Analysis: Dynamic Range

One of the D810's most significant features was its ability to shoot at ISO 64. There are two main factors that influence the Raw (saturation-based) ISO rating: efficiency and full-well capacity. Higher efficiency (a higher proportion of photons being registered) would push base ISO upwards, while an increased full-well capacity (capacity for electronic charge) pushes it down.

The D810 offered a lower ISO by increasing its full well capacity (or, at least, finding a way to squeeze a bit more out of it). This meant it was able to capture just as much of the light it was exposed to as the D800 but could tolerate more light before it started to clip. This meant it could be given greater exposure, which improves the signal-to-noise ratio and gives cleaner tones across the image: an improvement often interpreted as improved tonality.

This ability to tolerate an extra 2/3EV exposure is why we say that the D810 is able to compete with the latest batch of medium format cameras, whose 44 x 33mm sensors would capture 2/3EV more light at the same F-number and shutter speed.

So does the D850 manage to repeat this trick? Yes, it certainly looks like it.

Similar noise performance in the deep shadows suggests the same dynamic range, so long as it isn't clipping the highlights earlier. Bill Claff's data suggests it doesn't, and that the overall base ISO DR is just as good.

We further confirmed that dynamic range results remain identical in continuous drive modes, unlike many Sony ILCs that drop to 12-bit.

BSI benefits, High ISO Performance and ISO Invariance

We were always told that BSI sensors wouldn't offer much of an efficiency improvement in large sensors, since the pixel circuitry that might otherwise get in the way only makes up a small proportion of the light-sensitive area in large pixels. Instead, Nikon says the main benefit in the D850 was that it allowed them to include the additional circuitry needed to offer higher readout speeds without having to worry about it impinging on the light collecting part of the pixel.

Theoretically we might also see some benefit in performances in the corners with wide-angle lenses, but that doesn't come into play with the 85mm lens we use for these tests.

However, while we wouldn't expect the move to BSI to give a significant improvement in high ISO noise as a result of increased efficiency, we would expect to see some come from the D850's sensor's 'dual gain' design. This uses a different, higher gain read-out circuit for higher ISOs that slightly decreases the read-out noise but lowers the amount of the full well capacity you can use. Because, at high ISOs, even tiny amounts of read noise are multiplied many, many times, any reduction is valuable. At these settings, much of the sensor's capture is amplified to clipping, so a reduction in full well capacity is a trade-off worth making. As such, we see the camera's base ISO performance unaffected, but from ISO 400 upwards, we should see an improvement in noise performance.

This approach means the D850 is not ISO Invariant, but not in a bad way: it's not because the low ISO modes are adding a lot of read noise, but because the higher ISO modes are able to add even less.

Sure enough, just like the D810, if you brighten the D850's ISO 64 shots, they exhibit more noise than if you shoot at the same exposure using a higher ISO. Add a little more amplification at the bottom end, by starting at ISO 100 and the results are very similar to those you get by natively shooting at ISO 6400 (which are noisier than the ISO 64-4000 series of shots, since the latter were given more exposure).

Once you get above ISO 400, where the sensor switches to a higher pixel-level gain, there's essentially no difference between shooting at ISO 500 and brightening, vs shooting at ISO 4000.

That makes it rather unfortunate that Nikon - like most stills camera manufacturers - choose to brighten higher ISO shots with in-camera amplification as opposed to in the Raw converter. The latter method would offer multiple stops of potential highlight information the D850 - and most cameras - simply throw away. Many video cameras operate this way to afford you the maximum dynamic range and post-processing latitude at any ISO: it's generally called 'EI' mode.

The D850 throws away stops of highlights at ISOs above 500 for no benefit. Still camera manufacturers need to learn a thing or two from their video departments.

By ISO 4000 and 6400, you can start to see the D850's higher gain mode showing a slight noise improvement over the D810: a difference we'd expect to see as the ISO settings rise further. What's inexcusable at this point though is the additional amplification above ISO 500 that throws away many stops of highlights for no benefit whatsoever. Stills manufacturers need to learn a thing or two from their video counterparts.

Shadow noise

One factor not highlighted in these tests is a series of abnormally bright pixels in the deepest shadows of the file. All across the image are pixels that have recorded values 5-20x brighter than those around them. Invisible if left near black, these brighter pixels end up being pushed to clipping if you pull the shadows too far. They're masked in these tests by our use of Adobe Camera Raw's default chroma noise reduction, appearing as desaturated salt-speckled noise rather than bright red, green or blue individual pixels.

These sporadic bright pixels are not too visually damaging may slightly reduce the degree you wish to manipulate the files.