physical principles to calculate the illumination provided by moonlight at a given location on the earth's surface. The effect of atmospheric dimming of the incident moonlight is accounted for. The resulting predictions, made for a ten-day period bracketing the full moon at the time indicated, are given in one-hour increments. The predictions in each table are specific to the location indicated. and are for conditions of very clear air (greater than 50 mile visibility) at an elevation of 33 meters above sea level. Adjustments will need to be made to the predicted brightness levels to account for any appreciable haze in the atmosphere, however under the assumed conditions, tests have shown these predictions are accurate to within 1/4 stop of light. Note that the brightnesses tables account only for the presence of moonlight and starlight, no attempt has been made to correct for the presence of daylight.
The first and second column in each table provide the date and time . This is local date and time for the indicated time zone. The third column provides the lunar phase angle , which is the angle between the earth and sun as seen from the moon. The moon is full for values near zero, and new for values near 180 degrees. A value less than 1.5 degrees indicates a penumbral (partial) lunar eclipse, with an umbral eclipse (deep earth shadow) reached at about 0.9 degrees, and a full total eclipse (moon entirely within the deep earth shadow) at a phase angle of less then about 0.4. Phase angles less than 7 degrees bring lunar disk brightening from "the opposition effect", which can increase lunar brightness by 35% or more.
The fourth and fifth columns list the lunar altitude and azimuth, or the position of the moon in the sky (accurate only in the area for which the predictions were made). Altitude is in angular degrees above the horizon, with a negative value indicating the moon is below the horizon and not visible. Azimuth is measured in degrees from north, where east is 90 degrees, south is 180 degrees, and west is 270 degrees. This data gives you an idea of which direction the light will come from at any given time. One thing to note: if the moon is too high in the sky (more than about 40 degrees above the horizon), you will loose the sense of depth and shape provided by shadows on the landscape. However, if you want flat, even fill lighting as for the background landscape when doing flash fill or light painting on a foreground subject, you will probably want the moon to be high in the sky. The tables thus show you when such conditions exist.
The sixth column , labeled "18% Gray Surface Brightness (LV)", presents the surface brightness an 18% gray card would have if it were placed directly facing the moon. That is to say, this is what a light meter calibrated in Light Values (LV), or Exposure Value (EV) with film speed ISO 100, would read when pointed at a gray card under these conditions. You could also consider this to be the incident light level reading as if you were using an incident light meter, such as a Gossen Luna Pro, though capable of reading low light levels below LV=0. (If you desire photometric units rather than photographic units, you can convert the values using the methods discussed elsewhere on this site - that page also discusses the terms Light Value (LV) and Exposure Value (EV), if you are not familiar with the terms and their use.)
The seventh column labeled "Zenith Moon Brightness (LV)", gives the brightness of moonlight assuming the moon were directly overhead at the indicated time. This is useful if you are located somewhere other than the specific location for which the predictions were made. To use this, you have to adjust the brightness down to account for attenuation by the moonlight's passage through the earth's atmosphere, which absorbs a good part of the moonlight, especially as the moon approaches the horizon. The adjustment required can be anywhere from a half-stop to six or eight stops, depending on how close the moon is to the horizon, and how much haze is in the air. Atmospheric attenuation data can be found on the Atmospheric Attenuation page on this web site. You can also use the data from the Zenith Moon Brightness column as a maximum value, and then bracket exposures toward increasing exposure time (opening the lens as you go). Little bracketing is needed if the moon is more than 35 degrees above the horizon in clear air, since the error in that case is probably one-third to two-third stops. Be forewarned, though, that it only takes a little haze to dim the moonlight by several stops if the moon is near the horizon.
The eighth and final column provides time of sunset or sunrise , and the beginning and end of twilight each day, again accurate only for the location indicated in the table. Nautical twilight ends in the evening when a trained observer can no longer distinguish the horizon on a moonless night. Astronomical twilight ends when the sun is 18 degrees below the horizon, beyond which there is no longer any sunlight illuminating the upper atmosphere above you.
Exposures made using these brightness predictions should give you equivalent results to what you would get under bright sunlight conditions using the Sunny 16 Rule (shutter at 1/film speed at f-stop f/16). Calculating the brightness and camera settings required to give a sunlight-like rendering of the scene is merely a convention that gives a convenient and reliable point-of-departure. I find that a true daylight-like rendering of a moonlight scene does not give "the feel of ambient night", and usually give an underexposure of one-half to two stops to add the feeling of moonlight to the image. This can be done with bracketing the exposure until you learn what works best for the type of images you want to create. If you have never done this before, I suggest bracketing in 1/2 stop intervals toward less exposure by progressively reducing the lens aperture (selecting higher f-numbers).
If you have questions about these predictions, feel free to send me an email. If you should make use of these predictions, I would appreciate the return favor of letting me know how well they worked for you. I am always interested in feedback on these methods. Contact LunarLight Photography
Torrance, California, USA.
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