When shooting a long exposure, particularly at a higher ISO, the sensor is prone to producing "hot pixels". These are pixels that are stuck "on" and are already fully saturated, caused by the higher temperature of the sensor, and that's why they appear red, green and blue. They are particularly apparent in the shadows, especially when trying to lift the detail in post processing.
Preventing Hot Pixels?
There are measures you can take to reduce the number of hot pixels appearing but they can never be fully avoided:
Resetting your sensor may help
Enabling "long exposure noise reduction"on your camera - which effectively takes a second (same exposure time) dark frame and subtracts the hot pixels. This is more effective but unusable in some situations where you cannot have a gap in between shots (like star trails). A 10 minute exposure would mean your camera would be out of action for a further 10 minutes, not ideal in most situations.
Sony a7Rii & Zeiss 16-35mm f/4.0
3,753 seconds (total time) | f/4.5 | ISO 800
Removing Hot Pixels
There's a super simple method in Photoshop! Under "Filter --> Noise" you'll find "Dust & Scratches"
As you can see from the before and after image it is very effective at removing the hot pixels and retaining the detail of the image. You should play around with the peramiters of the filter but I have found setting the radius to 4 pixels and the threshold to 40 levels gives the best results.
I recommend creating a duplicate layer from your original image and applying the filter onto that. Then you can mask through to the original layer on sections that require the hot pixel removal.
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