Sigma AF 18-125mm f/3.8-5.6 DC HSM OS (Canon) - Review / Test Report
Lens Reviews - Canon EOS (APS-C)

Lens kindly provided by Thomas König!

Introduction

The Sigma AF 18-125mm f/3.5-5.6 DC HSM OS is one of the more interesting releases in the extreme zoom range league - it tries to keep at least a little rest of sanity with its "mere" 7x zoom ratio. While it may look harmless on paper today but it's still equivalent to a whopping "28-200mm" on full format cameras - during the film era (not so long ago) this was about as extreme as it got. The zoom range is just one aspect here though. The Sigma also offers HSM (an ultrasonic AF drive) and, more importantly, OS - Sigma's version of an image stabilizers. Combine all that with an affordable price of around 300€/US$ and we seem to have a winner, have we ?

The build quality of the Sigma feels surprisingly good on first contact - that's quite typical for the recent Sigmas actually. The lens body is made of quality materials and there's no significant sign of wobbling despite of the duo-cam design - the two inner lens tubes extend when zooming towards the long end of the zoom range. The broad, rubberized zoom ring operates slightly stiff whereas the focusing feels very smooth. The tested sample did not suffer from zoom creeping (which may develop over time) but Sigma implemented a transport lock (18mm only) just in case. The front element does not rotate so using an polarizer remains easily possible. According to the lens name the max. aperture should give you f/3.8 @ 18mm but EOS cameras will report f/4 here.

As mentioned the Sigma uses a HSM AF drive. This may sound promising but this incarnation is actually about as noticeable acoustically as a conventional micro motor and it's not really much faster either. The AF accuracy was good during the field tests with the EOS 50D. Full-time manual focusing is not available.

Sigma claims a benefit of up to 4 f-stops for its OS (Optical Stabilizer). However, this seems to be a bit optimistic. You may be lucky at times with shutter speeds that slow but it's more realistic to reckon with about 2 f-stops in field conditions.
There's only a single mode OS-switch (on/off) but the lens is able to detect panning (object tracking) which is different compared to Canon's IS implementation.
The OS seems to have a little side effect though. You may be aware that the active OS/IS/VR produces a "humming" noise caused by the system which shifts the OS/IS/VR element to compensate shake. The Sigma has the strange behavior to do that in non-OS mode as well. Even more strange - the image gets "lifted" into position when switching on the camera. Obviously the OS system remains active to keep the OS element in a static position in non-OS mode. This behavior is "normal" and no defect. This unnecessary power drain may be somehow annoying but, frankly, most users will not disable the OS anyway.

Specifications
Equiv. focal length29-200 mm (full format equivalent)
Equiv. aperturef/6-f/9 (full format equivalent, in terms of depth-of-field)
Optical construction16 elements in 12 groups inc. 1x SLD and 3x aspherical elements
Number of aperture blades7
min. focus distance0.35 m (max. magnification ratio ~1:3.8)
Dimensions74 x 89 mm
Weight490 g
Filter size67 mm (non-rotating)
Hoodsupplied, petal-shaped, snap-on type
Other featuresOS (Optical Stabilizer)




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