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By image stacking I mean the averaging of a number of separate photos to reduce noise, not focus stacking to increase depth of field. This requires the subject to be static and the use of a tripod. So I am wondering how image stacking compares with just using a longer shutter time with camera on tripod.

So for instance say we can properly expose with a 10 second exposure at ISO 100 or a 1 second exposure at ISO 1000 and we take ten 1 second photos and stack them. I did a quick test as in this example and didn't see any very noticeable difference. Possibly the stacked image was very slightly sharper but I'm not sure if that's due to the stacking itself or possibly some processing the program (Affinity Photo) does. A very slight sharpening of the 10 second exposure made the two images indistinguishable.

How are the results expected to compare? When would one use one method over the other?

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  • \$\begingroup\$ Long exposure time can make the sensor heat up and be noisy. Exposure stacking is better done with interval between shots long enough for the sensor to cool off. \$\endgroup\$
    – xenoid
    Commented Apr 3 at 0:26
  • \$\begingroup\$ Are you asking about astrophotography, primarily? \$\endgroup\$
    – scottbb
    Commented Apr 3 at 17:48
  • \$\begingroup\$ No, not astrophotography. It's just a feature of my program that I had never used before so was wondering where it would be useful. \$\endgroup\$ Commented Apr 3 at 19:15

4 Answers 4

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Some reasons why I would use it are:

  1. HDRI situations, mainly architecture photography.

  2. And several expositions to eliminate tourists or people, again, in architectural photography.

I am not too worried about noise reduction because you can simply apply a decent noise-reduction filter.

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    \$\begingroup\$ By image stacking I'm referring specifically to the process of averaging similar images to reduce noise, not High Dynamic Range imaging where photos are taken at different exposures. Your reason #2 is interesting - hadn't thought of that. \$\endgroup\$ Commented Apr 3 at 12:32
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You lose less when one of the photos is ruined. If you are doing astrophotography and want several minutes or hours of exposure you can have an airplane come through the shot. If you do one long exposure it is ruined. If you do many short exposures you can throw away the one with the airplane and use the rest.

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In terms of image noise there is a tradeoff between photon shot noise and dark current noise. Photon shot noise relative to the signal is reduced by longer exposures, and dark current noise relative to the signal increases with sensor temperature.

Sensor temperature varies with ambient temperature, the duration of the exposure (sensor energize time), and the number of exposures in quick succession.

In general, with CMOS sensors exposures longer than ~ 1 minute start to become problematic in terms of dark current, and stacking of multiple shorter images could be more beneficial. But there are too many variable to be more specific.

This graph shows dark current noise for various cameras during a series of 5 minute exposure (X1 begins at 10 minutes)(source).

enter image description here

Note that the electron count appears to be a factor of 10x low (i.e. the decimal place is off).

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I think this article describes one area of application quite well. With a 30s exposure, for example, the stars would move - due to the earth's rotation - and would not be dots but stripes. By stacking you might end up with more stars, but who would notice that ;)

Apart from that, I honestly can't think of any good reasons. I think it's just a niche technology. Those who need it can use it, but there isn't really THE reason to use image stacking in many areas.

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