21 November 2008

A one lens solution: the Sigma 17-70mm

Recently I managed to snag a mint Sigma 17-70mm f/2.8~4.5 zoom from someone moving over to another system at a very reasonable price. I had used the Pentax DA 16-45mm f/4 for quite sometime and in terms of focal length the Sigma is a very close competitor. I've had very positive experiences with the DA 16-45mm but after spending some quality time with the Sigma, my preference is slowly but surely shifting towards the Sigma as being the more versatile lens.



For a start, while the Sigma loses 1mm in focal length on the wide end to the Pentax, (which is noticeable), it nonetheless more than makes up for this by extending all the way to 70mm, a focal length that's good for portraits and the occasional isolated candid.

In terms of maximum aperture, it's a toss up between the constant aperture Pentax versus the variable aperture design of the Sigma. Unless one is shooting with strobes a lot where having a constant aperture is preferred, in practice I've not really been hampered with the variable aperture, whether in normal or low light shooting. Exposure with the Sigma is spot on on my K20D but for the DA 16-45mm, there is a tendency towards a consistent 1/3 to 1/2 stop underexposure, at least on my previous copy.


Capital Tower at night

In terms of optical performance, I've known the DA 16-45mm as a lens capable of even performance from the center to the edges. The Sigma is noticeably sharper in the center but edge sharpness is still good. In terms of distortion, the two lenses are about similar. Size wise, the Pentax is longer and narrower while the Sigma is fatter but shorter. The Pentax uses 67mm filters while the Sigma uses 72mm. The Sigma is about 90 grams heavier at 455 grams. Both have petal lens hoods that seems to be the norm today.


Floating barriers

Where the Sigma edges the Pentax is in build quality. The lens is solid and has a sensible design whereby the lens barrel extends at the 70mm setting. On the Pentax, the lens barrel extends fully at the 16mm setting, making use of the on-camera flash pretty useless at the wide setting. The Pentax has the useful Quickshift where one can manually adjust focus after the camera has locked autofocus. Focus is fast for both lenses. Something I've noticed with the DA 16-45mm is whether the build can hold up long term. I've seen the zoom and focus rings on some seasoned copies to be very loose with wobbly lens barrels. With the Sigma, the primary bugbear is lens creep when the lens is pointed down. There is however a zoom lock to prevent this.


Close-ups of Pentax 50mm lenses taken with the Sigma

One useful feature is the close focusing capability of the Sigma. While the Pentax goes down to 0.28m, the Sigma can focus as close as 0.2m, which in practical terms, means the ability to shoot up close with results that are almost as good as a dedicated macro lens. Overall, the Sigma makes a compelling case if you're looking for a single lens solution for walkabout or travel.

No comments: