a surprisingly good lens and well worth the money

I have never been a huge fan of third party lenses, mostly because I always felt that the stuff made by Nikon and Canon was superior in almost every way (except price). I still hold that position with equipment designed for film, but the recent advent of digital has created a demand for lenses that are designed with the APS-C sensor size in mind. Nikon has the entry level 18-55mm F/3.5-5.6, the mid level 18-70mm F/3.5-4.5 and the super pricey 17-55mm F/2.8 but I have been disappointed by every one of them. All three of those lenses have SIGNIFICANT distortion at the wide end, noticeable pincushioning on the long end, and inferior toy-like build quality (not the 17-55mm..that's a tank).

Lately there is this feeling that just because you can bump up the ISO on your camera that you don't need fast apertures. I find this to be total garbage and it’s an easy point to prove. Take your camera with your basic kit lens with variable aperture in a normal indoor home setting and try to take pictures by natural light (not right next to the big picture window, but in the rest of the room). You will find that even at 1600ISO and F/4.5 you are looking at 1/8 or 1/16-second shutter speeds. Another thing is that its not always desirable to shoot at 1600 ISO unless grain is your goal :-). Wouldn't it be nice to take natural light images at reasonably clean ISO's? If the lens were faster these would be hand-holdable and would result in less blur. You might argue that image stabilization solves the problem but look at the pics!...image stabilization won't stop motion blur caused by a moving subject. My point is that F/2.8 zooms and even faster primes are just as necessary now as they ever were In fact, I would argue that they are needed more than ever since the style today is for more natural light images, more editorial or "reportage" images and less flash....and they are needed for everyday people shooting everyday shots in their homes...kids B-day parties etc. If you want to shoot your kid doing something cute its hard to do it unobserved with a huge flash going off every 10 seconds.

sorry for the diatribe... so now you see why I always am looking for more speed and why I was so happy to find the Sigma 30mm F/1.4..exactly the lens for the job. This opened my eye up to the possibilities of using third party lenses and left me open to consider some of the wide-normal zooms out there as possible substitutes for the regular stable of Nikon zooms. As mentioned before I have never been totally pleased with the results of the Nikon digital zoom lineup and was VERY disappointed with the top-shelf 17-55mm F/2.8 especially considering the price. I am pleased to announce that I have found a worthy mate for my D200 that delivers the goods optically, has fantastic build quality AND a constant F/2.8 aperture.....enter the Sigma 18-50mm F/2.8

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Specs from the Sigma website:

Lens Construction 15 Elements in 13 Groups

Angle of View 69.3 - 27.9 degrees (SD format)

Number of Diaphragm Blades 7 Blades

Minimum Aperture F22

Minimum Focusing Distance 28cm/11.0 in.

Maximum Magnification 1:5

Filter Size Diameter 67mm

Diameter 74.1mm x 84.1mm 2.9 in. X 3.3 in.

Weight 445g/15.7 oz.

Corresponding AF Mounts SIGMA,
CANON,
NIKON,
PENTAX
KONICA-MINOLTA

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General overview...

The Sigma 18-50mm F/2.8 EX DC has the same field of view as a 27-75mm lens mounted on a 35mm camera. So this qualifies as your traditional midrange zoom. It has a constant F/2.8 aperture at all focal lengths. Most digital zooms have variable apertures that can be as dim as F/5.6 at even modest focal lengths as 50mm! That difference can definitely make the difference between making the shot or getting a blurry mess. It uses 2 aspheric elements for control of color aberrations and uses coatings similar to the ED coatings found on Nikon glass. It uses Internal Focusing just like the better Nikon lenses which is faster than traditional methods and does not turn the front element when focusing or extend the length of the barrel.

Construction Quality is SUPERB!!! It's not as tank-like as the Nikon 17-55mm F/2.8 but other than that one lens, it is better constructed than ANY OTHER DX NIKON GLASS I HAVE EVER USED. The other Nikon zooms in the same focal lengths feel like cheap toys in comparison. When you zoom this lens there is NO WOBBLE OR EXCESS MOVEMENT in the barrel. Zoom and focus feel very precise and the whole barrel is very well machined. The 18-50mm and 18-70mm Nikkors both feel very loose in comparison. In addition the zoom ring is where it belongs...in the back of the lens barrel. It is large, rubber coated and very precise. The focus ring is smaller in comparison and located forward of the zoom ring. Other than the Nikon 17-55mm F/2.8 I definitely feel this "feels" best in the hands of any lens I have used for the DX Nikon. For those of you who own the 12-24mm F/4....this lens puts that one to shame in the construction quality department.

Optical quality,,,this is the exciting part. The lens is ( of course) free of any noticeable color aberrations like most well made lenses. It shows mild pincushion distortion of the long end and mild barrel on the short. But after owning the 18-70mm lens for over a year I feel that the Sigma is better controlled on the wide end and that the pincushioning on the long end is only noticeable when you go out of your way to find subjects to expose it...like a picket fence shot dead-on to make sure all the slats are parallel...other than that you never really see it...and its minor anyway. I have used the Nikon 17-55mm F/2.8 extensively and I honestly feel that the Sigma outperforms it. Its not that it is distortion free...but the distortion is easily correctable in Photoshop, while the distortion on the Nikon lenses is not perfectly spherical and as a result can require a lot of fiddling to get perfection. Compensating for the lens distortion on this lens is trivial. There is one thing I do have to mention. At 18mm at F/2.8 there is definitely light falloff in the corners and it disappears the second you stop down to F/4. I don't notice it at other focal lengths.

Observations:

The Sigma 18-50mm F/2.8 is a joy to use. It's exactly what I was looking for. It is surprisingly compact for an F/2.8 lens ( same size as a Nikon 18-70 or 18-50mm) yet has FAR SUPERIOR construction. I also feels it outperforms any of the Nikon mid range zooms. It is also super sharp.

There were a few things that I feel bear mentioning.

1. it uses 67mm filters...not the more desirable 77mm, but it does tolerate a step-up ring very well

2. the lens hood is fantastic compared to the cheaper Nikkor zooms and is large enough to actually be useful

3. While this may be an internal focus lens it is not as fast at focusing as most Nikkor lenses I have used and it is much noisier when focusing than the Nikons

Overall this is a fantastic lens and is well priced. I have found it for as low as $410 and it is WELL WORTH the price especially when you have to spend three times as much to get anything close to its performance and build quality in the Nikon lineup. It is a worthy upgrade over the kit-lens 18-50mm and 18-70mm lenses and I feel it makes a lot more sense than the VERY expensive 17-55mm F/2.8, which has better construction but is worse distortion.

This is the second Sigma lens I have added to my stable. Unless Nikon steps things up a notch I might be adding more. Most of the Nikon lenses I still use are 35mm designs that have no equivalent counterpart in DX designs (in speed and construction)... if I had to build a lens setup from the ground-up I would definitely give Sigma a long look.