Sigma 15-30 EX DG

This lens has a maximum f/3.5 aperture at 15mm, f/4.5 at 30mm. The minimum aperture is f/22 at 15mm and almost f/32 at 30mm. Focusing reaches as close as 11.8 inches (0.3m). Depth of field markings are provided for f/5.6, f/8, f/11, and f/16. The design has 17 elements in 13 groups, at least one of which is aspherical.
The front element does not move during focus changes, though this doesn't do you any good, since it sticks out like the bottom end of a plastic cola bottle and doesn't accept screw-in filters. A built-in "flower hood" attempts to block off extraneous light, but don't count on it--the front element curvature is so extreme that the middle of the lens is extremely prone to catching extraneous light.

The focus ring is wide, but uses the same rubber pattern as the zoom ring, which is narrower and close to the back of the lens. By feel alone you can't distinguish between them, but it's not a big deal.
At over a pound (1 pound 6 ounces), this is a substantial lens. It's also quite large (almost 6 inches front to back and about 3.5 inches in diameter). The size and weight tends to surprise most people, as they seem to expect something smaller, especially when they hear that the lens has a variable f/3.5-4.5 maximum aperture.

Handling
I really doubt you'll be manually focusing this lens other than setting hyperfocal distance--you get almost no benefit from focusing manually due to the huge depth of field (which is approximately 1.25 feet to infinity at f/16!)--but if you do, you'll find the action quite stiff. The lens is put into manual focus by pulling the focus ring backwards towards the camera body, and this action is somewhat "reluctant" on my sample.
Autofocus on my N90s, F100, F5, and D1x is reasonably fast, but slower than the Nikkor 17-35mm f/2.8 AF-S. I've never seen the lens hunt on autofocus. The focusing is quiet, but not "silent wave" quiet.
The aperture ring is very nice, about the best I've felt on a Sigma. Click stops are definite, but you can easily set almost an infinite set of intermediary settings.
The front element is very significantly exposed, so you're going to get fingerprints on this beauty if you aren't careful.

Performance

Performance is a mixed bag.
From a sharpness and contrast standpoint, the lens does a quite fine job. On a 35mm body, there's visible softening of the corners wide open, and the lens starts to show softness again (diffraction caused?) at very small apertures (f/22 and higher). In the mid-range apertures the lens is pretty darn sharp from corner to corner. On a D1x, the lens is sharp at everything but the widest and smallest apertures.


Copyright 2007 (ICM) Fusion

Given Sigma's excellent performance with previous wide angles, I expected no rectilinear distortion. After all, the 14mm renders pretty darn straight lines, even when you place them at the frame edges of a 35mm body. Unfortunately, at 15mm this zoom shows significant barrel distortion even on a D1x (I haven't done any architectural work with a 35mm body and this lens, but I would expect the same). What's "significant?" It might be as much as 2 percent on a D1, slightly higher on a 35mm body. I fully expected light falloff in the corners, but this common wide-angle problem is fairly well controlled on this lens except at the maximum aperture. What little falloff there is I find pleasing enough not to be bothered by it. And most of the falloff is at the very edges, so D1 users won't see much, at all. But...the extreme front element curvature produces a penalty you need to be aware of: flare. If any stray light hits that big hunk of glass, contrast is greatly reduced, making most images unusable. Worse still, direct hits of light on the element sometimes produce a strange bit of flare--essentially a hot spot in your image (the depth of field is so great at smaller apertures and close focus distances, that at times it can be at the front element, meaning the light hitting the lens is rendered in the image!). This isn't just a matter of sun being in the frame. On my D1x, any hot spot that is rendered "beyond white" has a tendency to produce a flare spot.

Drawbacks

Optical quality issues. A bit of barrel distortion, edge sharpness issues on 35mm bodies at the extremes, and contrast/flare control problems keep this from being a top-of-the-line lens. If you're expecting this lens to be equivalent to a wider Nikkor 17-35mm AF-S, downgrade your expectations a bit

The Lens Cap. Because it slips over the built in hood, the unique cap has a tendency to come off in my tightly packed camera cases. Moreover, that strange cap on the cap sometimes pops off, leaving the front element exposed.
Positives

Value. For it's price, this lens performs quite well. When I consider my star ratings on performance, I consider the cost/benefit, and this lens delivers well on the positive side of the ledger.

Digital Near Nirvana. Until Nikon introduced the 12-24mm DX, this lens was the widest zoom you could get to combat the 1.5x angle of view reduction. If you're scrimping on pounds but need wide, this lens is your clear choice.


Copyright 2007 (ICM) Fusion