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1.2 Exposure Times
One reason why long exposures are hard: there are many things that can ruin your picture... like a late-night car driving where it shouldn't be.(Logarska Dolina, Slovenia) Visit author Dan Heller
Because of the different sources of light and the great effects such subtle changes can have on a very long exposure, you can't really "calculate exposure times" here. The camera's light meter is irrelevant, especially for exposures that are going to be well into the minutes, if not hours. This requires setting the shooting mode to "manual" or "bulb" and using a cable release. Some advanced cable releases have timers built into them, whereas manual versions require you to push the cable yourself. If you think that's not so bad, keep in mind that you're going to have to be around (and awake) in several hours when you want to release that button. Every camera manufacturer has different cable releases to choose from, but after having done this for a while, I can speak confidently that having one with a timer is well-worth the money.
At this juncture, you must now experiment and rely on trial and error to learn the ropes. Just compose what you might think would work, release the shutter, and go get coffee (or go to sleep).
It's tempting to want to use a higher ISO setting to brighten photos due to the lower light conditions, but the side-effect of higher ISO is higher noise. I use ISO 100 to keep the "digital noise" down, which is more pronounced in darker areas of an image than lighter ones. Your mileage may vary, as different camera manufacturers deal with noise differently. Even though some cameras may perform better than others, all cameras will produce much better images at lower ISOs than higher ones.
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1.3 Powering the Camera
3 Hour Exposure Facing East (North Star is upper-left, southern point is below horizon on bottom-right.)(Morocco) Visit author Dan Heller
Almost any film-based SLR will expose long enough because battery consumption is low. But digital cameras have a tougher time because digital sensors eat power. So much so, that most cameras won't do much more than 30-40 minutes on a single charge, regardless of the battery type or camera model. It therefore becomes part of the evening's landscape choice to find a location where you can use your power cable. When staying in hotels, I often choose rooms that face towards the darkest part of the sky and that have the least amount of ambient light (usually the decorative hotel lights). This way, I plug into the wall, and place the camera (on tripod) either on the balcony, or shoot through the window.
When camping, I use an AC power adaptor plugged into the car or a nearby power supply. (A house, cabin, etc.) Obviously, this may not always be possible. In this case, true night-photo nuts go out and buy generators or battery packs that can keep a good exposure going for quite some time.
To get more images in less time—and to make the most of battery power if you're not using a power adaptor—opt for simpler night pictures of shorter star trails, or just star fields. As stationary objects, it's still a pretty amazing site.
1.4 Fogging Lenses
During a full moon, exposures can only last about 8 minutes before over-exposing.(Patagonia) Visit author Dan Heller
A common problem with night photography is dew fogging the glass because ambient air is warmer than the lens itself. What causes dew isn't the temperature, it's the amount of humidity in the air (though cold fronts tend to have drier air). Metal lenses will always be colder than the ambient temperature, but even lenses with plastic barrels can suffer from the problem simply because of the glass components inside the lens. The solution is keep the lens warmer than the air. How you do that is the challenge. The most fool-proof way is to get a bulky battery pack and wire it up to your lens. Places that sell astronomy equipment make these for larger telescopes, but they are too large for normal camera equipment—it'd be like shooting a mouse with an elephant gun. The solution "works" and won't damage anything, but it's an overkill.
A more common and inexpensive way to prevent humidity from building up on the glass is to use one of those hand-warmers that you put inside gloves or boots for winter activities, like skiing. One particular brand is at www.warmers.com. They cost about $1-2 each, and while that site sells in bulk, you can buy them one or two at a time at most outdoor outfitters like www.rei.com. Wrap one or two around your lens with a rubber band, and it'll last anywhere from 4-8 hours. Of course, if it's really humid, like at the top of mountains in Peru, where clouds hover around your tent, it's darn near impossible to avoid the dew.
