GettyStargazing lenses Group Test
HOW WE TESTED THESE LENSES
In addition to our usual range of real-
world and lab-based testing, we looked at
aberrations that affect the performance
of lenses for astrophotography at their
widest available apertures. Towards
the corners of the image frame, these
include vignetting; ‘coma’, which gives
stars and other celestial bodies the
appearance of having a comet-like
tail; and ‘astigmatism’, which
turns dots of light into lines.
Coma and astigmatism together
can give an irregular shape known as
‘batwing coma’. Spherical aberration can
occur throughout the image frame, giving
points of light a soft halo effect. All of
these aberrations tend to be greatly
reduced when you narrow the aperture
by an f/stop from its widest setting.
For zoom lenses, the results of lab tests
shown on the following pages highlight
performance at their shortest focal
length. This is generally their preferred
zoom setting for astrophotography,
and the closest equivalent to the focal
lengths of the prime lenses on test.