Image stabilizer
Both lenses have built in image stabilizer,
which compensates the hand shake in certain shutter speed. Canon EF-S
17-85mm f/4-5.6 IS USM provides the three-stop compensation; whereas
Canon EF-S 18-135mm f/3.5-5.6 IS gives an additional stop compensation, making it a
four-stop stabilization. To me, both lenses work well in low light situation
and enable clear shots in low shutter speed. In comparison, Canon EF-S
18-135mm f/3.5-5.6 IS triumphs in this area.
Closet focus distance
As neither of them are Macro
lenses, they both have long closet focus distance, making the magnification of the
object small. Canon EF-S 18-135mm f/3.5-5.6 IS has a 45cm closet focus
distance, which can be troublesome in occasions where you try to shoot a
close-up shot on nearby objects. You are likely to lean back and shoot the object from a further distance; Canon EF-S
17-85mm f/4-5.6 IS USM however, lets you shoot 10cm closer to the object, which makes a big difference. Therefore, Canon EF-S 17-85mm f/4-5.6 IS
USM’s 35cm closet focus distance enables higher flexibility shooting indoor or within limited spaces.
Ultrasonic motor (USM):
this technology allows fast and quite focusing, which is an exclusive technology from Canon, giving Canon EF-S 17-85mm f/4-5.6 IS USM the ability to
focus quickly and silently on the object, increasing the possibility of
capturing a moving object; on the other hand, Canon EF-S 18-135mm f/3.5-5.6 IS is not only slightly
slower, but also produces mechanical noises. That could be an annoyance.
Full-time manual focus
This feature allows users to rotate and adjust
the focus ring while the shutter button is half-pressed. For example, I focus on the subject by half-pressing the shutter, (similar to the normal camera) before
I fully press it (capture the image), while at the same time, i could adjust the focus distance to
my desirable spot by manually rotating the focus ring. It is useful in situations where I could not get an accurate focus point shooting a close object, thus adjustments
are needed. Canon EF-S 17-85mm f/4-5.6 IS
USM serves me well in this aspect while Canon EF-S 18-135mm f/3.5-5.6 IS lacks.
UD glass
 |
| Chormatic aberration of Canon 17-85 |
 |
| Chromatic aberration of Canon 18-135 |
Ultra-low dispersion glass is used primarily for correcting chromatic aberration. In my understanding, chromatic
aberration appears at the edge of the object when shooting against light. It
usually appears as the purple colour("purple fringing") along the contrasting light. Based on the sample pictures, Canon
EF-S 18-135mm f/3.5-5.6 IS has better control of chromatic aberration as it is equipped with the UD glass.
Verdict
In my opinion, both are great as entry level lenses. They
have their own strengths and weaknesses, but perform as well as the other. Canon EF-S 18-135mm f/3.5-5.6 IS
has longer zoom range and better technology as it was released in 2009. While Canon EF-S 17-85mm f/4-5.6 IS USM features the more advanced Ultrasonic
motor despite being released earlier in 2004. The USM focus system enhances the photo shooting experience in sports and low light situations; in addition, the low noise level could be beneficial for video recording. Personally, I think Canon EF-S 17-85mm f/4-5.6 IS USM is the better choice.
