Sky and Telescope - February 2017 - 67

S A rule of thumb is that a camera's realistic upper limit is two ISO settings below the maximum the camera can provide. The Pentax can shoot at ISO
25600 (far left), but in practice ISO 6400 is its top end, producing good image quality in nightscapes where high ISOs are often required.

image. This provides increased resolution, because luminance information
is now sampled at each pixel position
and not synthesized from a blend of
adjacent pixels as usually happens when
"de-Bayering" RAW data from the sensor. Noise also goes down, because four
images are being merged to create a
single image.
In test shots of night scenes, I did
ﬁnd lower noise in pixel-shifted exposures versus single shots, and slightly
better resolution. The drawback to using
Pixel Shift is that most RAW developing
software (such as Adobe Camera Raw or
Lightroom) will produce odd checkerboard artifacts on any parts of the image
that moved during the four-shot acquisition. RAW ﬁles are also four times the
size, up to 130 megabytes each.
For deep-sky photographers, this incamera drizzle-stacking sounds attractive, but unfortunately it only works on
exposures up to the camera's internal
maximum of 30 seconds. It doesn't
work with Astrotracer shots or on long
exposures taken using the Bulb setting.
At best, Pixel Shift is useful for night-

scapes, for increasing resolution and
decreasing noise in the foreground, but
only if nothing is moving. It's not for
moonlit shots of waterfalls!

Noise Performance
The K-3 II has tiny 3.9-micron pixels,
which are great for high resolution
but risk high noise. However, I was
pleasantly surprised to ﬁnd that the
K-3 II exhibited no worse noise than
the 18-megapixel Canon 60Da with its
larger 4.4-micron pixels, providing very
good results at ISO 3200 and acceptably
low noise even at ISO 6400.
However, the Pentax, like Canon
DSLRs (but unlike Nikons and Sonys),
does not have what is called an "ISOinvariant" sensor. Images have to be
properly exposed for the best signal-tonoise performance.
Even so, while noise in the Pentax was
higher than in full-frame cameras with
larger pixels, the cropped-frame Pentax
still performed very well at high ISOs
and in long multi-minute exposures.
One oddity was that the Pentax's
Slow Shutter Speed Noise Reduction

SThe K-3 II's Pixel Shift mode moves the sensor by one pixel over four exposures to improve
resolution and reduce noise in exposures 30 seconds and shorter. However, moving targets such
as stars can exhibit pixelated artifacts in RAW files, either when developed in Adobe Camera Raw
(left), and to lesser extent even in Ricoh's own RAW developing software Digital Camera Utility that
comes with the camera (middle).

option took internal dark frames that
were consistently only 75% of the duration of the light frames, compromising their ability to accurately reduce
thermal noise.
Nevertheless, the in-camera darks did
help eliminate hot pixels. But shooting a
dark frame separately that was the same
length as the light frame, then subtracting it later in processing, did a better job
of cleaning up most thermal artifacts.
I also found that the Live View function so critical to focusing astronomical
images presented quite a dark image.
There seemed to be no "Exposure Simulation" function as other cameras have
that brightens the view in dim scenes.
Pentax's Live View requires a very bright
star or planet to focus on.
Perhaps the sensor-shifting functions were responsible, but on my test
nights I found I could get only about 90
to 120 minutes of continuous shooting out of a battery, compared to 3 to 4
hours with other cameras.
Despite its drawbacks, the Pentax K-3
II offers an attractive array of unique
features useful for astrophotography.
If the Astrotracer function had worked
more accurately, the camera might be
attractive enough even for non-Pentax
owners, especially for shooting nightscapes. As it is, I would consider this a
camera of interest ﬁrst and foremost to
Pentax fans wanting to try their hand at
astrophotography.
■ A member of The World at Night
(TWAN), Contributing Editor ALAN DYER
is author of the ebook How to Photograph & Process Nightscapes and TimeLapses, available on his website at amazingsky.com/nightscapesbook.html.

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* FEBRUARY 2017

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Sky and Telescope - February 2017

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