A few weeks ago a friend and colleague at Royal Botanical Gardens, Bill Kilburn, asked me a deceptively simple question. After looking at some of my photographs of the moon, he asked me what it really looked like. I had mentioned that the surface of the moon was actually very dark. Our natural satellite looks very bright to us at night, and it’s easy to over-expose a photo of the moon, especially if you set your camera to an automatic exposure setting.
Bill’s question challenged me to try to take a photograph of the moon that has an appearance more in keeping with our understanding of the moon’s natural reflectivity, or albedo. Before starting to take photos, I needed to think about the matter of light and exposure.
Normally, cameras are calibrated with one of several roughly equivalent systems of standardization regarding their sensitivity to light. The actual standards are established by the International Standards Organization, or ISO, and there are several ways that cameras are calibrated according to the standard. The standards were first established for photographic film, but today the standard (technically, it’s called ISO 12232:2006) applies to the camera itself: its complex systems of light-recording sensors and tiny computers that process the images they record.
The system is designed to allow photographers to match up the sensitivity (the “ISO number”), the shutter speed of the camera, and the aperture or “f-stop” to achieve a properly exposed image. For any given ISO number (higher numbers mean more sensitive to light), proper exposure will be a function of both the shutter speed (faster speeds mean less light gets in) and f-stop (higher numbers mean less light gets in). While that sounds complicated, there is a simple old photographer’s rule of thumb that applies very well: the “Sunny-16 rule.”
Sunny-16 states that on a sunny day, for any ISO number, the image of an object in sunlight will be correctly exposed if the f-stop is set to f/16 and the shutter speed is set to the reciprocal of the ISO number. Still sounds complicated? Here are a couple of examples:
If ISO=400 and the lens is set to f/16, then the correct shutter speed is 1/400.
If ISO=100 and the lens is set to f/16, then the correct shutter speed is 1/100.
Makes more sense now? Working from this idea, that a sunny scene should be properly exposed under the Sunny-16 rule, I reasoned that if I set my camera to ISO=400, shutter speed = 1/400 seconds, and aperture to f/16, then the image of the moon (which is fully exposed to the sun) should be recorded by the camera as though it was sitting in front of me on a sunny day. There is a big assumption in this, but really only one: that the full thickness of the earth’s atmosphere is actually as transparent to visible light as is a few dozen or hundreds of feet of air would be for a terrestrial scene on a sunny day.
So, armed with these three settings (IS0 400, 1/400 of a second and f/16) I set up my camera and telephoto lens and took some photos of the moon. Here is a frame recorded at that setting without any changes “post camera” to brightness or contrast:
The technical details of the moon’s brightness are available from NASA (http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html). There are a couple of ways of measuring the albedo, or reflectivity, of a surface, but either way, the moon reflects to us between 11% and 12% of the light that strikes it. This is similar to the albedo of slightly weathered asphalt. There are some areas on the moon that are darker, and some lighter, than this. The relatively new crater Aristarchus, for example, is considered to be the brightest feature on the moon’s surface.
It must also be noted that your own computer monitor or tablet may not display the image “exactly right.” Unless it’s been calibrated for brightness, the gray scales represented in the image may look a little different to you.
The same image, adjusted to bring out detail and make the moon look a little brighter, looks like this:
This is a bit more like we’re used to seeing it. Part of the difference lies in contrast at night. Once our eyes are dark-adjusted, we’re much more sensitive to light than we are in the day, and the moon – coal-dark as it is – appears much brighter than we think it does in the daytime.
© 2012, David Allan Galbraith