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* Please note this review is for the 'Macro' version of the lens. There are two versions of the Sigma 18-50mm 2.8 EX DC. The earlier version did not have a 'Macro' designation. Also my experience is based on using the Canon mount of this lens however I am not aware of any differences on the other mounts of this lens (apart from the Nikon with HSM) *
I bought the Sigma 18-50mm EX DC f/2.8 as an upgrade to my previous lens (Canon EF 28-135mm IS) as it was not wide enough on my Canon 450D. Having had the luxury of such a relatively long zoom I was in two minds whether the Sigma would be long enough. For this aspect of the lens I cannot really comment, an acceptable zoom range varies greatly depending on a persons needs. Personally most of the time I have found it an adequate zoom range but there have been times that I wished it had more reach. The Sigma is designed for crop sensors though it does use an EF mount and so it will fit on Canon APS-H & full frame bodies. You would probably getting heavy vignetting and you'd have to watch the mirror and the wide angles.
Features of the Sigma 18-50mm EX DC f/2.8 Macro at a glance:
Lens Construction - 15 Elements in 13 Groups
Minimum Aperture - F22
Maximum Aperture - F2.8
Minimum Focusing Distance - 20cm
Maximum Magnification - 1:3
Filter Size - 72mm
Dimensions - Diameter 79mm X Length 85.8mm
Weight - 450g
Special Low Dispersion (SLD) and Extraordinary Low Dispersion (ELD) glass elements for protection against colour aberrations.
The first thing you'll notice about this lens upon opening the familiar Sigma style box is the amount of accessories you get compared with Canon lenses. Sigma provide a lens hood as standard (in this case a petal shaped hood) and it also comes with a padded pouch that has a zip fastening top for transporting or storage. Also worthy of note is the lens cap, it's a centre pinch cap which is much easier to use than the Canon caps that you pinch from the outside.
The image quality of the Sigma is very good. Whilst it is not on a par with the Sigma DX range of lenses or the Canon L range, it is definitely a step up from the Canon & Nikon kit lenses. Specifically, it has better contrast, colour and is sharper at wider apertures. The Sigma does suffer from noticeable distortion at the wide and long end of the zoom, perhaps more so than the Canon EFS 18-55mm (which I owned previously), however it is correctable using any software that has a lens correction feature. It must be said that it is usually only noticeable where there are straight vertical lines in an image, such as a church spire or tall buildings for example and when using the lens extremely close up to the subject. Chromatic aberration does rear its head but it's not overly obtrusive and not excessive.
Sigma have given the 18-50mm f/2.8 a 'Macro' designation due to its ability to focus extremely close to the front element of the lens (be aware it is not a true macro lens!). Its minimum focusing distance is 20cm (from the sensor plane not the front element of the lens). In practice, when zoomed out to 50mm you can focus so close to your subject that you'll worry about touching the front element . This gives up to a maximum 1:3 magnification (1:1 is true macro). This is impressive and opens up the lens to a multitude of uses that a lens of this range wouldn't normally be able to accomplish.
The lens has a high quality matte finish. It is solidly built and feels good when using it. Crucially the front element does not rotate when focusing, this makes using a polarising filter much easier. There are two switches on the lens, one the obligatory Autofocus/Manual Focus. The 2nd switch is a zoom lock, which can be employed to stop the zoom accidentally extending when you're walking around *Note this can only be activated when the lens is in its fully retracted position*. There is no distance scale on the lens but they're getting quite rare these days.
The weight of the lens is 450g. It's about average for a lens of this type but it will be noticeably heavier if you're upgrading from an 18-55mm kit lens. One often criticised feature of this lens, is that there is no HSM focusing motor. HSM (Hypersonic Motor) is Sigma's answer to Canon's USM (Ultrasonic Motor) for quick and quiet focusing. However whilst it may be slower and noisier than a lens with USM/HSM, I have never particularly noticed this when shooting. Sigma have released a Nikon mount of this lens which does have HSM. It is currently the only mount of this lens to feature HSM.
If you're upgrading from a 'kits lens' (18-55mm range) then there are two major points to consider, a pro and one a con.
First the con, there is no Image stabiliser on this lens. Whether that will be a deal breaker for you is heavily contingent on what you intend to use the lens for. If it's primarily for landscapes then I wouldn't even give the lack of IS a second thought. If it's primarily for social snaps in low light (and without the usage of flash) then the IS may be missed (due to the low shutter speeds involved).
The 'pro' of this lens... The constant f/2.8 aperture throughout its zoom range. Where it lacks with no IS, it makes up for in its much wider aperture compared to the kit lenses. Shooting at f/2.8 will give you faster shutter speeds than you would get with a kit lens and as a consequence you may indeed not miss the IS of the kit lens. However there are two points to be aware of when shooting at f/2.8. One is the shallow depth of field, f/2.8 is a wide aperture and as such you do not get a lot of depth of field. However unless you're shooting very close to the subject it isn't likely to be too much of an issue. The 2nd point is that the lens is at its least sharpest at f/2.8 but I have found it fine for portrait work.
Sigma do have a small reputation for having quality control issues with some of it's lenses. I initially had to send my first copy of this lens back because it was decentering at the wide end. It was frustrating at first but I wouldn't let this put you off.
I find the Sigma to be the better lens for me compared to the kit lens but not everyone will. I actually miss the reach of the 28-135mm more than the IS!