Digital SLRs offer a function called Dark-Frame Subtraction usually called “Long Exposure Noise Reduction” or “High ISO Noise Reduction” in your camera’s settings menu. Dark-Frame Subtraction exposes a camera’s digital sensor less any external light for a matching duration of a photo taken to detect digital noise and electroluminescence. When the Dark-Frame Subtraction feature is active hot pixels, amp glow and other aberrations are subtracted from your photo. This feature works very well in most instances, but does tie up your camera limiting your ability to take a photo in quick succession. For example if you make a 2 minute exposure with this feature on, immediately after your exposure is completed another 2 minute exposure will be made to generate a dark-frame. A net total of 4 minutes will pass before you can review your photo or take another.
Dark-Frame Subtraction is a great thing to take advantage of if you have the time, patience and battery life.
For more on long exposures check out my ebook Photographing the 4th Dimension – Time
