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Testing a lens can be fun an informative,
but it is not that easy: you have to be very
careful if you want to get reliable results,
and you need a lot of patience. In this
article, I have explained how I review a
lens, why I give more importance to some
aspects rather than others. This page is
useful both to understand better my reviews,
and to help you if you want to do your own
tests!
Even though the basis of my review
process are always the same, in these years
I have made many small improvements to make
the reviews more precise, useful and
informative. The information in this page
describes the review system I use from late
2009.
Specifications
Specs are useful to get an overview of
the lens. Some notes: there are mainly two
types of AF, the "Ultrasonic AF motor" (very
fast and silent) and the old "AF motor",
that is slower and it does not have full
time manual focus. In the comparison table,
I always list the type of AF actually used
by the lens, regardless of the manufacturer's
claim: many manufacturers are not very
honest about this aspect, and they call
"Ultrasonic" even lenses that use the cheap
"micro-USM" AF motor, that is
identical to the old non-USM AF motor. For
example, the Canon 300mm f/4 L IS USM has
the true ultrasonic AF motor, while the
Canon 70-300mm f/4-5.6 IS USM has the cheap
non-USM AF motor.
The production field lists the year of
introduction of the lens and, in case of
lens that are out of the market, the
year of discontinuation. The date can be
useful if you are evaluating the purchase of
the lens, because old lenses are more likely
to be replaced soon by a new version than
recent lenses. The prices are in United
States dollars and they are based on the B&H
prices (www.bhphotovideo.com).
Description (built
quality, autofocus and stabilization)
In this paragraph I describe the main
aspect of the lens, and I compare its
features and built quality with other
similar lenses. I don't do in-depth tests of
autofocus or image stabilization because it
is nearly impossible to make a "scientific"
evaluation of these aspects; my
consideration are based on my experience
with these lenses. (when I testing some
lenses, I try to use them as much as
possible, to get an idea of their AF and
IS).
The postage stamps
test: Sharpness, contrast, chromatic
aberration
Sharpness is one of the most important
aspect of image quality, for this reason I
carefully test sharpness (and contrast) of
every lens I test, and I publish 100% crops.
These crops are useful even to evaluate
chromatic aberration, too, but this is not
as important: CA is one of those things you
can correct with PS, while you can't do much
for sharpness. (sharpening helps a bit, but
it won't turn you 70-300 in a 300 2.8!)
To test the sharpness, I use a 24x36
centimeters test target covered with postage
stamps that have fine detail (for long
lenses), while for wide-angles I prefer to
use a much wider test target, that is about
3 x 4.5 meters. It does not make sense to
test a wide angle with a small test target
because the majority of times you will use
it for wide views, so I think a 3 x 4.5 meters
target provides a more accurate result.
Standard zooms and tele instead are often
used even for relatively small subjects
(portraits, birds) so an approximately A4
test target is ok.
Of course the test target must be flat
and it must be perfectly parallel to the
sensor! I focus carefully using live view
and 10x magnification, and I take the photos
using a stable tripod, mirror lock up and
self timer, to avoid any vibration that may
invalidate the results.
I always repeat this test at least two
times, to be sure there are not focus errors
or other imprecisions. Currently I use two
cameras for the tests: a Canon 1Ds III
(fullframe, 21 megapixels) to review the
majority of FF lenses except teles, and the
Canon 7D (APS-C, 18 megapixels) to review
APS-C lenses and teles. I prefer to use the
7D for long lenses because the majority of
times these lenses will be used on a crop
body, and usually when you take a photo with
a tele the corner performance does not
matter much, while the sharpness in the
center is important.
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One of
my wide-angle test targets |
One of
my "long lens" test targets |
In the majority of reviews, I compare
the lens with other similar lens: it is
very useful, because it allows to see
how the lens compares with other lenses
that have similar specification, so you
can evaluate the alternatives. Other
than that, many times it allows to see
if a pro lens is really worth the money
or if it is overpriced.
Distortion and
vignetting
Sometimes (for wide-angles) I add a test
of distortion, but personally I don't give
much importance to this aspect, you can
easily correct it with Photoshop. The same
for vignetting: it is easily correctable
with PS, so I don't give much importance to
this aspect.
Flare
Flare is one of those things you can not
correct with PS, so it may be important in the
evaluation of a lens. While for teles it is
never a problem, with standard zooms and
wide-angles it can make a big difference, in
particular if you take often photos that
include the sun. That said, it is very
difficult to make a good, scientific test of
flare, so generally I don't include a test
of flare in the reviews.
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