Sky & Telescope - December 2024 - 71

cannot collect as many photons over
a given exposure time. As the signal
goes down, the all-important signalto-noise
ratio gets worse. For example,
I performed same-night tests with my
45-megapixel Canon R5, which I have
used with good success on nightscapes.
But, as expected, its images were
noisier at high ISOs. The extra detail
it could theoretically capture got lost
amid the higher noise.
ISO Invariance and
Dynamic Range
Early reviewers of the Z6III discovered
the camera suffers from a lower dynamic
range than previous models in the series.
This is the range of tonal values a camera
can capture, from the dark shadows just
above the noise floor, to just below the
point where bright highlights are clipped.
How can a new camera be worse than
earlier generations?
The answer is that camera makers
now value readout speed above other
key specs. The Z6III's sensor design is
said to be " partially stacked, " referring
to additional layers of signal
processing circuitry behind some of the
photosites to speed up readout times.
Its electronics can transfer a complete
24-megapixel image in just 14 milliseconds,
according to DPReview.com. Fast
readout speeds allow higher frame rates
in continuous modes (the Z6III can
fire up to twenty 14-bit RAW files per
second) as well as faster auto-focusing
during continuous shooting and higher
frame rates in videos. These are all useful
capabilities for wildlife and sports
photography and for capturing solar
eclipses. However, making a sensor
faster for high-speed shooters comes at
the cost of dynamic range for us slowmoving
astrophotographers!
The Z6III also has what is called an
" ISO invariant " sensor. With such a
design it's possible to shoot at a lower
ISO then stretch the results during
processing to get results similar to
shooting at a higher ISO. However,
despite the Z6III's ISO invariant sensor,
my testing on nightscapes revealed that
the camera has about one EV (Exposure
Value, equal to one f-stop) less dynamic
range than its predecessors. Where the
original Z6 could be underexposed by
-4 EV and still recover good details in
the shadows, I found the Z6III's limit
is an underexposure of no more than
-3 EV. With any greater level of underexposure,
boosting the shadows in
processing resulted in excessive noise
and image artifacts.
A Secret Base ISO
The negative dynamic range trait is
partly mitigated by a sensor specification
Nikon and other manufacturers
rarely publicize: what the camera's base
ISO settings are. Like most models, the
Z6III uses a base ISO of 100 then electronically
amplifies the signal to create
a brighter image at higher ISOs.
But for still images, a second set of
electronics kicks in at ISO 800, making
the Z6III's sensor " dual-gain " with a
second, higher base ISO. I demonstrate
this in the series on page 72 that starts
with a properly exposed 8-minute image
at ISO 100. I then cut the exposure time
down by 5 EV to 15 seconds for severe
underexposure. Brightening the image
in camera by increasing the ISO produced
awful results. Until ISO 800. At
this second base ISO the image suddenly
exhibits a huge improvement in quality
and lower noise, despite the image still
being underexposed by -2 EV. From
ISO 400
2 minutes
ISO 800
1 minute
ISO 1600
30 seconds
ISO 3200
15 seconds
ISO 6400
8 seconds
ISO 12800
4 seconds
ISO 25600
2 seconds
Each image in this starlit scene was shot at the same equivalent exposure. The dark ground
has had the shadows boosted by 100% in processing. As expected, noise gets worse with each
increase in ISO, notably so above ISO 6400.
ISO 3200
30 seconds
0 EV
ISO 1600
30 seconds
−1 EV
ISO 800
30 seconds
−2 EV
ISO 400
30 seconds
−3 EV
ISO 200
30 seconds
−4 EV
ISO 100
30 seconds
−5 EV
Beginning with a properly exposed shot at far left, each image progressing towards the right
was underexposed by -1 to -5 EV by lowering the ISO, then brightened during processing by +1 to
+5 stops to compensate. An underexposure of -3 EV proved to be the Z6III's limit when lifting dark
shadow content.
sk yand tele scope .o r g * DECEMBER 2024 71

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Sky & Telescope - December 2024

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