Sigma 18-50mm f/2.8-4.5 DC OS HSM

ePHOTOzine's expert Gary Wolstenholme takes a long look at one of Sigma's zoom lenses.

The Sigma 18-50mm sports a wider aperture than many 'kit' lenses and optical stabilisation, so may be a popular choice for those looking either for a direct replacement for such a lens, or an upgrade. It is a little larger and heavier than most kit lenses, possibly due to the faster aperture of f/2.8. This is two thirds of a stop faster than the typical f/3.5 aperture found on many lenses of this level and will help being able to take pictures in low light conditions. The optical stabilisation feature also promises to help with low-light shooting by taming camera shake.

Sigma 18-50mm f/2.8-4.5 DC OS HSM: Features and handling
This lens is largely constructed from plastic with a brass lens mount. The exterior has a powder coated finish typical of Sigma lenses. This finish adds a quality feel to the lens when new, but in my experience it soon marks with normal use. Much of the lens barrel is taken up by the zoom and focus rings, which have nice wide rubber grips on them. The focus ring rotates during autofocus and can sometimes catch fingers as it whizzes back and forth..

Zoom and focus are performed internally, which took me by surprise – a pleasant one. The length of the lens does not change, ever, and the front does not rotate, which makes it perfect for those using filters. The zoom ring is nice and light to adjust and because everything is done internally, there are no problems with the zoom creeping forward when pointed downwards. Focusing is quick enough due in part to the HSM motor, although it's not silent, due to the movement of the focus ring.

Most will find the handling of this optic better than standard 18-55mm kit lenses, but I find some of the manufacturer's own offerings at a similar level handle better, such as Nikon's 18-105mm, which costs around the same. Pentax users have the 16-45mm f/4 available for a few pounds more than this lens, but that lens is one full stop slower than the Sigma at the wide end. Canon and Sony users will find this lens more compelling, as there currently isn't much or anything available at this price point with these features for them.

Sigma 18-50mm f/2.8-4.5 DC OS HSM: Performance

For this review, the lens was tested on a 10Mp Nikon D80 using Imatest.

At 18mm, the Sigma 18-50mm turned out good resolution figures in the centre of the image, with the corners improving as the lens is stopped down to f/11. The sweet spot for image quality is at f/8 for this lens at the wide end. As the lens is zoomed in, the sweet spot moves to wider apertures as this lens appears to suffer greatly with resolution reducing diffraction at small apertures. F/5.6 gives the overall highest resolution between 35mm and 50mm.

Resolution at 18mm

Resolution at 35mm

Resolution at 50mm

Light falloff towards the corners of the image area are fairly well controlled, with the corners being 1.5 stops darker than the image centre at 18mm and f/2.8, stopping down to f/5.6 evens things out nicely. At 50mm and f/4.5, the amount of falloff is roughly the same and this disappears by about f/8.

Chromatic aberration at 18mm

Chromatic aberration at 35mm

Chromatic aberration at 50mm

Distortion is quite well controlled with only 2.5% barrel distortion being recorded at 18mm and only 1.22% pincushion at 50mm. This lens is very well corrected, and will not cause many issues for shots with dominant straight lines.

Building at 18mm.

Building at 50mm.

When shooting images with strong point sources of light in the frame (such as the sun or street lamps), flare can be an issue with this lens, with strong flare patterns extending across the image centre. With light sources out of the frame, there is very little ghosting or flare. The supplied petal-shaped hood further improves this aspect of performance, so I would be inclined to leave it on all the time.


Sigma 18-50mm f/2.8-4.5 DC OS HSM: Verdict
Overall, this lens isn't a bad performer and will suit those looking either for a replacement upgrade to their standard 18-55mm kit lens, or a solid starter lens that covers the standard focal ranges.

Nikon and Pentax users have an alternate choice at a similar price point available from the manufacturer, but still may find this lens is worth a look, especially if shooting wide at 18mm in low light conditions is their priority. Canon and Sony users will find this lens fits into a gap in their manufacturer's lens offerings and may well be a good value upgrade for them.

FEATURES
HANDLING
PERFORMANCE
VALUE
OVERALL

Sigma 18-50mm f/2.8-4.5 DC OS HSM: Pros
Internal focus and zoom
Optical Stabilisation
Good resolution at wider apertures
Low distortion
Sigma 18-50mm f/2.8-4.5 DC OS HSM: Cons
Fingers tend to catch on rotating focus ring
Resolution falloff at smaller apertures

Sigma 18-50mm f/2.8-4.5 DC OS HSM: Specification

Price	£250
Contact	Sigma Imaging
Filter size	67mm
Format	APS-C/DX
Construction	16 elements in 12 groups
Angle-of-view	69.3° - 27.9°
35mm equivalent focal length	27-75mm (Nikon/Sony/Pentax), 28.8-80mm (Canon)
Internal focusing	Yes
Image stabilisation	Yes
Minimum focusing	30cm
Maximum aperture	f/2.8-4.5
Minimum aperture	f/22-32
Weight	395g
Size	88.6x74mm
In the box	Petal shaped lens hood

The Sigma 18-50mm f/2.8-4.5 DC OS HSM lens costs around £234 and is available from Warehouse Express here:

Sigma 18-50mm f/2.8-4.5 DC OS HSM

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