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CAPTURING THE UNIVERSE
text and photography by tony puerzer
cHooSing a lenS
for aStropHotograpHy
is it possible to find camera optics that combine speed and quality with low cost?
S
peed, qualiTy, priCe: pick two.
you’ve probably heard some version
of the above phrase if you’ve ever
hired a contractor or had a mechanic work
on your car. Basically, it’s a variation of the
familiar adage “you can’t have it all.” and
yet when it comes to selecting lenses for
astrophotography, you really can have it
all—but only if you choose carefully.
balancing factorS
in astrophotography, optical speed is the
single most important factor. a “fast” lens
is one with a low f-stop number, which is
simply the ratio between its focal length and
42 SKYNEWS • NOVEMBER/DECEMBER 2017
WIDER THAN WIDE a fish-eye lens affords the widest possible view of the night sky, capturing
the dramatic sweep of the milky way. the canon 15mm fish-eye used for this photo offers a 180degree
diagonal field of view when mounted on a full-frame camera, such as the canon 6d. the
camera was set to iSo 3200 with the lens wide open at f/2.8. an ioptron Skytracker mount was used
to prevent the stars from trailing during the 2-minute exposure.
aperture. For example, a 28mm lens with an
aperture of 10mm would be an f/2.8 optic.
your digital camera probably came with an
inexpensive “kit” zoom lens with an f-ratio
that varies from f/3.5 to f/5.6. That’s fine for
general use, but if you’re looking to up your
astrophoto game, you’ll need to consider a
lens with an opening of f/2.8 or larger.
The lower the f-stop number, the shorter
the exposure you’ll need to record dim
objects. Keeping exposure times short is
essential if you want to use a fixed tripod
and avoid star trails. But such a lens has
important benefits even when your camera
is mounted on a tracking platform or an
equa torial mount. Generally, the longer the
exposure, the more likely you’re going to
get pictures that show errors from imprecise
polar alignment or slop in the mount’s
gears and motors. and since digital cam
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