Originally posted by Huck Finn That's not what I was suggesting. I'm not saying to divide a 10 seconds exposure in 5 2 seconds ones. I'm saying that if you tick the "Auto EV adjust" check box the camera will average out the images, therefore obtaining the same result of an exposure as long as the sum of all the exposures made.
The result will still be multiple exposures interrupted by short periods of non exposure. Of course it depends on what you want to achieve with your 30s exposure. Imagine a cityscape at night where you want to capture the trails of car headlights. Whether the exposures are averaged out of added doesn't really make a difference as the combined light trails will not result in continuous lines either way. For this you really need to use one or more ND filters. If your total exposure does not include one continuous movement or if the interruptions won't be visible in the end result there is no problem with your approach of course.
Another difference is impact of mirror movement and alignment between frames. I found that a very sturdy tripod is required, more than when using a single long exposure with ND. The mirror going up and down between every exposure increases the impact of mirror shake compared to working with an ND, and if you eliminate this by using 2s MLU timer that increases the lag between exposures even more. Mirror movement - even if the impact on individual frames is eliminated with (2s) MLU - may also slightly move the position of the camera on the tripod (if not mounted properly) resulting in significant loss of sharpness because of misalignment between the frames (esp between the first and the last). This may sound massively exaggerated, but I have actually experienced this with the camera seemingly properly mounted on the tripod. If the camera is mounted level the risk is small, but if not you would be surprised how easily gravity can pull ever so slightly on a camera and change its position a fraction, and vibration resulting from mirror movement helps this process along. A misalignment of very few pixels is enough to result in significant loss of sharpness.
Originally posted by Huck Finn BTW this will also have a good effect on any digital noise you may have in dark areas, since the noise is randomly scattered and it will be smoothed over by the multiple exposure.
For the noise that is random this is true. However, some noise is not random, e.g. the noise that is eliminated by DFS. Note BTW, that if you leave Slow shutter speed NR turned on, the interruptions between exposures will always be longer than the exposures themselves once you use 1 second or more.
Originally posted by Huck Finn Actually this works better than a physical ND filter, since this kind of filters (at least the darker ones) often induce color dominants in the picture. To have a truly neutral ND filter you have by a quality, expensive filter.
True, filter quality is paramount. But that goes for every type of filter, actually for every optical element you add to the optical chain between your subject and the sensor. Good quality (expensive) filters are the only way not to compromise your camera's IQ imho, but that's a personal opinion... Good quality ND filters exist and should be preferred imho...
hth, Wim
Last edited by Ishpuini; 12-23-2010 at 01:42 AM.