Hello, im new to digital capture and post process, photoshop ect. Previous work 6x7 and 4x5. My question is should i turn my 5dmII LONG EXP. NR on or off ? I shoot RAW, MANUAL MODE EXP. RANGE IS 1 SEC. TO A FEW MINUTES ( NO NIGHT PHOTOGRAPHY ). I shoot this way 99% of the time. AM I BETTER OFF TURNING IT ON ( OR OFF AND USING CS4 TO WORK ON THE NOISE ? ) THANKS, MARK
Long Exposure Noise reduction is, at the root, the subtraction of a dark frame (a shot of the same exposure length taken with the shutter closed) from the image frame. It will definitely give better results than simple post processing because the noise observed in long exposure is mostly a fixed pattern.
There are even slightly better (and eventually quicker) methods, if you can use astronomical imaging software (lots of them are freeware): shoot your own dark frames. As long as the sensor temperature doesn't change a single dark frame can be used to reduce noise in all the pictures taken with the same exposure duration. Even better, you can take a number of dark frames and average them into a "master" dark frame in order to diminish the variation (noise!) in the noise itself and come closer to the true fixed pattern noise. And for the very dedicated, you can also take bias frames (shortest possible exposure with shutter closed) that you can average and then use in the processing pipeline of the dark frames... How much work and care to put into this depends on your goal
- if you just want a better picture, without hot pixels, and with decent noise reduction, you can use Long Exposure NR and you'll be satisfied. It will be hard to filter pattern noise without losing image quality in Photoshop, and will be time consuming to tackle hot pixels one by one...
- if you want dozens of them, with roughly the same exposure length, shooting dark frames and then applying the correction to multiple images is faster, but may be a bit difficult for the photographer who isn't comfortable with astro/scientific software and file formats.
- if you want the best signal to noise ratio for very dim objects, the astro pipeline (eventual stacking, master bias, master dark) is the way to go.
One important thing to keep in mind when using a library of dark frames is that they should be taken at the same temperature as the images. Noise in sensors is directly proportional to temperature (roughly doubles every 6° C)
Note: this is by no means a complete and authoritative answer but rather a quick summary - whole books have been written about these techniques, and the ways cameras subtract dark frames and carry out additional processing (such as muting the red channel) are black boxes.
Edit: checking if Photoshop offered special tool for dark frame subtraction, I stumbled upon this
http://www.pearsonfaces.com/fpsurgeon/2007...raction-in.htmlwhich could be a better method for photographers, combining independent dark frames and photoshop.
