Etant équipé Pentax, j'ai de l'info dans mes archives et je possède un Sigma 18-200 loin d'être "pourri" et pas cher ; sinon entre temps sont sorti les 18-250 !
Voilà l'info :
(From Tamron lens literature) A lightweight, compact and ultra high power zoom lens designed exclusively for digital SLR cameras
with APS-C sized imager sensors. The AF 18-250mm f/3.5-6.3 Di-II LD Aspherical (IF) Macro is the ultimate high power zoom lens boasting
the world's greatest zoom ratio* of 13.9X, a milestone that Tamron, the pioneer of high power zoom lenses, has achieved by commanding its
technologies to further expand the capabilities of high power zoom lenses.
To prevent the lens from becoming bulky, the design concept of the AF 18-200mm f/3.5-6.3 XI Di-II LD Aspherical (IF) Macro (Model A14),
a popular lens among the world's digital SLR users since it is the ideal high power zoom lens, was the basis for this new lens that features
an expanded focal length to 250mm at its tele-end. With the new AF 18-250mm zoom lens that provides enhanced image quality,
Tamron has achieved an astounding 13.9X zoom power, the world's greatest in the class of zoom lenses; yet the increase in size is confined
to a mere 0.2mm more in its maximum diameter and just 0.6mm in overall length, in a lens that offers a 388mm ultra telephoto focal length
(diagonal angle of view of 6.38 degrees) when converted to the 35mm film format.
The new zoom lens covers up to a 250mm telephoto focal length, the longest focal length among zoom lenses that start with an 18mm wideangle
focal length available on the market for exclusive use with digital SLR cameras. Thus, it provides an overwhelmingly impressive telephoto effect
that is entirely different from the effect obtainable at 200mm on conventional high power zoom lenses. The new zoom lens will satisfy
the expectations of photographers that are used to the 300mm tele-end of Tamron's AF 28-300mm Di (Model A061) when used on a digital SLR camera.
* As of September 2006. Based upon Tamron's research of lenses for exclusive use with digital SLRs equipped with APS-C sized image sensors.
Note: Tamron has announced a motorized Nikon version of this lens compatible with the popular D40 and D40x digital SLRs. See the Tamron press
release for more details.
Test Notes
Long-ratio zoom lenses like the Tamron 18-250mm are frequently referred to as "vacation lenses," as that's arguably the situation where
you'd most want to cover the range from wide angle to telephoto in a single lens, without the bother of lugging multiple lenses along,
or changing them back and forth between shots. This Tamron 18-250mm takes the genre farther than ever before, with its exceptional 13.8x zoom range,
from a good wide angle to a pretty long tele. (Note that this is a digital-only lens, with a reduced image circle.
This means it can only be used on cameras with sensors having crop factors of 1.5x or higher. It will vignette strongly on full-frame cameras.)
It competes against other long-zoom designs, both in the form of the original Tamron 18-200mm, as well as the Sigma 18-200mm f/3.5-6.3
and the Nikon 18-200mm f/3.5-5.6 VR (vibration reduction) optic. While it's clear that a $500 ultra-zoom will never compete with higher-priced,
lower-ratio zooms, let alone prime lenses, we were pleasantly surprised by how well the Tamron 18-250mm stacked up against the competition.
Read on for all the details...
Sharpness
This was the area where we observed the most dramatic improvement over its 18-200mm predecessor. It shows noticeable corner softness across
its focal length range when shot wide open, but center sharpness is always surprisingly good, vastly better than that of the 18-200mm.
One notch down from maximum (representing a range from f/4 to f/8 across the zoom range), the corner sharpness improves significantly,
and almost the entire frame is surprisingly sharp from 35-50mm. At maximum tele, the corners never get really sharp, even stopped down to f/11,
but there's a sweet spot in the center that stays quite sharp, no matter what. Pretty impressive, particularly in light of the 13.8x zoom range
and street price of less than $500.
Chromatic Aberration
Chromatic aberration ranges from low to high, depending on where you're working within the zoom range. CA is highest at maximum telephoto,
somewhat lower at maximum wide angle, and best in between the extremes, and with the lens stopped down a notch or two. The Tamron engineers
appear to have split the difference in terms of CA and focal lengths: The 18-250mm does better than the earlier 18-200 at the ends of its range,
but not as good in the middle of its range. I think this was a good trade-off to make though, as it delivers reasonable performance across the entire
range, rather than a range from excellent to poor, as was the case with the 18-200mm.
Shading ("Vignetting")
Shading is a little high with this lens at the 18mm end of its range, hitting a maximum of 0.6 EV wide open, dropping to 1/4 stop from f/5.6 onward.
At all other focal lengths though, shading is never more than 1/3 stop wide open, and generally 1/4 stop or less as you stop down. All in all,
a very good performance in this respect.
Distortion
Distortion is perhaps this lens' weakest point, with a maximum value of 1.29% barrel at 18mm (among the highest we've seen). As is often the case
in long zooms, the barrel quickly shifts to pincushion as you zoom out, reaching 0.59% at 35mm, and then decreasing gradually at longer focal lengths,
finally flattening out at 0.36% at 200mm. These distortion figures are higher than we like to see, but the good news is that geometric distortion
is relatively easy to correct in Photoshop and other image-editing tools, for those really critical shots.
AF Operation
The Tamron 18-250mm uses a conventional focus motor (as opposed to an ultrasonic design), and the elements have a fair distance to travel when
the lens is zoomed to its telephoto position. As a result, the maximum AF slew time (the time to go from closest macro focus to infinity) is on
the order of a second and a half at telephoto (but only about a half a second at wide angle). This may not be your first choice if you're a dedicated
sports shooter, but it should be more than adequate for most other uses. (If you're using it to shoot your kids ball games, just learn to anticipate
the action, focusing ahead of time on the base they'll be sliding into, the goal they're heading toward, etc.)
The focus motor is quite audible, but I wouldn't go so far as to say that it's obnoxious. It's fairly typical of the non-ultrasonic motors I've heard.
While the Tamron 18-250mm works well enough in AF mode, like many modern lenses, it's less than stellar for manual focusing. This is because
the entire focus range from maximum macro to infinity is compressed into only about an inch and a half (a bit over 3cm) of focus ring motion.
This makes it very ticklish to set focus precisely when working manually. Not a show-stopper for most users, but well worth mentioning for those
who like to focus manually.
Macro
The Tamron 18-250mm carries a "Macro" designation, and it does surprisingly well at close focusing. The closest focus distance at 250mm is about 21cm
(8 inches) from the subject to the front of the lens element, at which point it captures an area about 73mm (~2.9 inches) wide on our Canon EOS-20D
test body. (A 1.6x crop factor camera.) That's a pretty small area, and the 8 inches of working distance would be very welcome in working with some
subjects.
Build Quality and Handling
Our sample of this lens operated quite smoothly, with virtually no play between any of the elements, and the zoom adjustment was nice and smooth.
You can definitely tell that it has a plastic rather than a metal body, but it always felt very solid when working with it. Pointing straight up
or straight down, the zoom ring is just stiff enough to keep the zoom from creeping in or out: Any vibration though, and it'll wander. Carrying it
on a camera slung about your neck with the lens barrel pointed down, you're likely to find it creeping out on you as you walk along, but there's
a small locking tab that you can slide into a recess on the lens body to keep it locked at its wide angle (retracted) position.
This is very decidedly an external-zoom lens, meaning the lens barrel extends a lot (79 mm, about 3.1 inches) as you zoom from wide angle to
telephoto. This is also an internal-focus design, which means that the front element doesn't rotate during focusing (or while zooming, for that matter),
a nice feature for use with polarizers and graduated neutral-density filters. A standard bayonet mount hood comes with the lens, doing a good job of
preventing flare at the wide angle end of the range, but a considerably poorer one at the telephoto end. (As you'd expect, any hood that doesn't crop
your images at 18mm is going to be almost entirely ineffective at 250mm.) Jim found the hood a bit of a bother to put on and take off, but it didn't
bother me that much. (And in practice, you'd rarely find a need to remove it anyway.)
All in all, this was a very nice-handling zoom, and one that felt very nicely balanced on our Canon 20D test body.
The Competition
As noted above, the Tamron 18-250mm competes with three other lenses currently on the market. These are: The original Tamron 18-200mm,
the Sigma 18-200mm f/3.5-6.3 and the Nikon 18-200mm f/3.5-5.6 VR (vibration reduction) lenses. Let's take a quick look at how the Tamron 18-250 mm
stacks up against each of these.
Tamron 18-200mm f/3.5-6.3 XR Di II LD Aspherical IF AF (~$350 street)
This is perhaps the easiest comparison to make, as the differences are pretty striking. Given its shorter zoom range, you'd expect the original
18-200mm lens to outperform the new 18-250, but that proved not to be the case. The 18-250mm beat its 18-200mm predecessor all hollow when it came
to sharpness. Wide open, the 18-250 was much better behaved, and sharpness was markedly better across the board. The 18-200mm did better in the
chromatic aberration department at medium focal lengths, but the 18-250mm was better at both ends of the range in that parameter.
Shading and geometric distortion performance was similar between the two lenses, but the 18-200mm did slightly better in both categories at
maximum wide angle. Looking mainly at the differences in sharpness between the two lenses, the 18-250mm is a significantly better optic, easily
justifying the roughly $100 difference in street price between them.
Sigma 18-200mm f/3.5-6.3 DC (~$310 street)
Sigma lenses have a well-deserved reputation for sharpness, and the 18-200mm f/3.5-6.3 DC follows suit, but the Tamron 18-250 edges it out for
wide-open center sharpness across most of their shared focal length range. Interesting things happen in the corners between these two lenses though.
As Jim points out in his Tanner Report, the average corner sharpness of the two lenses is virtually identical. That said though,
the Sigma's corner sharpness varies a lot more than that of the Tamron. While the Sigma is significantly better at some focal length ranges,
it's also radically worse at others. The average comes out too close to call, but the Tamron is more consistent when shooting wide open across
its focal length range. Stopped down, both lenses improve, but the Tamron is arguably flatter across the board, with the possible exception of 200mm
at f/11, where the Sigma is slightly flatter across the frame. The Sigma does win in the area of chromatic aberration though, particularly in
the middle range of focal lengths. Shading is very similar between the two lenses, the Sigma doing slightly better overall, while distortion numbers
favor the Sigma somewhat at 18mm, but the Tamron from 35mm on. All in all, a very close-fought race between the two lenses: I'd give the nod to
the Tamron for sharpness, but the Sigma is close behind, and has better CA over much of its range. The Sigma is also quite a bit cheaper,
with a street price as of this writing (June, 2007) a good $150 less than that of the Tamron. If you need the greater focal length range,
the Tamron wins hands down, and you can be happy about its somewhat greater sharpness. If you're budget constrained though, the Sigma will win out,
and you can be happy with its better CA performance.
Nikon 18-200mm f/3.5-5.6G IF-ED AF-S DX VR (~$890 street)
Here, we get into a rather different category of lens, as this Nikon lens incorporates Nikon's excellent VR (Vibration Reduction) technology,
to reduce the effects of camera shake when shooting at slow shutter speeds. This increases the lens somewhat, as does the higher (apparent)
build quality of the lens. The net result is a lens selling for street prices ranging from $800 to $1,100, as compared to the roughly $430 - $499
range of the Tamron. While the Nikkor optic wins on build quality and VR capability though, the Tamron very much holds its own optically.
Wide open, the two lenses are quite similar in their sharpness characteristics, with similar center sharpness and roughly equivalent corner
performance: The Nikon generally edges the Tamron on corner sharpness, but gets a little wild at 35mm. The average corner sharpness across the focal
length range probably favors the Nikkor slightly, but the differences are fairly slight. The Nikon also does better across most of its range
(at all but maximum wide angle) in terms of CA, sometimes bettering the Tamron's performance by a good margin. -
But when you look at shading behavior, the Tamron comes back into the ring and handily beats the Nikon lens. Turning to distortion for
the final round of the competition, Nikon beats Tamron (slightly) at 18mm, but Tamron beats Nikon (slightly) from 35mm on. All in all then,
the contest between these two lenses is nearly a tie optically. If you're a Nikon shooter, want VR, and are willing to pay for both it and
the Nikkor's higher build quality, then the Nikon 18-200mm f/3.5-5.6G DX is a great vacation lens. If you have less money and are willing to forego
the VR, the Tamron delivers very similar optical performance for almost half the cost.
Tanner Report
Jim delved into this lens and some comparisons with others in his inimitable, meticulous way, with charts and graphs galore. Where I focused
exclusively on other vacation lenses, Jim also took a look at how the 18-250mm stacked up against a Canon 18-55mm lens (the USM version of
the common 18-55mm "kit" lens) at the short focal length side of its range, and against a 70-200mm Canon "L" lens at the longer end.
No surprise, the L glass wins hands down, but the 18-250 beats the 18-55 in many characteristics, supporting its use as an all-in-one lens:
At least you don't lose anything in the 18-55mm range relative to the kit lens you might otherwise use. Read his Tanner Report for all the details.
Conclusion
We tend not to expect much from "vacation" zooms, so were pleasantly surprised by the Tamron 18-250mm f/3.5-6.3 Di II LD Aspherical IF Macro AF.
It maintained good center sharpness across its entire focal length range, softness in the corners was average to better than average, and
things flattened out nicely when we stopped it down just a bit. Other optical characteristics are in line with the rest of the field, and
its build quality was quite good for its price range. All in all, a very nice lens, a great lens to bring, if you're bringing only one!
PS : Suite à une opération "Carrefour", il y en a régulièrement (Sigma 18-200) sur leboncoin, encore garanti 1an 1/2 entre 150 et 200€
Ven 11 Fév 2011 - 23:38
mais je pense que les pentakistes pourront me renseigner 
) dans 2 mois pour un trek de 15 jours donc avec une capacité limitée pour trimballer mon matos...
not possible malheureusement. c'est bien sur lui que mon choix c'était porté au départ.
Tu as modifié ton post ; au début tu parlé de "Transtandard"...
D'ailleurs je suis en train de réfléchir à bidouiller un système, pour accrocher directement mon reflex aux bretelles du sac à dos ou à la ceinture. De cette manière, il serait facilement accessible, tout en étant porté par les épaules ou les hanches, au lieu du cou.
et à maudire les éléments 
avec 5-6 h de marche par jour au tonkin dans le nord vietnam) donc je peux mettre le matos sans que le poids soit un réel problème. la question est comment équilibrer le tout, avoir accès au matériel et qu'il soit protégé correctement (mais, ça se gère à condition d'y penser suffisamment tôt et de faire quelques essais. c'est pour les 15 prochains jours). 
ne seront pas de trop! 
