The Canon EF 17-40mm f/4 L USM
This lens was introduced in May or June (depending on where you live)
2003, and is one of Canon's "L" ("professional" lenses). This lens is
smaller than the Sigma, although the hood is quite bulky. On an EOS D30,
D60, or 10D, this lens acts like a 27-64mm f/4. It's not quite
as wide as the
"24-48mm" Sigma, but it's still a very useful range.
Like many of the recent L lenses, there is a rubber gasket around the lens
mount. While this is of most relevance to use on an EOS 1D/1Ds/1V with
their weatherproofing everywhere else, it does provide extra protection
against dust and moisture even for the D30/D60/10D. Some lenses
expand and contract during operation (for example the large "push-pull"
zooms) and thus can draw dust and moisture into them when used "in the
elements". Not so with this lens. Actually, the front element moves in and
out slightly during focussing, but this is inside the filter thread. I have a UV
filter permanently attached to mine to provide protection for the lens glass,
and this also seals the moving element in.
The controls on the lens are simple: the zoom ring, an AF/MF switch, and the focus ring. Like most USM lenses the lens offers
"FTM" (full-time manual focus override). The focus ring is not directly attached to the internal focus mechanism, but sends
electrical signals to the same mechanism controlled by the camera's AF. Once the camera has achieved focus via AF you can
tweak the focus simply by turning the focus ring. On my cameras I have set a custom function to move AF from the half-
pressed shutter button to a button under my thumb, and then can AF with my right thumb or MF with my left hand. This gives
me great flexibility, especially when using a telephoto lens, and having the same system on this lens makes things easy to
work with.
Comparing flare
The next photos compare the performance of these lenses in a fairly extreme condition.
I carefully set up this scene to try to
maximise the opportunity for flare
without actually having the sun in the frame.
The camera was set up on a tripod quite low to the ground, with a 420EX flash on a cord off to the left to fill in the shadows in
the foreground. The camera body was not moved between exposures or when the lenses were changed. Both lenses were
focussed at or close to their minimum focus distance.
Canon 17-40mm @ 17mm
1/125s, f/11
In this image you can see flare effects (yes, the 17-40mm can suffer from flare although
this is an extreme situation). A pink streak in the upper right, with some green haze below
it, and a green blotch in the lower left. Click on the image to see a larger version if you can't
make them out, and then use your browser's BACK button to return.
The sun was shining full on the front element of the lens, so the hood was hardly providing
any benefit. The lens had a Hoya HMC (multi-coated) UV filter attached. I left this attached
as this is meant to be somewhat of a "real world" test.
Sigma 15-30mm @ 15mm
1/125s, f/11
The extra hood was attached to the lens.
The perspective is slightly different here for two reasons. In the top half of the frame you
can see that we have a wider view due to the use of 15mm instead of 17mm, but in the
bottom of the frame you can see that we almost seem to have a narrower view. The cause
for this is simply that the 15-30mm is a longer lens than the 17-40mm, so the front element
is physically further forward into the scene.
We can see a few obvious examples of flare here: a large red blob in the lower left, and
blue and purple dots against the dark tree toward the upper right. As before, click on the
image to see a larger version if you can't make them out, and then use your browser's
(10 pages)
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