The short answer that will take you longer: Google "dark frame subtraction," you'll have ample reading material... I believe the D70 also has dark frame subtraction built in, you'd have to look it up in the manual.
The procedure for doing it manually, in a nutshell:
1.) Shoot your long exposure shot. Use manual settings so they don't change in step 2. this includes white balance.
2.) Put on lens cap, shoot again. This is your dark frame.
3.) Open the shot and the dark frame in PS
4.) Copy the dark frame into the same PS document as the photo, as another layer (above the photo). Change the blend mode to "difference."
5.) Zoom in too 100% (or 200%) and adjust curves, moving the white point control point either left along the top or down the right side until the effect is correct.
Note that step 5 is typically necessary, even though it really should not be. Most times, I find that I need to adjust. TTo judge when it's right, look at the "grain" in the dark areas. As you increase the "exposure" of the dark frame, you'll see that darker spots appear, and vice versa as you decrease exposure (remember you're subtracting). When the luminosity of the "hot pixels" matches the background luminosity, it's right.
Technically, what you want to do is minimize the entropy in the shot, as judged by its compressibility, which will be maximized when entropy is minimized. Since you can't watch the compressibility change, you're stuck doing it visually.
There are specialist software packages out there for this, but they are not a simple learning curve. The PS technique works OK, if it is not avilable in camera.
3.)
Message edited by author 2005-09-03 23:11:55.
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