One of the things I’ve found so difficult about night sky photography is keeping everything sharp. Because of the earth’s rotation, exposures that are too long make the stars appear to be moving. They cause streaks or, with exposures that are marginally too long, little fuzzy football like spots. It’s certainly not what we see when viewing the sky with the naked eye.
The Best Conditions For Astrophotography
Before we talk about what equipment is necessary and what numbers to apply to aperture and shutter speed, we need to be starting with a dark sky. That is, a sky with no moon. It’s also nice to have a minimum of clouds. That’s why astrophotography can be so much more successful in the winter months. The air is colder and the humidity is usually lower. That’s a factor in giving us the crystal clear sky we want. Sometimes, if you want your foreground lit, a half moon can be a good thing. Your stars will be sharper with no moon, however.
What You Need For Astrophotography
- You will need a sturdy tripod. Exposures will frequently be from 15-30 seconds.
- A camera that is able to shoot in “manual mode” and has exceptional low light capability will allow for the best results.
- A fast wide angle lens with a maximum aperture of 2.8 or faster is best.
- A cable release is necessary because you don’t want to be touching the camera during these long exposures.
Getting The Sharpest Photos Of Stars
Unless you want star trails or the little fuzzy footballs we discussed earlier, there’s a little math necessary to get the sharpest results. The formula we would use is commonly known as the “500 Rule”. You divide 500 by the 35mm equivalent focal length of your lens. That is, if you are using a full 35mm sensor camera and a 24mm lens (or 16mm on a crop sensor camera), you would divide 500 by 24. The answer is 20.83. For our purposes, we’ll round down to 20. This is the maximum number of seconds we can use for an exposure without getting movement from the stars.
Since the exposures aren’t going to be very long it’s best to shoot with your lens wide open. Your exposure time should be the longest possible as determined by the “500 Rule”. The starting ISO should be 6,400, if possible, and reduced incrementally with each exposure. Keep your lens aperture wide open.
Don’t forget to dress warm! The best results often happen on the coldest nights.