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 Star Trails Tutorial | |
Shooting the Twilight:
For twilight images I like to take an exposure during the last glow of pink in the sky, and then come back an hour later when the stars are out in full force. Then I will take another exposure. Then after returning home I’ll put the two images in a stack in photoshop, and use a lighten blend mode to bring out the stars. There’s a lot more details on this later on in the article.
”Twilight Blues” ~ Mt. Rainier National Park, WA
The Tech: Canon 5D, 17-40mm
Exposure: 4 exposure blend:
1. (for the foreground) iso 400, f/11, 2 minutes
2. (for the mountain) iso 400, f/11, 2 minutes, shifted focus from the foreground exposure
3. (for the sky) iso 400, f/11, 25 sec
4. (for the stars) iso 1600, f/4, 15 sec, shot 1 hour after the previous three
Processing: first 3 layers blended normally, last layer in \'lighten mode\'. Due to the poor quality of the stars, I went in and manually retouched the stars with a paintbrush to make them look pretty... now with a 16-35mm f/2.8 and 5DII hopefully I\'ll be able to get some better quality stars to start with 
Taking the Picture – Star Trail Techniques:
With film you could simply flip up the mirror, go to bed, and come back a few hours later and you’d have yourself a photo (after making sure of the sunrise/sunset, moonrise/moonset, and picking a clear moonless night of course). With digital, there are some more hoops to jump through. So why use digital at all? With film there were problems with reciprocity, meaning the films sensitivity to various kinds of light changes with longer exposures, and it can be hard to predict unless you know your film well. But for really long exposures, it might make your life a lot easier. I like the control of digital, and while I have considered trying some film for these applications, I must admit that having grown up with digital, I’m no film user. Fortunately those hoops we have to jump through with digital are not insurmountable.
Noise is the number one enemy you will face. Once you start to push your exposure beyond a few minutes, the noise starts to increase significantly – long exposure noise. Also, different parts of the camera will heat up and affect how the sensor works. Most (if not all) cameras will exhibit some kind of magenta colored bloom in a consistent spot (different for different cameras). There are a number of solutions to consider.
As the temperature drops, the signal to noise ratio gets better, and your noise will decrease. So if it’s -15 degrees F outside, you might very well get away with a 2-hour straight exposure without much noise! But often times it’s not that cold (thankfully!). So for those nights you have two other options: long exposure noise reduction (available in most cameras, or it can be done manually using dark frames), or taking multiple exposures and stacking them. I have not found the long exposure noise reduction to be very effective at all, so I use multi-exposure stacking.
Shooting Multiple Exposures:
The idea here is to sequentially shoot lots of shorter exposures; the length of each individual exposure will depend on the tradeoff between noise and processing time/power required to assemble your final image. I find 5 minutes to be a nice number (so for a 2-hour exposure you’ll need 24 exposures). Note that for most cameras anything beyond 30 seconds needs to be done through the ‘bulb’ command. If you plan on trying this, I recommend getting a remote for you camera that lets you program such commands so you don’t have to manually sit there and hit the shutter (for Canon that would be the TC-80N3). For the TC-80N3 you’ll need to set the time between exposures to 1 second (it doesn’t shoot multiple exposures if you leave it at zero).
Note: turn off long exposure noise reduction, and mirror lock up. You want as seamless a series of images as you can get for star trails.
Power Constraints:
I should note here that batteries don’t last forever. Your battery might only last you for an hour, or two at the most. Using a battery grip or external power supply like from http://www.digitalcamerabattery.com/, might be a good idea. Otherwise you have to be really quick with replacing a dead one! Cold temperatures will reduce their efficiency as well.
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