As a rule made to be broken, you don’t put your subject between you and the light source. I don’t think I was ever successful with a backlit subject using film. Digital automatic settings make it easy. Here are two of my most dramatic photos:

But the one below didn’t work, even though the conditions are nearly identical with the first one above:

One reason to avoid backlighting is to avoid flares and other artifacts, but even those can be artistic in limited amounts.

ISO is short for International Organization for Standardization (why not IOS?). In the context of photography, ISO refers to light sensitivity of media, film and digital. Media that is more sensitive to light needs less light to expose an image and has a higher ISO number. Media with an ISO of 100 is less sensitive – requires more light – than ISO 400, which is 4 times more sensitive (requires only 25% as much light). More or less light can also be a function of more or less time: as ISO increases, less time is needed for exposure. Therefore, low ISO is called slower and high ISO faster.

Keep in mind that ISO is one of three intertwined variables in exposure. The other two are aperture (lens opening) and shutter speed. Aperture determines how much light enters the lens at once – wide for a lot, narrow for a little. Shutter speed determines how long that amount of light enters – slow for more, fast for less. A higher ISO makes it possible to use smaller apertures (increased depth of field) or faster shutter speeds (frozen movement). Available light obviously affects choices here, but so does the use of a telephoto or zoom lens, which requires more light than a wide angle and usually has more limited aperture. Handheld shots in low light may require higher/faster ISO, whereas using a tripod may allow for lower/slower ISO.

Ignoring depth of field and blurring – hard to ignore – one can produce nearly identical exposures through myriad combinations of ISO, aperture, and shutter speed.

But higher ISO has a dark side: noise (electrical interference). Consider both pairs of photos:

 
above: ISO 125. below: ISO 1000.

 
left: ISO 250. right: ISO 1250.

In both pairs, the higher ISO wasn’t necessary. These examples were chosen to highlight noise, an increasing problem as ISO increases. However, in low light without a flash you may have no choice but to increase ISO, but no more than necessary.

ISO relates to Shooting the Moon. Most automatic settings choose a higher ISO when the scene is mostly black. But then the area of the moon is overexposed and the whole image is grainy. A low ISO (100, 125) that would not work on a moonless night or in a dim room works with the bright moon and provides an even blacker, smoother surrounding space. ISO 1000 ruins this photo.

High ISO film appears grainy because larger grains of of silver halide are used – the higher the ISO, the larger the grains. There are no such grains in digital photography, yet noise has a similar effect. One way cameras adjust ISO is by amplifying the signal in the image sensor, which also amplifies the inevitable noise, much like cranking the stereo to 11 increases hissing.

The moon rises nearly full tonight. These photos are from other months:

Here is an excerpt from my book, Digital Cameras & Photography for Dummies (2nd edition):

Shooting the moon (Chapter 6)

The moon is a particularly captivating subject, waxing and waning as it transits the sky. The moon isn’t necessarily any more difficult to photograph than other night-time subjects. There is, however, one important thing to remember about the moon: It’s very bright when it’s full. Another obvious pearl of wisdom, yet, when you shoot at night, you prepare for underexposure. When you shoot the moon, you get overexposure unless you choose your setting wisely. (Or have a terrific Full Moon mode.)

Although the moon is fascinating by itself, including the horizon, buildings, or other objects provides context and scale. The moon looks largest at the horizon.

Consider these other suggestions:

• Use a tripod to eliminate all motion. Shooting handheld can work, especially if you increase ISO to allow a faster shutter speed. Using a tripod also makes it easier to shoot multiple shots.
• Turn off Image Stabilization (IS). On a tripod, IS may be unnecessary, even interfering, and it may slow down the process of taking a picture. If your owner’s manual doesn’t say whether to use IS with a tripod, experiment with IS on and off and compare the results. The IS symbol is usually a waving hand on a button or menu.
• Any movement of the camera will be a problem. Use your short self-timer so you can press and release the camera.
• Focus may be a problem on such a distant object at night. Set Manual Focus to infinity. (Technically, the moon is only 250,000 miles away, not infinitely far.) Focus control may be through a button or menu item and may not be available in Auto or scene modes.
• Get as close as you can. Zoom all the way in unless some other feature fits the scene. (A long zoom exacerbates problems with camera movement. Another vote for the tripod.)
• Because the moon is so bright, you don’t need a particularly high ISO (avoiding the noise that is a common problem in low light), unless you’re not using a tripod. Use between 100 and 200 ISO on a tripod.
• Try a shutter speed between 1/100 and 1/200 and an aperture above f/5.2. The image on-screen should be a little dark.

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Your camera may have an option to bracket shots. This means that when you take one photo, the camera will take several, each with exposure settings lighter or darker than the settings you choose. Bracketing increases the odds that you’ll get a photo under difficult exposure conditions.

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If you have to use a Scene setting, don’t use Night Sky because that will probably overexpose the moon. Try Snow or Beach scene modes.

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The easiest way to avoid overexposing a moon shot is don’t shoot at night. Seriously, the moon rises nearly full in late afternoon sunlight a few days before full and sets nearly full in the early morning light for a few days after full. Don’t pin all your hopes on just a few minutes every 28.25 days. Check out www.sunrisesunset.com for sun and moon rising and setting times. If you own a GPS unit, it may list rising and setting times, as well.

PS: I have a gripe about when the moon is full. Seriously. A calendar will tell you that the full moon in May, 2011, occurs on 5/17. Yes, at precisely 5:09am MST. That’s about 9 hours after sunset 5/16 and 15 hours before sunset 5/17. Which sunset will the moon be closer to full? On the other hand, tonight’s nearly full moon rises 20 minutes before sunset and tomorrow’s 45 minutes after, making for completely different exposures. Get out there. Don’t forget to howl.