So as some of you might know if you've been following me, I'm on a road trip. I'm currently in Moab, Utah near Arches NP. I would love to do some long exposure shots of the Milky Way above the Arches, but I'm having some trouble. My camera, (the D3000) is very bad when it comes to low light/high ISO shooting. There's grain everywhere. I discovered a technique, that if you over exposure 2 stops, and then bring it down the 2 stops in Photoshop, then you will get the same shot, but with almost no grain. Now the problem I have is understanding stops. I need to expose 2 stops over 30 seconds, which means I have to be in manual mode (not a problem), but I don't know the actual exposure time. Does any one know anything about this, and could fill me in? That would be great!
Each stop increase (or decrease) to your current exposure means you want to double (or halve) the amount of light being captured. With time, this makes it really easy -- to lengthen your shutter speed by two stops, simply double the time twice (e.g. 30 * 2 = 60 * 2 = 120 seconds).
Note that it sounds to me like you're doing short-exposure star photography, rather than long exposure star photography (where you use a lower ISO and an exposure time of typically 30 minutes to several hours). The appeal of short-exposure star photography is that you get the stars (and hopefully galaxy) 'frozen', without any of the typical arcs that you see in long-exposure star photography.
In this case, you should remember the 'rule of 600', which is a way to ensure that your star 'points' won't turn into trails. The rule of 600 says that the longest shutter time you can use before the stars turn into trails is 600/focal length. As always, that's for full-frame cameras, so for crop cameras you need to adjust. As an example, if you are using a 10mm lens of a crop-sensor camera (equivalent to a full-frame 15mm for Nikon), the longest exposure you should use is 600/15 = 40 seconds. As you can see, this means that your 120 second exposure will cause the stars to turn to trails with any reasonable lens you might own.
Grain is a huge issue in these kind of photos, and it's best to try and live with it at the right shutter speed rather than try and work around it. You'll notice that even people who take shots on full-frame low-noise cameras with super-fast lenses rarely show them at large sizes, because the noise is so problematic. If you are willing to accept that you will blur your star trails and lose all colour information from the stars (because you're massively overexposing them), then give your technique a try, but take some shots 'normally' as well, as you might be happier with them when you come to process them, especially if you can use a recent version of Photoshop's Camera Raw or Lightroom to process the noise.
We were there close to sunset and I really want to go back for at least a few days. I agree, you need a lower ISO. My husband recently showed me some star shots from there, and they photographer had a light source on the arches and did some long exposures. He's not home, to ask him about the link.
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Note that it sounds to me like you're doing short-exposure star photography, rather than long exposure star photography (where you use a lower ISO and an exposure time of typically 30 minutes to several hours). The appeal of short-exposure star photography is that you get the stars (and hopefully galaxy) 'frozen', without any of the typical arcs that you see in long-exposure star photography.
In this case, you should remember the 'rule of 600', which is a way to ensure that your star 'points' won't turn into trails. The rule of 600 says that the longest shutter time you can use before the stars turn into trails is 600/focal length. As always, that's for full-frame cameras, so for crop cameras you need to adjust. As an example, if you are using a 10mm lens of a crop-sensor camera (equivalent to a full-frame 15mm for Nikon), the longest exposure you should use is 600/15 = 40 seconds. As you can see, this means that your 120 second exposure will cause the stars to turn to trails with any reasonable lens you might own.
Grain is a huge issue in these kind of photos, and it's best to try and live with it at the right shutter speed rather than try and work around it. You'll notice that even people who take shots on full-frame low-noise cameras with super-fast lenses rarely show them at large sizes, because the noise is so problematic. If you are willing to accept that you will blur your star trails and lose all colour information from the stars (because you're massively overexposing them), then give your technique a try, but take some shots 'normally' as well, as you might be happier with them when you come to process them, especially if you can use a recent version of Photoshop's Camera Raw or Lightroom to process the noise.
We were there close to sunset and I really want to go back for at least a few days. I agree, you need a lower ISO. My husband recently showed me some star shots from there, and they photographer had a light source on the arches and did some long exposures. He's not home, to ask him about the link.