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The maximum magnification of the Tamron SP AF17-50mm F/2.8 XR Di-II LD
ASPHERICAL [IF] is :1:4.5 or 0.22x and is obtained at 50mm. Not bad for a wide to normal
zoom, but not really exceptional. The Canon EF-S 18-55 is slightly better at 0.28x and the
Canon EF-S 18-85 is slightly worse at 0.2x. Obviously the Tamron SP AF17-50mm F/2.8 isn't
a "macro lens", but 0.22x is still useful for closeups.
The above image was shot at the minimum focus distance at the 50mm focal
length setting
Benefits of a fast lens
Though the tests above concentrate on technical image quality, there are several
benefits of a fast lens over a slow lens which aren't related to lp/mm or MTF
measurements!
The first is the ability to shoot at faster shutter speeds. While slow lenses with
Image Stabilization can often be handheld at slower shutter speeds than would be possible
with a faster lens, those slower shutter speeds may not be fast enough to freeze movement.
For example let's say we have a Canon EF-S 17-85/4-5.6 IS and a Tamron SP AF 17-50/2.8.
The IS gives you a 3 stop advantage in terms of the ability to handhold the lens, but the
f2.8 lens is one stop faster. This means that at 17mm and f4 you can probably handhold the
IS lens at a shutter speed of about 1/4s and get sharp images. Under the same lighting
conditions you'd be shooting at 1/8s with the f2.8 lens at f2.8 and that's too slow for
sharp handhold images. You'd really need to be shooting at about 1/30s with a
non-stabilized lens to be reasonably certain of getting a sharp shot. So in this case, for
a static subject, the IS lens would likely yield a sharper image.
On the other hand, let's say you're shooting action at 50mm. While the IS lens might
let you shoot at, say, 1/20s at f5.6, the non-stabilized lens would allow you to shoot at
1/80s at f2.8. For a moving subject, 1/20s might be too slow to freeze movement (resulting
in a blurred image), while 1/80s might freeze the action enough to give you a sharp shot,
so in this case the faster (f2.8) lens might yield the sharper image.
The second advantage of a faster lens is the ability to shoot with a smaller depth of
field and to better blur any distant background (note these two effects, while similar,
are not the same). Below is a series of images shot at f2.8, f4 and f5.6. As you can
clearly see, the background is far less distracting in the f2.8 shot than in the f5.6 shot
due to the greater degree of background blur. If you're shooting portraits (and a 50mm
f2.8 lens on an APS-C DSLR makes a good portrait lens), the additional background blur can
make a significant improvement in your images.
At f2.8 the background is blurred and it's hard to tell what the red
object is!
At f4 you can see more background detail and things are coming into focus
slightly,
At f5.6 the background is becoming slightly distracting and you might
guess that the red object is, in fact, a red wheelbarrow standing on end!
Conclusions
The Tamron SP AF17-50mm F/2.8 XR Di-II LD Aspherical [IF] is a good lens. It's pretty
sharp at all focal lengths, even wide open, but contrast does go up a notch when stopped
down from f2.8 to f4. Chromatic aberration and distortion are generally well controlled,
though there is noticeable barrel distortion at the wide end of the zoom range. Chromatic
aberration is worst at 17mm, though you can see from the test image above that even at
17mm in the corners of the frame it's pretty good. At longer focal lengths you'd need to
look quite hard to see any problems. The Tamron SP AF17-50mm F/2.8 did exhibit some
flaring with the front element in direct sunlight or the sun in the frame, but that's a
pretty severe test. Under normal conditions, with the sun out of the frame and using the
supplied lens hood, flare seemed well controlled.
At $450 it's quite reasonably priced, especially if you compare it with the Canon EF-S
17-55/2.8 IS USM ($1200). Of course it doesn't have Image Stabilization, but it is $750
cheaper! It's closer in price to the EF-S 17-85/4-5.6 IS USM ($550). Optically the Tamron
SP AF17-50mm F/2.8 XR Di-II LD Aspherical [IF] is better than the Canon EF-S 17-85/4-5.6
IS, and though 1-2 stops faster, it lacks IS and the zoom range is smaller. I
haven't yet had a chance to look at the Canon EF-S 17-55/2.8 IS USM, but at $1200 it would
have to be really good to justify the price.
As usual the decision about which lens to buy is a matter of compromise. You can have
good, fast, optics, you can have IS, you can have a lens for about $500, but you can't
have all of them in a single lens! If you're after high optical quality, a fast lens
which can blur backgrounds and stop action but you don't want to spend more than about
$500, the Tamron SP AF17-50mm F/2.8 XR Di-II LD Aspherical [IF] is a lens you could be
very happy with.
