How To Do Star Trails Via Image Stacking
Since I get asked this, here is a quick page on how. The old film way consisted of simply using a tripod and a cable release for you camera and holding the shutter open for however long you wanted. Problem with this is you get light build up. And with digital now, it gets damn noisy fast that way.
So the best way to do it now, with digital, is image stacking. Here's a run down on what you need and to do.
2. Cable Release for your camera.
3. Put your camera, hopefully a DSLR, into consecutive shooting mode. Somewhere on there it should have single shot, consecutive shots, and timer shooting. Consecutive is where you can hold the shutter down and it will fire as fast as it can, or in this case whatever shutter speed you pick, repeatedly.
4. Put your lens on manual focus and set it to infinity somehow. Some lenses have markers. Many DSLR's now have Live View which works great to set focus. Turn live view on and zoom in 10x or whatever on the lcd and focus. It's not wise to try using just a light source, like a star or street light. Best to have something near a light with detail. If you make the light the smallest point of light you can, it's often wrong. So I avoid using the actual light source, but something near it. The one exception is if you have pretty dim stars around brighter ones, that as you move the focus and watch live view, those can barely be seen when you get it set just right. Then they quickly go back out of focus and vanish. Those points of light work for precise infinity focus setting. I guess maybe one can stop down for the brighter ones and get it to work the same way. I do that when the moon is out, stop down till I can see the detail and use that.
5. I put the camera in JPG mode for star trails. I use RAW for everything else. Using JPG will save time later on this as you take tons of shots for the stack.
6. Go into custom functions and turn on high iso noise reduction. Standard often works nice enough. (this does nothing to a RAW file obviously) This is the other reason I use JPG mode for star trails. The in-cam noise reduction is awfully damn good.
7. Set the white balance to something other than auto. Since you'll have many photos over time and have to stack them later, you won't want the color balance changing around. Using tungsten works great 99% of the time.
8. Put the camera in manual mode.
9. Set aperture to its widest possible setting. F3.5 is as wide as my 10-22 EF-s will go. F2.8 would be an expensive wide angle lens. Smaller the number, the faster the lens is, letting in more light. I see people mentioning stopping down and well just don't do that. Go wide as you can.
10. Set the ISO or camera sensitivity. Like a wide open lens lets more light in, higher ISO's make the camera more sensitive to that light. The higher the ISO, the more stars you'll pull out. I wouldn't bother with less than 400 ISO. I have some examples below.
11. Set the shutter to something that works for the rest of the scene's light. A longer shutter really isn't going to pull out more stars, because the stars are moving. I use anywhere from 10-30 second shutters. Say we're using 30 seconds, set it to that.
12. Put the cable release in and lock it down. After 30 seconds is up, since you are in consecutive shooting mode, it will fire off another. On and on till you stop it or the battery dies.
13. I'd go at least 15-20 minutes to get some trail. If you aren't going to go real long, there's only so much use in going longer than say 30 minutes. That gives you a good bit of star trail. You can set up another shot or let it go.
14. Take the camera home and dump the images to the computer. Then you'll need a program to stack the images. You can do this in photoshop too. Stack one image on another and select "lighten" for the layer. What that does is finds which pixel is lightest and shows that one. So really the only change is it's letting the star through for the next image. That is all the stacking programs are doing. Compares the two images and lets the lightest pixels through. Then flattens that and does it again on the next image. I use this program.... http://www.startrails.de/html/software.html
Here are some samples at different ISO levels. The above was using 400 ISO. It was under moon-light and 30 second exposures. The landscape was really too dim. I should have shot this at 800 ISO 30 second stacks, as the foreground could have used and handled that and more stars would have came out as well. But note the amount of stars at 400 ISO, F3.5 30 second stacks.
Now this was 30 second stacks at 800 ISO F3.5. Notice the increase in stars. No moon on this one, so the foreground is darker obviously.
And this one was using 1600 ISO and I think 20 second shutters at F3.5. See even more stars get pulled out.
The only other thing to really consider is where the north star is. If you shoot east or west away from the north star, you don't have to go as long as those are moving faster. The north star is easy to find. Find the big dipper. Go in the handle and around on the back edge of the bucket. The two stars point at the north star.