Should long exposure noise reduction be turned off for stacked star-trail shots?
Asked 9/4/2014
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I want to try shooting star trails with an intervalometer and stack the images later. When long exposure noise reduction is enabled, my camera pauses after each exposure for dark-frame processing, sometimes for nearly as long as the exposure itself. For a continuous star-trail sequence, will that create visible gaps? If I turn long exposure noise reduction off, can I shoot continuously enough for stacking, and is the extra noise usually manageable when stacking star-trail images?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
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Long Exposure Noise Reduction takes a dark frame of the same duration to subtract the noise from the image. That's why there is a long delay; a 30-second exposure plus a 30-second dark frame plus processing time plus the time to write the image to the memory card. There will be obvious gaps in the star trails.
If the gaps are minor, they can be corrected in software like StarStax, for example. A 30-second shot followed by a 30-second plus gap is unlikely to be correctable. Even if you turn Long Exposure Noise Reduction off, there will be a tiny, probably correctable gap because of the time required to close the shutter, record the image to memory and then open the shutter for the next shot. In my experience, those are seamlessly correctable in software.
A technique commonly used in astrophotography is to take just one dark frame, with the lens cap on in the same conditions and exposure time, at the end of the shooting and then use dark-frame subtraction in Photoshop, Lightroom, GIMP and other software. Essentially, you are subtracting sensor noise from your image.
This also makes it easy to see the result with and without dark-frame subtraction.
Originally by user13846. Source · Licensed CC BY-SA 4.0
user13846
11y ago
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Yes—if you’re shooting star trails to stack later, you’ll usually want long exposure noise reduction turned off.
That feature works by taking a second “dark frame” of the same length after each exposure and subtracting noise from the image. So a 30-second exposure can mean roughly another 30 seconds of downtime, plus processing/write time. For star trails, that creates obvious gaps between frames.
With long exposure NR off, there will still be a very small gap while the shutter resets and the file is written, but that gap is typically minor and often handled well by star-trail stacking software.
Noise will increase somewhat without in-camera long exposure NR, but for stacked star-trail sequences this is usually the better tradeoff than having large breaks in the trails. Stacking itself is mainly for combining the trail movement, not a full replacement for dark-frame subtraction, but it’s generally the standard approach for continuous star-trail shooting.
In short: turn it off for continuous star-trail stacks unless you specifically need the in-camera dark-frame correction and can tolerate the gaps.
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