Another Winner From Sigma

Here's a lens that I've owned for a couple of years now, and it is one that is with me most of the time.

The Sigma EX lens series are an attractive, robust and durably finished line of full frame lenses. The construction is solid and predominantly metal based. The EX finish, a dappled matte grey, is very durable and should keep the lens looking nice for years. The slim gold band near the focus ring really sparkles. The lens comes with a nice case, front and rear caps, and petal shaped hood.

Within the wide prime model line up are the 20mm, 24mm and 28mm focal lengths, all sporting the maximum f/1.8 aperture. All are designed for 35mm full-frame coverage but they will of course work on a digital SLR. In fact, optical performance of a given full-frame lens tends to improve when used with a DSLR, as the digital sensor uses the "sweet spot" at the center of the lens' glass. (The edges of the lens' field of view are where distortion and softness first appear due to the curvature of the glass.) The latest versions of this lens and its ilk sport a "digital' DG coating for reduced reflection off the rear element next to the camera CCD. To clarify a major difference in lens designs, Sigma also makes DC lenses which have a smaller image circle coverage, specifically for the digital APS-C sensor only. These DC lenses are not compatible with film cameras. 

The Sigma 20mm f/1.8 wide angle lens is the lens I reach for when shooting most of my landscape and typical wide angle photos. The 20mm translates to 30mm on my KM 7D which isn't super wide by any stretch, but quite sufficient. This crop/sensor factor is my main reason for choosing the 20mm lens over either the 24mm or 28mm. The others are cheaper, but for maximum coverage, I opted for the widest lens of the three. Furthermore, with its wide focal range, I like to carry my Minolta 28-135/4-4.5 as a main shooting lens outdoors, but this lens lacks a wide enough angle of view on digital at the bottom end. This is where both the 20/1.8 and my other ultra wide angle Sigma14/2.8 come in to play. (24mm and 28mm are pretty redundant when you consider my lens line up.)

In terms of useage, I think the 20/1.8 sees more time in the shooting position for landscapes than any other prime lens in my bag for a number of reasons, but one of the most important is the fact that I can put a circular polarizer out front and this was a big positive consideration for me. In my opinion, a CPL is a necessity for shooting outdoors - it's the one filter effect that even Photoshop CS2 cannot replicate.  Many wide angle lenses have a bulging front element (eg. Sigma 14/2.8) and as such, will not accept front filters. I have a set of Cokin P graduated neutral density filters that I  often use attached to this lens for sunsets and other high contrast landscape images. The non-rotating front element is useful for Cokin filters and CPL use.

Pro:

• Quick AF
• Rear internal focusing element
• Ability to accept filters up front, albeit 82mm monsters
• Bright viewfinder w/ f1.8
• Very good optical properties when stopped down
• Can be used in low-light
• Nice out-of-focus bokeh
• Relatively inexpensive lens

Con:

• Reported vignette of approx 1 EV at f/1.8 in bench tests
• Fair MTF resolution at f/1.8 (worse at edges)
• Some CA (purple fringing) in challenging lighting
• Heavy

A few specifications:

• 82mm filter
• 13 elements in 11 groups
• 9 blade aperture
• 520g,  89.5mm x 88.6mm
• 20cm minimum focus, 1:4 Macro

I like this lens a lot. It offers a good trade off in terms of more than acceptable image quality and price. The Minolta version of this lens, (smaller size and f/2.8) will cost  $300 - $400 used, while this lens should be available for about $200-$250 on Ebay.  A nice bonus is the 20cm close focus capability of the 20/1.8, which opens up all kinds of creative options for composition. If I need more coverage, or if I am in a tighter location, I will opt for my 14mm/2.8 Sigma.

The few compromises that come with this lens are not, in my opinion a big deal. First off, they are noticed at the widest aperture and disappear after a stop or two. Photoshop can repair vignetting of corners easily if you need to shoot at f/1.8 and you do detect any light drop off in the corners of your photograph. Edge softness at a lens' widest apertures can be seen in most consumer and prosumer lenses on the market and is only noted when "pixel peeping" at 100% crop. Furthermore, these documented shortcomings are mostly picked up while the lens is "bench tested" in a lab. I've read a number of user reviews of this lens that indicate a very high degree of satisfaction in normal use. It has been proven countless times that it takes more than measured low distortion and high MTF scores to produce a great lens. Due to the complexity of wide and ultra wide angle lenses, you can't expect perfection. The Sigma 20/1.8 comes awfully close, and these quibbles are very minor and in my opinion do not affect my image quality to a noticeable degree.

Only the Minolta G lenses are sharp throughout their entire aperture range. And how many lenses on the market even open up to f/1.8?! I consider the speed of this lens a bonus even if it is only 99% perfect at f/1.8. This Sigma 20mm is the widest lens on the market to incorporate such a large maximum aperture. In any case, except in low-light, I find I use this lens at f/8 to f/16 most of the time where it is very sharp, but it is nice to have the opportunity to open up to a wide aperture when necessary.

Full Frame Wide Angle vs. APS-C Lenses

Why add a full frame 20mm lens to your bag when there are several APS-C zoom lenses available that cover this focal length? True, if you like zooms, you may get greater utility and possibly even a wider zoom angle at the low end. Unfortunately, with the increased complexity of these wide and ultra wide zooms comes a trade off in smaller maximum apertures (which isn't really much of a trade off), but also increased levels of distortion and increased vignetting. If you can live with these shortcomings, there are some very nice zoom lenses in this category. In the "plus" column, these zooms minimize lens changes and sensor contamination, are very lightweight and can optimize compositional framing. It's simply a matter of which features and conveniences you put a higher utility on.

It is rumoured that Sony has a full frame DSLR prototype that will be released sometime in the next year. (Don't hold your breath on the timeline, and ensure you are sitting when you see the price tag!) Because the APS-C sensor bumps up your lens' focal length by a factor of 1.5x, it would be nice to have a full frame sensor camera for use with wide angle lenses. With full frame lenses like the 20/1.8 (and 14/2.8) you'll be able to take full advantage of the full-frame sensor by shooting at the true focal length of the lens. If you plan for the future, as I have, you might want to keep this in mind when purchasing a wide angle lens and go with a "full frame" (non APS-C) model. 

In any case, getting a good wide angle prime lens (or zoom), whether be full frame or APS-C only compatible makes good sense if your photographic interests put a lens like this into the "must have" category. Regardless of format, perhaps try to invest in the widest, good quality lens that you can afford. I think the Sigma 20mm f/1.8 offers a level of performance and price that make it an excellent choice for a wide angle lens.

Hi-res images can be seen at flickrdotcom/photos/seagr112