Review of the Nikon Z6


Introductory Comments

This is very much a technical review, so rather than describing the handling of the camera and its various features (which can all be found elsewhere) I am more concerned about the technical characteristics that will affect long exposure astrophotography of deep sky objects. In many cases I will be comparing it to the Sony A7S and Canon EOS R cameras, which I also own.


Read Noise Gain etc.

I don't intend to duplicate Bill Claff's excellent work at PhotonsToPhotos because I obtain very similar figures:
Regarding the DxOMark Derived Characteristics, note that Bill says that fitting the DXO data has its limitations, affecting estimates of gain, read noise, QE etc.

Note the sharp drop in read noise (from 5.1e to 1.6e) at ISO 800 where the camera switches to HCG (high conversion gain) mode. However, for long exposure RGB imaging the read noise is usually swamped by the noise of the background sky so read noise is not a concern. On the other hand, low read noise might be important if short exposures are being used or if narrowband filters are being used.

The bias level of the Nikon Z6 is 1008 at all ISOs.


Dark Current and Camera Self-Heating

My methodology for testing dark current and self-heating is to put the camera in a dark room at 20C ambient temperature and leave it to acclimatise for an hour or two. I then switch the camera on and immediately run a couple of hours of consecutive 5 minute dark frames with the rear LCD folded out from the rear of the camera to allow heat dissipation. By subtracting each frame from the previous one, the thermal noise can be calculated and ultimately converted into dark current.

Here's the result for a few cameras, measured in electrons per pixel per second:



However, a lot of the difference is caused by the size of the pixels, so I find it's more meaningful to display the graph in terms of electrons per square micron per second:



It can be seen that the Z6 has low dark current and remarkably low self-heating during an extended session of long exposures. Remember also that during an actual imaging session the self-heating should be lower because the camera is thermally attached to a potentially heavy telescope and also the wind will be circulating the air around the camera body, both of which will have a cooling effect.

However any time spent in live view will potentially heat the sensor quite rapidly so live view should be kept to an absolute minimum to prevent any heat building up. Beware if you are controlling the camera from a laptop via USB because live view might be on continually. I therefore switch the tethered camera off unless actively focusing or framing the shot.


Histogram Gaps and RGB Channel Scaling

In common with most other Nikon cameras, the Nikon Z6 performs digital scaling (known as white balance pre-conditioning) to the colour channels, which leaves regularly spaced gaps in the histogram of the green and blue channels.

For instance, here's the histogram of a flat frame at ISO 800:



Counting the number of gaps leads to a good estimate of the scaling factor applied. In this case it is approx 1.18 for both the red and blue channels. I have confirmed this by calculating the camera gain values from flat frames and bias frames. The scaling factor is sufficiently high that it will not have adverse effects on astro-imaging.

The blue channel and one of the green channels also have regularly spaced spikes. It's not clear what might cause this.


Raw data filtering

Raw data filtering in cameras can cause a range of effects including: Raw data filtering generally caps outlier pixel values to prevent them causing obvious noise in an image. It can be detected by comparing every pixel value against the maximum of their neighbours.

Here's an example plot from a 5 minute dark frame of a camera without raw data filtering (the Canon EOS R). Every pixel value is plotted against the maximum of its 24 neighbours in a 5x5 square block surrounding the pixel:



In contrast here is the plot from a 5 minute Nikon Z6 dark frame:



Note the absence of the strong vertical "arm" on the left. The pixels in the vertical arm represent isolated bright pixels and their absence shows that they have been capped to some other value, which is evidence of spatial filtering. The obvious diagonal line is another indicator of spatial filtering and represents pixels whose value has been capped to the value of one of its neighbours.

The good news is that this filtering has no obvious effect on the shape or colour of stars. The effects on master dark subtraction are also minor.
Here is a test of how well master dark subtraction works on the Nikon Z6:



It would appear that the dark subtraction has worked very well, with just a few bright pixels left behind. These are bright pixels whose values were capped in the dark exposures but remained unfiltered in the light frames. The master dark is therefore unable to remove them.

As expected, LENR (long exposure noise reduction) has performed much better because it does an in-camera dark subtraction.

All in all, the darks on the Nikon Z6 are very clean and dithering is likely to be just as effective as taking darks. This is good news for those too lazy to take darks!


Electronic Shutter

The Nikon Z6 has an electronic (silent shutter) mode. On some cameras this can compromise the bit depth of the data, giving 12 bits instead of 14 bits. This is not the case on the Nikon Z6 (nor the Canon EOS R). It is therefore safe to use electronic shutter with no fear of loss of image quality or unwanted image artefacts. The electronic shutter prevents wear and tear on the mechanical shutter and it also allows very accurate fast exposures e.g. for taking flat frames against a bright sky.

However, beware that the slow scan speed of the electronic shutter (reported to be around 1/15sec) can cause horizontal banding with the fluctuating cycle of artificial lights. This is one reason I use sky flats.


QE (Quantum Efficiency)

On a clear sunny day I was able to test the Nikon Z6 against the Canon EOS R on exactly the same DIY spectrometer (a cardboard tube with a razor blade slit, diffraction grating and built in lens) pointing at a white target illuminated by direct sunlight.

The pixel values are converted into electrons using the gain for that colour channel and then adjusted for pixel area so we are comparing like with like i.e. photons recorded per unit area of sensor. Here is the result:



A note of caution is needed when interpreting this chart. The chart cannot be used to compare the QE of the sensor at the blue end of the spectrum against the red end. The blue channel climbs higher than the red simply due to the spectral composition of the daylight being reflected off the white (or whitish) surface the spectrometer was pointing at. However, at any individual wavelength the chart can be used to compare the response of the Nikon Z6 Sony sensor against the Canon EOS R.

The overall shape of the curves is very typical of the difference between Sony and Canon sensors. The sensitivity of the Nikon Z6 peaks higher than the Canon EOS R in all three colours channels and has a 25% higher response at the all important H-alpha wavelength.

Bill Claff calculates some QE figures from the DxOMark data on PhotonsToPhotos.net and the difference I see between the Z6 and EOS R is broadly in line with what he calculates. It is sometimes claimed that DSLR and Mirrorless cameras have lower QE than dedicated one-shot-colour astro-cameras by erroneously comparing PhotonsToPhotos QE figures against the figures quoted by the astro-camera suppliers. But astro-camera suppliers are quoting peak response whereas the figures on PhotonsToPhotos are necessarily some kind of average response over the green channel.


Concentric Coloured Rings

After using the Nikon Z6 for a couple of successful imaging sessions, the next stacked image (Iris Nebula and nearby dust clouds) showed some strange background coloured banding, together with an obvious left/right discontinuity:



Interestingly, a similar pattern was observed in the corresponding pixel rejection map produced by the image stacking:



To make clear what I'm referring to, the positions of the coloured bands are highlighted in the image below:



After a lot of further testing, it became apparent that these coloured bands were produced by an undocumented hardcoded image correction applied to the raw data, which cannot be switched off. It occurs when the camera is used telescopes as well as with lenses, so it's not simply a lens correction.

It turns out that a large number of Nikon cameras are affected by the same issue.
More details can be found here: Hardcoded Image Correction

The only way to mitigate the issue is to use a combination of long enough exposure and high enough ISO so that the peak of the back-of-camera histogram is halfway across or slightly more than halfway. But even this solution is not perfect.


Sample Images

Here are a couple of successful Nikon Z6 images I have shot and processed. Click on them for more information and for larger versions:


Veil Nebula - even with this image the concentric rings and left/right discontinuity are just beginning to appear.


NGC7000 North America Nebula


Conclusion

The existence of the rings caused by Nikon's hardcoded image correction makes the Nikon Z6 unsuitable as camera for imaging very faint nebulosity and faint dust clouds.
To be fair, the Z6 may be fine for astro-landscape work but I still hesitate to recommend it because it is impossible to know when the concentric coloured rings problem might appear.

Last updated by Mark Shelley: 2 April 2023 (astro@markshelley.co.uk)