Last night I figured I would do an experiment on the differences of stacking versus ultra-long exposures. I see why now that stacking is so prevalent. The increase in contrast is much better. Both of these images total 40 minutes of exposure. The first image (landscape mode) is 40 minutes with a single exposure. The second image (portrait mode) is 4 images of 10 minutes exposure each stacked in registax. (I just downloaded DSS but can not open the files ). Both images were taken with a 5D, ISO 125, AV 5.0, Lens 24-70mm @24mm. Noise reduction was on with all images.