Recently, I discovered a weird 'glitch' caused by Long Exposure Noise Reduction on the Nikon D750. Note, this only applies to astrophotographers and night photographers taking images with stars in them. If your image does not have stars, you shouldn't notice any problems.
When LENR is enabled, the processing causes stars to lose their true colors and turn green, yellow, cyan, and purple. After doing numerous tests, I've confirmed this problem is directly related to the LENR processing. The comparison below should give you a good look at the problem.
When LENR is turned on, your camera will take 2 exposures. The first is a normal photo, the second is a dark frame. This dark frame is mainly used to find Hot Pixels. Once the camera has both images, it will automatically remove the Hot Pixels and save one final RAW or JPEG image. You can try this process manually. For example, take a normal photo. Immediately after that finishes, put your lens cap on and take another image with the same settings. This will be your dark frame. Using various Astro photo stacking programs, or just Photoshop, you can subtract the Hot Pixels from your normal image. However, this might not always work well. Therefore, I like using LENR in-camera.
At some point during this process, the D750 is shifting the colors and causing artifacts. After conversing with other astrophotographers, and scouring the internet, this is the first time I've seen this problem reported. Thankfully, we might already be close to a fix.
As of now, there are 2 ways to fix this problem in Post-Processing. The first requires Nikon's Capture NX-D software. Once you open your RAW image, find the Noise Reduction panel on the right-side. You should see an Astro Noise Reduction checkbox. Once clicked, it should automatically remove any hot pixels and fix the color shift issue reasonably well.
Of course, using Capture NX-D isn't part of most photographers' workflow. Personally, I use Adobe Camera RAW to initially edit my RAW photos. By increasing the Color noise reduction slider, and decreasing the Color Detail slider, the color shift is drastically reduced. However, this may negatively impact real colors in the image.
I'm currently in contact with Nikon Service Reps about this issue. They are now aware of the LENR issue affecting Nikon D750 images. The recent D850 Firmware Update states: "Photos taken with On selected for Long exposure NR would sometimes have increased noise or shadows with a greenish cast." I was actually able to get my hands on a D850 with the 1.00 firmware update installed. There is a heavy green color cast when LENR is turned on, however, the stars do not exhibit the same issue as seen in this blog post.
I will update this blog post as soon as I have more information from Nikon.
If you have a Nikon camera with the ability to enable LENR, do you notice this problem? In order to test, take an image of the night sky with LENR turned off. Then take another image, same settings, with LENR turned On. I noticed this issue mainly on my wider angle lenses, like the Sigma 35mm and Nikon 14-24mm. If you have a star tracker, try using that to take a longer exposure, which might help exaggerate this issue.
If you find this same problem on another Nikon camera model, please let me know! Either mention it in the comments, or send an email to firstname.lastname@example.org with sample images.