High ISO Noise Reduction reduces visible noise in high-ISO shots, but the trade-off is loss of fine detail. Stronger settings like High apply more aggressive smoothing, which can mistake real texture for noise.

The main downside is that in-camera processing is typically baked into the image, so you have less control later and may not be able to undo it. If you prefer to fine-tune noise reduction yourself in software, turning it down or off is often better, especially if you want to preserve as much detail as possible.

In general:

• Off/Low: keeps more detail, leaves more noise
• Normal: a balanced default
• High: cleaner-looking files, but higher risk of smeared detail

A common approach is to avoid heavy in-camera processing unless you need ready-to-use JPEGs straight out of the camera. If you post-process your images, doing noise reduction on the computer usually gives better control and results.

Also note this is different from Long Exposure Noise Reduction, which uses a dark frame to remove hot pixels in long exposures; that feature is often more useful and not as easily replicated unless you shoot your own dark frames.