WunderPhotos®

Thanks for the comment!

It's not that you "need" dark frame exposures, but it's a good idea. The dark frame subtraction method basically erases your hot and cold pixels that form in long exposure photography. Those are the red and blue pixels that show up in your photo. It also tends to even out (somewhat) the overall luminosity of your photo.

Hot pixels (the red ones) and cold pixels (the blue ones) form as your camera's sensor heats up during long exposure photography. During normal photography, you shouldn't have any.

To do the dark frame subtraction, you simply put your lens cap on the camera and take the same length photos as you did for your light frames...in the same ambient temperature as the light frames. So, for this photo, as soon as I finished gathering all 300 light frame exposures at 10 sec each, I put the lens cap on and took 8 photos of blackness at 10 sec each...before moving my camera inside or anything of the sort. That allows the camera to record the exact same hot and cold pixels that are recorded in your light frame exposures. Since the whole image is black, except for the recorded hot and cold pixels, the program I used knows to take those particular pixels out of my photo. I usually take about 6 dark frames, but for whatever reason I decided to take 8 this time. The more you take, the more are removed from you photo by the programming software's algorithms and stuff. I've never taken or seen taken more than around 10 or so, as it's not necessary to take as many dark frame exposures as light frame ones.

Again, it's not "needed" because you can go in and manually clone out the pixels, but if you have a lot that could take an awful lot of time...a lot longer than the minute or so that it took me to take the dark frame exposures!! Then in a program like Star Trails, you load up your light frames first, then your dark frames. The program combines your light frames then your dark frames and you can literally watch as almost all the hot and cold pixels disappear. Those few (if any) that might be left can be easily cloned out in just a couple minutes.

Hope that answers your question!