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Below are two night sky images taken with what appear to me to be identical camera settings: f/4.0, 8s shutter speed, ISO 800, focal length 93mm. The only difference I can spot in the EXIF information is that the drive mode was "Self-timer Operation" for the first image, and "Single-frame shooting" for the second.

But I cannot think of a reason why this would result in the much lighter image.

Sorry, this is a copy-and-paste of a screenshot of the image due to upload size limits, but I'm confident that nothing in the process of displaying it on Stackexchange has altered the apparent brightness.

darker image

lighter image

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    \$\begingroup\$ Are these screenshots of JPEGs, or raw? What is the screenshot of (an image viewer, or Lightroom, or Photoshop, or ...)? How were these processed? Unfortunately, their are possibly too many unknowns in the images as screenshots. For upload size purposes, it would be better to export as a lower resolution image, rather than upload screenshots. \$\endgroup\$
    – scottbb
    Jan 17, 2022 at 22:35
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    \$\begingroup\$ They are screenshots of JPEGs. The JPEGs were created in camera - no RAW files involved. No further processing other than displaying in Geeqie. \$\endgroup\$
    – jl6
    Jan 17, 2022 at 22:38
  • \$\begingroup\$ @jl6 So why not make a smaller version of the actual JPEG, rather than taking a screenshot? The difference could be due to a myriad of things that happened when taking the screenshots, such as your camera changing exposure based on reflected light on the screen. \$\endgroup\$
    – Michael C
    Jan 18, 2022 at 21:26

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To my eye it looks like there may have been more water vapor/moisture in the air between the camera and the stars in the second shot than in the first. If the moon was anywhere above the horizon, or there were any ground sources of light pollution in the area, that light would be reflected and disbursed by any thin clouds overhead.

I've been out shooting at night and had very high, very fast moving clouds that aren't even visible to the naked eye show up in photos. They're easier to see in wide angle views when the entire frame is not affected, but the blurrier stars in the second image look a lot like what happens to those stars behind a very thin cloud.

enter image description here

When viewed full screen in a dark environment, the thin clouds can be seen in this view of Orion and surrounding constellations taken March 12, 2013. Those near the horizon are easier to see due to ground based light pollution. But if one looks carefully, one can see a few tendrils of clouds over the area between Orion's belt and Taurus.

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