đánh giá sigma 17-50 f2.8


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đánh giá by Klaus Schroiff, published July 2010IntroductionOver the recent years Sigma struggled to take off in the fast standard zoom aremãng cầu for APS-C DSLRs. This segment has been dominated by the Tamron AF 17-50mm f/2.8 (VC) with its highly attractive sầu price/performance ratio and the all mighty but expensive sầu Canon EF-S 17-55mm f/2.8 USM IS owning the unique crown. However, it may make sense to lớn rethink this order of things now - Sigma just released their all new AF 17-50milimet f/2.8 EX DC HSM OS. That alone may not be all THAT exciting - we"ve sầu seen many fast standard zoom lenses by Sigma come & go - but this one is a bit more special. Recently Sigma announced a couple of lenses featuring their brvà new FLD glass và the new 17-50mm f/2.8 OS is aý muốn them. FLD glass has an optical characteristic similar to lớn fluorite glass. Such glass elements can be used to compensate optical aberrations (defects) more efficiently than conventional "special" elements such as Sigma"s more commonly used SLD glass. We were already very impressed by the performance of the Sigma AF 8-16mm f/4.5-5.6 DC HSM which also benefits from FLD elements so there"s some well founded hope that the AF 17-50milimet f/2.8 EX DC HSM OS stands out from the ordinary as well.
The build unique of the Sigma lens is pretty good although it stays a little short of the high expectations that we associate with a Sigma EX ("Excellence") lens. It"s made of tightly assembled combination of metal and high quality parts so there"s nothing wrong here. Like its predecessor (AF 18-50milimet f/2.8 HSM DC EX macro) it uses a mono-cam system (one inner lens tube) khổng lồ extkết thúc the lens when zooming towards the tele setting. The front element does not rotate The zoom ring has a smooth, pleasant action whereas the focus ring has basically no friction in manual mode. The focus ring is coupled lớn the AF mechanism so it rotates during autofocus - this is somewhat disappointing for a high over product actually.
The 17-50milimet f/2.8 EX DC HSM OS features the slightly simplified version of Sigma"s HSM ("hypersonic AF motor"). It is still fast & nearly silent but full-time manual (FTM) override is not possible in one-shot AF mode. The AF tốc độ via in contrast AF (Live-View) is on the slow side. Manual focusing is delicate due khổng lồ the extremely short focus path - it just takes a turn of ~30 degrees lớn focus from infinity khổng lồ the cthất bại focus limit. This approach improves the AF tốc độ for sure but you need khổng lồ have sầu some haptic skills if you want lớn do it all manual. The focus accuracy of the tested sample was fine on our demo DSLR.A major new feature is, of course, the OS ("Optical Stabilizer"). As already reported in previous Reviews it is impressively efficient & as good as the corresponding genuine manufacturer implementations. Sigma claims a gain of 4-stops which seems to fit here based on our field impressions.Finally here"s a comparison of the "gang of three" fast standard zoom lenses with image stabilization support:
Sigma AF 17-50mm f/2.8 EX DC HSM OSCanon EF-S 17-55milimet f/2.8 USM ISTamron AF 17-50mm f/2.8 SP XR VC Di II ASPH
equivalent full format focal length and aperture (depth-of-field)"27-80mm f/4" "27-88mm f/4" "27-80mm f/4"
Special Elements2x FLD + 2x aspherical2x UD + 2x aspherical2x XR + 2x LD + 2x aspherical
Image Stabilizer Efficiency (official)4 f-stops3 f-stops4 f-stops
Aperture blades7 (circular)7 (circular)7 (circular)
Min. focus (magnification)0.28m (1:5)0.35m (1:6)0.29m (1:4.8)
AF motorHSM (ultrasonic) without FTMUSM (ultrasonic) with FTMconventional micro-motor without FTM
Filter size77mm77mm72mm
approx. Price (EUR)~650€/US$~850€/US$~450€/US$

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