Nikon D750 High ISO NR
Nikon D750 High ISO Noise Reduction
Like most Nikon pro and prosumer DSLRs, the D750 offers four High ISO Noise Reduction settings: Off, Low, Normal (default), and High, allowing you quite a bit of flexibility in choosing how you want to make the trade-off between subject detail and noise levels in JPEGs. The Nikon D750 manual doesn't say at what ISO High ISO Noise Reduction begins to be applied, so we've included crops from ISO 50 on up here. Note that the Nikon D750 user manual says when Off is selected, the amount of noise reduction is always less than the Low setting, but doesn't provide any additional details.
See for yourself how the Normal and Off settings work compared to RAW files with no noise reduction (or sharpening) applied. Click on any of the crops below to see the corresponding full-sized image.
As you can see by comparing the Normal and Off high ISO noise reduction settings, the D750 applies noise reduction at all ISOs. You can also see the Off setting still applies some minimal noise reduction, just as Nikon says. Note that noise in low to moderate ISO JPEGs appears higher at the Low setting than noise in the converted RAW images with no noise reduction because no sharpening was applied to the RAW files. Also note that ISO 50 is an extended low ISO, and ISOs 25,600 and 51,200 are high extensions.
The Nikon D750's default high ISO noise reduction generally does a good job at reducing luminance noise and a very good job at reducing chrominance noise, however as you can see, there can be quite an impact to areas with subtle tone on tone contrast, particularly in our red-leaf fabric already at moderately low ISOs. This is often the case with more aggressive chroma noise reduction and something to mindful of. You may want to try the Low setting if you're finding too much detail is lost at high ISOs.
Still, really excellent high ISO performance overall for its resolution.