Sigma 10-20mm f/4-5.6 EX DC HSM Review

There are currently few lenses 
on the market that can match the Sigma 10-20mm’s impressive ultra wide field of view. It’s designed specifically for DSLRs with crop-type sensors, in other words those with an APS-C size sensor – typically with a multiplication factor of 1.5x or 1.6x.
It’s recently been introduced for Four-Thirds SLRs too. As you’ll most likely be aware, the multiplication factor of APS-C type sensors is useful when shooting distant subjects, like wildlife and action, but is a major drawback when shooting scenery and architecture. To address this, a new breed of wide-angle zooms has emerged and this Sigma zoom is one of them, offering a particularly wide field-of-view, giving a focal range equivalent of 15-30mm on my Nikon.
Because this lens is only compatible with DSLRs with an APS-C type sensor, it would cause vignetting throughout its range if attached to a full-frame model. To field test the lens, I used my Nikon D300 and D200 DSLRs. The first thing that I noticed when handling the lens is that considering its 
wide focal range, it is remarkably compact and, whilst it feels solid and well made, 
it isn’t particularly heavy. The zoom ring is marked at 10mm, 12mm, 14mm, 17mm 
and 20mm and zooming extends the lens length by barely 5mm.

Focusing is 
achieved using a fast and quiet HyperSonic Motor – the elements don’t have to move far, so focusing is practically instant. The filter size is 77mm, which is comparatively small for an optic this wide and internal focusing means the barrel doesn’t rotate during AF, which is a bonus for filter users.
The front element doesn’t protrude, making it easy to attach filters, either directly or via a holder. This is very relevant, 
as the majority of potential buyers will probably require this focal range in order to 
shoot scenic images and, therefore, will want to attach a filter holder.
Vignetting is a major concern when using super wide angles. Realistically, an 84/85mm filter holder – like the Cokin P system – is too small for the diameter of this lens. I use a 100mm Lee Filters holder, but vignetting was still apparent at 10mm – even using a dedicated wide-angle adapter ring. However, at 11mm vignetting disappeared, so it is obviously a very fine line. If you buy this lens and intend to use filters, I would suggest you do your own test shots. This is because, unless your viewfinder boasts 100% coverage, any darkening of the corners may go unnoticed.Optically, this lens is impressive. Chromatic aberrations – a common problem with ultra-wides – are well controlled with the help of three Special Low Dispersion (SLD) elements. Images are consistently sharp and crisp, betraying its focal range and price tag.
This lens has clearly been optimised for use at its widest end, as this is the range where it performs best. I found that at around 15mm, there is some softening at the edges and at 20mm slight pin-cushioning is noticeable, although it is not significant. At 10mm, there is minor barrel distortion, but very good edge to edge sharpness. As you might expect, at its widest end, the lens does need to be used with care to limit the effect of perspective distortion.
This form of distortion is unavoidable when using such wide focal lengths, creating the impression that elements at the far edges of the frame, like buildings and trees, are leaning. The effect of converging verticals is exaggerated if you point the camera upward or downward.

However, this effect isn’t always undesirable and by using the wide end of the 10-20mm to shoot buildings, trees or even people from close-by, this ‘distortion’ can be exploited to achieve some dramatic results.
Before using the Sigma, I hadn’t previously worked with a lens with such a wide perspective. I was amazed at how wide 10mm looked through the viewfinder. It allows you to include so much within the image area, and whilst I prefer shooting landscapes, it is equally well suited to interior photography and architecture. In fact, the field of view is so wide, that it’s possible to accidentally include a tripod leg at the bottom of the frame if shooting in vertical format – so bear this in mind.
The short minimum focus makes it possible to position it very near to foreground interest, helping to create dynamic compositions bursting with impact and depth. Achieving sufficient depth-of-field isn’t difficult, as it is extensive using such a short focal range – use a small aperture and hyperfocal focus to maximise it.

Verdict
Using such a wide angle-of-view presents a whole host of fresh creative possibilities and since buying this lens, I rarely use any other focal range to shoot landscape images.
As you would expect from one of Sigma’s EX range, the lens is solid, well constructed and a joy to handle. It may have a relatively slow maximum aperture of f/4-5.6 – but scenic photographers will normally employ an f/stop of f/8 or smaller anyway.
Considering its ultra-wide focal range, it is remarkably compact and it is designed with a practical filter size. Compared to the Nikkor 12-24mm f/4 and Canon’s 10-22mm f/3.5-4.5, it represents a healthy saving of over £200, yet it isn’t a compromise on quality.
If you use an APS-C-type DSLR (most of us do) and presently in the market for an ultra wide-angle lens, then this may well be the optic that you are looking for.  It comes highly recommended.