The Canon Magazine 107
astro lenses
Centre Edge Corner2500200015001000500
Sharpness
f/1/8f/2.8 f/4 f/5.6 f/8 f/11 f/16f/22TokINA AT-X 11-20mm
f/2.8 AF Pro DX £529/$469
A rarity among APS-C formats, the Tokina combines
ultra-wide viewing angles with a fast aperture
ltra-wide zoom lenses
with fast aperture
ratings for APS-C
format cameras are few
and far between. The main
competitor to this lens is
Tokina’s own AT-X 14-20mm
f/2 AF Pro DX, which is
pricier, has a faster aperture
rating and delivers impressive
quality for general shooting
and astrophotography.
However, its max viewing
angle is restrictive compared
with this 11-20mm offering.
The sophisticated optical
path includes two all-glass
moulded elements and three
SD (Super-low Dispersion)
elements. The autofocus
system feels basic, powered by
an electric motor that’s rather
noisy, but not lacking in speed.
As with most Tokina lenses,
there’s a ‘One-touch Focus
Clutch Mechanism’. This relies
on you snapping the focus ring
forwards or backwards to
engage AF or manual focus
respectively. It works well as
an alternative to the manual
override offered by most
alternative autofocus systems.Typical of Tokina’s ‘AT-X
Pro’ lenses, build quality feels
very solid and robust, and
there’s a weather-seal ring on
the mounting plate. The zoom
and focus rings operate
smoothly, the latter including
a focus distance scale.Performance
Sharpness is good but drops
off a little more than usual in
the corners when combining
the shortest zoom setting and
widest aperture. It’s similar to
the Tamron 15-30mm in this
respect. Vignetting, distortion
and spherical aberration are
well controlled, and there’s
little coma and astigmatism.FeATUres
VerdIc T
01
The design includes
an 82mm filter
thread and
detachable hood.02
Two moulded glass
elements and one
precision-moulded
element included.03
The lens has Tokina’s
typical push-pull
focus ring for
swapping between
auto and manual.04
AF is driven by a
relatively basic
electric motor.05
The mounting plate
has a rubber
weather-seal ring.FeATUresBUIld & hAndlIngPerFormAnceVAlUeoVerAll01030204U
tand on the equator and you’re travelling at 1000mph, even when you’re
standing still. That’s the speed that the Earth spins on its axis. Similarly,
the stars are constantly moving across the night sky, so long exposures
will turn them into streaks of light. It’s great for creating star-trails but a major
problem when you want stars to retain a natural appearance.
The governing factors are the length of the exposure and the focal length of the
lens. The ‘500 rule’ acts as a rough guide. The idea is to divide 500 (300 for
APS-C cameras) by the focal length or zoom of your lens. The result gives the
longest exposure time that you should use, measured in seconds. For example, it
works out to about 20 or 30 seconds with a 24mm or 16mm lens, on a full-frame.THE 500 rULE
This astronomical rule of thumb helps to deliver streak-free images
S
Use the number 300 instead of 500 with an APS-C
camera, to account for the crop factor, and adjust your
ISO setting to enable a sufficiently short exposure05Denys Bilytskyi