bob elkind
Veteran Member
Here are some threads and links which explain (better than I could do) the concept of using photoshop (several images stacked and averaged) to effectively duplicate the effect of using a ND filter to create a single long-exposure image.
First link to a thread, with examples: http://forums.dpreview.com/forums/read.asp?forum=1034&message=27839108
Here's how to stack and average images in PS. Averaging will reduce image noise (because averaging is a low-pass temporal filter), and will also effectively sum the shutter times of the stacked images (i.e. a low-pass temporal filter!). Link: http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm
Here's another reason to use this technique (or use a ND filter). In trying to lengthen exposure, so as to blur or soften motion, the aperture is closed down. Of course, this increases DOF. Well, what if you want to maintain foreground-background separation? One approach is to use one or more ND filters, but you need more and more ND filters stacked to lengthen the exposure more and more. The multiple-exposure stacking approach is extensible to any effective (cumulative) exposure time you wish, and you can open the aperture (a shorten DOF) as much as you wish. Go ahead and shoot at f/2.8 and 1/20 sec. Take 10 of these shots at 1/20 to effectively achieve 1/2 sec exposure time, while still maintaining the shallow DOF that f/2.8 affords.
This can also keep the DX folks out of that nasty diffraction effect on sharpness that comes with stopping way down.
Hope this helps. Not my idea, this has been posted several times in these Nikon forums, and the first time I saw this described I was in total awe of the fundamental cleverness of this approach.
-- Bob Elkind
Family,in/outdoor sports, landscape, wildlife
photo galleries at http://eteam.zenfolio.com
my relationship with my camera is strictly photonic
First link to a thread, with examples: http://forums.dpreview.com/forums/read.asp?forum=1034&message=27839108
Here's how to stack and average images in PS. Averaging will reduce image noise (because averaging is a low-pass temporal filter), and will also effectively sum the shutter times of the stacked images (i.e. a low-pass temporal filter!). Link: http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm
Here's another reason to use this technique (or use a ND filter). In trying to lengthen exposure, so as to blur or soften motion, the aperture is closed down. Of course, this increases DOF. Well, what if you want to maintain foreground-background separation? One approach is to use one or more ND filters, but you need more and more ND filters stacked to lengthen the exposure more and more. The multiple-exposure stacking approach is extensible to any effective (cumulative) exposure time you wish, and you can open the aperture (a shorten DOF) as much as you wish. Go ahead and shoot at f/2.8 and 1/20 sec. Take 10 of these shots at 1/20 to effectively achieve 1/2 sec exposure time, while still maintaining the shallow DOF that f/2.8 affords.
This can also keep the DX folks out of that nasty diffraction effect on sharpness that comes with stopping way down.
Hope this helps. Not my idea, this has been posted several times in these Nikon forums, and the first time I saw this described I was in total awe of the fundamental cleverness of this approach.
-- Bob Elkind
Family,in/outdoor sports, landscape, wildlife
photo galleries at http://eteam.zenfolio.com
my relationship with my camera is strictly photonic
