Canon T6i High ISO NR
Canon T6i High ISO Noise Reduction
The Canon T6i's four High ISO Noise Reduction settings ("High, "Standard", "Low" and "Off") provide good flexibility in choosing how you want to make the trade-off between subject detail and noise levels. The camera also offers a Multi Shot Noise Reduction option which takes 4 shots, and then aligns and merges them into one image to average out noise.
See for yourself how the Canon T6i's "Standard" and "Off" High ISO NR settings compare to RAW without noise reduction under daylight-balanced lighting. (Note that these RAW images also have no sharpening applied, so they're a little softer than camera JPEGs at low ISOs.) Click on any of the crops below to see the corresponding full-sized image. Since the Canon T6i user manual states noise reduction is applied at all ISOs, we've included crops from the base ISO on up.
As you can see, Canon T6i's default "Standard" High ISO NR setting is quite effective at reducing noise, but it struggles to maintain subtle contrast detail at higher ISOs, especially in our red-leaf fabric (as many cameras do). We'd probably shoot with lower settings at higher ISOs and reduce noise ourselves with post-processing (or just shoot RAW). Note how the "Off" setting doesn't disable noise reduction entirely even at low ISOs, which is true for most cameras.