Nikon D5100 High ISO Noise Reduction
The Nikon D5100's four noise-reduction settings provide good flexibility in choosing how you want to make the trade-off between subject detail and noise levels. The Nikon D5100 user manual doesn't say when High ISO NR kicks in, so we've included crops from the base ISO in the tables below.
The crops below show the effects of the Nikon D5100's four levels of noise reduction available, under the studio HMI lighting we use to simulate daylight. See for yourself how the noise reduction works under daylight-balanced lighting. Click on any of the crops below to see the corresponding full-sized image.
To our eyes, the default "Normal" level of noise reduction strikes a good balance between noise and subtle subject detail, although the red fabric swatch presents a real challenge at higher ISOs, as it does for virtually all cameras we test. Cranking the noise reduction setting up to "High" produces smoother-looking images, while still maintaining good detail in areas of high local contrast (as seen in the circular scale), but a lot of detail is lost in areas of subtle contrast (the red fabric swatch is a particularly challenging subject, and the detail loss there is worse than you'd find in most real-world examples). At the other end of the scale, the "Off" noise reduction setting (which doesn't totally disable noise reduction at ISO 1,600 and above) leaves more noise in the image, but also leaves a lot more subject detail: Use this setting if you have good third-party noise reduction software; with appropriate processing, the result could be quite good. You should be able to do even better working from the RAW files.