Since I’ve seen a few posts requesting how to do those star trails pictures I thought I would do a quick post on it.
So first thing Star Trail pictures are really easy. What you need is a camera that supports a bulb mode (usually marked B). Most point and shoots do not support a bulb mode. The second thing you need is a tripod. Finally you need a locking cable release or intervalometer. Optional items would be a captains chair or some other folding chair a jacket and a mocha.
Now for the hard part. Set up your tripod point the camera so there is some sky showing. Lock the cable release on, have a seat in the chair and enjoy your mocha. After sometime (the longer you wait the long the star trail) close the shutter. That’s basically it. As long as there is no ambient light except stars you can leave the shutter on as long as you want you have no light to blow out anything.
The few gotchas are the aperture, the ISO rating and the direction.
First the aperture set the aperture to the optimum of the lens. The optimum is usually f8. You set the aperture to the optimum because you are going to set the focus to infinity. With the focus on infinity you need the aperture set to best setting for the lens.
Next the ISO setting. There are trade offs with the ISO. The lower the ISO the less stars will be distinguished but the less any ambient light will be captured. During my playing the best ISO setting I’ve found is 400. Usually the reason to lower the ISO as low as you can is to reduce noise but since the shutter will be open for minutes or hours the noise will come from the long exposure more than the ISO setting.
Finally direction. The “Great Circle” of stars is centered around Polaris (the North Star) in the northern hemisphere. The North Star can be located by first finding the little dipper. The two stars furthest from the handle, the outside wall of the dipper points (from the bottom up) to the North Star.
The other things to look out for are to make sure it’s as dark as possible. Even the light from a street light can become too bright when the shutter is open for 30mins. If the moon is up you will probably have to limit your exposure time to 20mins.
So if you are stuck taking a shot with ambient light you can figure out how long you can leave the shutter open fairly easily. If you’re like me you don’t have a light meter that will work in the dark. What I do is set the camera to aperture mode dial the ISO up to the max (3200 in my camera) and then adjust the aperture until the shutter adjusts down to 30sec. From there I can drop the ISO down to 400, each full ISO setting down doubles the time (1600, 800, 400). So down to 400 would increase the shutter time from 30secs to 4mins. Since the aperture is usually at 2.8 to bring it up to f8 that’s three full stops (f4, f5.6, f8). So from 2.8 to 8 the time moves from 4mins to 32mins (8mins, 16mins, 32mins). That’s how I figure out the maximum time I can leave the shutter open without blowing things out. The thing to keep in mind is with that little light you can usually double the time because depending on the response of the sensor or film it takes more light than normal to start registering. The other thing to remember one stop beyond 32mins would be 1hr 4mins (each stop is a doubling of time) and one stop again would be 2hrs 8mins, etc.