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ISO Sensitivity / Noise levels (contd.)

In-camera High ISO noise reduction

Noise reduction is set using the 'High ISO NR' option in the 'Shooting' menu. The D3S offers four settings: Off, Low, Normal and High, with Normal being the default. When turned on, high ISO noise reduction only kicks in when shooting at ISO settings of 4000 (Hi 0.3) and above, but some noise reduction is applied at ISO settings higher than Hi 0.3 even when high ISO noise reduction is turned 'off'.

  High ISO Noise Reduction compared
Off
Low
Normal
High

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis.

The highest readings at all noise reduction settings are for chroma noise, and the biggest jump in measured noise levels in these four graphs is that between noise reduction turned off, and on 'low'. The difference between the three different 'on' settings is more subtle, but the effect of high noise reduction on image quality can be inferred from a comparison of the 'nr off' and 'nr high' graphs shown here. The images shown below provide a better indication of how the noise reduction settings actually impact upon image quality.

High ISO Noise Reduction Crops

  ACR RAW NR Off JPEG NR Off JPEG NR Normal JPEG NR High
ISO 3200
ISO 6400
ISO 12800
ISO 25600
(Hi 1.0)
ISO 51200
(Hi 2.0)
ISO 102400
(Hi 3.0)

Noise reduction and fine detail

Noise reduction is an art, and some manufacturers have mastered it better than others. Nikon's approach has traditionally been to pay most attention to chroma (color) noise, and take a more conservative approach to the monochromatic grittiness that we associate with luminance noise. Whether you like this philosophy is a matter of personal preference, but in general, we'd take a sharper, but grittier file in preference to a smoother but smudged one.

The differences between the D3S's different noise reduction settings are obvious from these images, and the main effect is to reduce chroma, as we would expect. Luminance noise is smoothed as well, and the effect is intensified at the higher NR settings, but images still display a noticeable graininess, even with noise reduction turned up to 'high'. The plus side is that high contrast detail is retained reasonably well at all noise reduction settings. As we'll see in the next set of images, however, low-contrast detail doesn't fare quite so well.

To check the effect of noise and noise reduction on low contrast detail we shoot our box scene, framed as shown here, with the new added feature of feathers. The very fine detail in the feathers will help to better judge the effect of noise reduction on fine detail. The ACR crops have noise reduction set to its lowest setting and use the 'Camera Standard' profile to provide more consistent color

Raw vs in- camera JPEG noise reduction low contrast detail comparison

The crops below show the effect of the various noise reduction settings on a low contrast area of a representative scene. Remember that noise reduction doesn't kick in until ISO Hi 0.7, so we haven't shown anything lower than ISO 3200, which is included here as a reference. The raw images shown here are converted using Adobe's Camera RAW plugin for Photoshop with all noise reduction turned down to zero. Although this isn't strictly 'raw' data (which is difficult to show in a useful way) these images represent the sensor's output before noise processing as closely as possible whilst still being meaningful as photographic images.

  ACR RAW NR Off JPEG NR Off JPEG NR Normal JPEG NR High
ISO 3200
ISO 6400
ISO 12800
ISO 25600
(Hi 1.0)
ISO 51200
(Hi 2.0)
ISO 102400
(Hi 3.0)

Here we can clearly see that low contrast detail suffers pretty badly when high ISO noise reduction is turned up to its maximum 'high' setting. In general, we would recommend keeping noise reduction at 'normal' between ISO 3200 and 6400, and maybe even turning it off when shooting at ISO settings above this point. Although the resulting images are certainly noisier, fine detail is preserved in low contrast areas much more effectively.

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