Sky and Telescope - February 2017 - 66

S&T Test Report

With both stacking modes, the result
is a single 14-bit RAW ﬁle, in either
Pentax's proprietary PEF format or in
Adobe's more universal DNG format.
Pentax is unique in offering DNG as a
ﬁle format choice.
These in-camera compositing modes
worked great. Only Olympus and Sony
(the latter through its camera apps)
have anything similar; Canon and
Nikon do not. However, these functions are buried deep in the K-3 II's
menus, and their scant descriptions
in the instruction manual make their
beneﬁts for night-sky shooting less than
obvious. I suspect most beginners would
never discover them.
S The K-3 II's Interval Composite: Bright mode created this star trail automatically by shooting
and stacking fifty 30-second exposures in the camera, producing a single RAW file as a result.

2,000 frames at shutter speeds up to 30
seconds. At that exposure, setting the
interval to 33 seconds ﬁred the shutter as quickly as possible, minimizing
gaps in the trails. The Pentax stacks the
images on the ﬂy as it shoots.
While this "star-trail" mode worked
very well, it produces an image with
trails of uniform brightness. Specialized stacking programs and Photoshop
actions can produce striking "comet
trail" effects, with trails that taper in
brightness from beginning to end by
stacking images at decreasing opacities.

mode did indeed exhibit much less noise
than did single images, identical to
what's possible by stacking images later
in processing.
While Average mode stacking can't
be used with Astrotracer, you can stack
multi-minute exposures taken with
the camera riding on a conventional
tracker. In that case exposures must be
timed by an external intervalometer set
to take the same number of images as
you've told the camera to stack.

Pixel Shifting
A more publicized feature is what Pentax calls "Pixel Shift," a feature unique
to Pentax. It's similar to what astrophotographers do when drizzle-stacking
images, only the camera is doing it
automatically and precisely. No need to
align exposures yourself.
With Pixel Shift turned on the camera uses its electronic, not mechanical,
shutter to take four exposures, shifting
the sensor by one pixel in each of the
four directions. This allows a pixel of
each color in the sensor's Bayer ﬁlter array to sample every point in the

In-Camera Compositing
The Interval Composite stacking mode
also has an "Average" option, to mathematically average the images as they
stack internally. It's the same technique
astrophotographers use to reduce noise.
In this case, it's done in the camera.
This function allows shooting and
stacking several images to smooth
noise in untracked nightscapes, perhaps for an image that contributes the
ground in a ﬁnal composite. In that
case, the sky would need to come from
a separate single image, possibly taken
using Astrotracer. However, Astrotracer
cannot be used in conjunction with the
Interval Composite mode to internally
stack "astrotraced" tracked images.
Images stacked with the Average
66

S Using the Interval Composite: Average mode (left) to internally stack eight 30-second ISO 3200
exposures produces a much smoother image than the single 30-second exposure at right.

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

Sky and Telescope - February 2017

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