When laypeople talk about extremely wide-angle lenses, they will invariably use the word "fish-eye" in the same way that many people colloquially use the word "migraine" to describe any type of major headache. In both cases, the intent is commonly understood but the accuracy of the terminology is suspect. "Fish-eye" is properly used to describe a specific type of wide-angle lens in the same way that "migraine" is medically descriptive of a specific subset of headaches.
Ultra-wides are the go-to lens below the 16mm APS-C (24mm full frame) focal length. They are eminently more-practical than fish-eyes, if somewhat less dramatic. "Practical" is a good term to use to describe the Sigma 10-20mm f/4-5.6 EX DC HSM, which upholds the traditional virtue of costing less than the equivalent first-party equivalent. However, traditional Sigma lenses have also not met the same performance standards as those set by Canon and Nikon. To that end, does the benefit of better affordability hamper the quality of this lens?
Build, Design and Handling
The Sigma 10-20mm f/4-5.6 EX DC HSM typifies the construction of Sigma's lenses pre-Global Vision. The construction is in keeping with what you would find with a mid-level DSLR like a Nikon D7100 or a Canon 70D. The construction quality is better than that found in the kit lenses, but not overbuilt in the way that the Nikon gold ring or Canon L lenses typically are. The matte black appearance is utilitarian and far from flashy. The zoom ring, though not overly wide, turns with an appropriate amount of resistance; more substantial than with a kit lens, but with less drag than that found on a more expensive lens like the Sigma 35mm f/1.4.
|Sigma AF 10-20mm f/4-5.6 EX DC HSM|
Unlike the current trend towards longer-and longer retrofocal designs, the Sigma 10-20mm is pleasantly compact when mounted on either a entry-level or serious-enthusiast DSLR. There is a practical consideration to this, as ultra-wides will typically see less use than normal-zooms; as such, a smaller physical size is useful as it allows the lens to pack away discretely in a camera bag. As a rule of thumb, lenses that are too large to sit comfortably alongside a camera body and mounted lens will often be left at home.
|Sigma 10-20mm on Canon EOS T5i|
|Sigma 10-20mm on Nikon D7100|
Not depicted: This lens comes standard with a petal-type hood.
As a HSM lens ("Hypersonic Motor"), focus operation is quick and silent. You can manually override autofocus as with a Nikon AF-S or Canon EF-S lens. Note: The Nikon version of this lens omits the focus selector switch but full manual focus is still possible by changing the settings on the camera body. The Sony and Pentax versions of this lens are the screw-drive type and cannot have the autofocus be manually overridden in full AF mode. The front element takes 77m filters. Though large filter sizes are unavoidable with ultra-wide lenses, be warned that this pushes up the cost of items like circular polarizers, where the cost increases noticeably with size.
Usage and Optical Characteristics
The Sigma 10-20mm f/4-4.5 EX DC HSM in casual usage has a modicum of distortion if you aren't paying close attention. However, at it's widest setting (10mm), the distortion pattern is on the complex side. There's a fairly pronounced amount of barrel distortion at the edges of the image, but the central portion is fairly neutral. In the middle of the zoom range, the distortion leans slightly towards the pincushioned side, while at 20mm the image is fairly distortion free. For most casual users, this won't be significant problem, but it is important to remember that the automatic JPEG distortion correction features on Canon and Nikon DSLR's don't work with third party lenses.
These are examples of the differences in field of view between widest and longest focal lengths:
|Sigma 10-20mm: 10mm focal length, f/8|
|Sigma 10-20mm: 20mm focal length, f/8|
However, in real world usage, the image element to pay attention to is not necessarily distortion, but rather exaggeration. At the ultra-wide focal lengths, the near-far relationships are exaggerated beyond what you would be used to with normal vision. This places a slightly heavier onus on setting up an image during composition... especially if you have recognizably straight horizontal and vertical lines. You can see this in the first shot above (10mm), in which camera is not quite square with the back wall. It was pointed just a bit to the left, which pulls the right side of the scene closer to the viewer, but which also pushes the left side away. Here is an example of what a minor 5-10 degree rotation to the left does to the visual balance of the image:
|Sigma 10-20mm, 10mm and centered|
|Sigma 10-20mm, 10mm, slight left rotation|
Vignetting is about what you would expect for this price-class. It's not egregious, but it is visible and palpable at 10mm, and though it's never completely gone, it's less of an issue at the longer end of the lens and/or with the lens stopped down. Lateral chromatic aberration tends to rise as you stop down the lens.
The above samples show the obvious potential for an ultra-wide lens to accommodate width, but there is greater dramatic potential when lenses like the Sigma 10-20mm is used to accentuate depth:
|Sigma 10-20mm at 10mm|
|Sigma 10-20mm at 20mm|
To give perspective (so to say) about how much the depth of the scene is exaggerated, the tiles of the mall courtyard are square; you can see the perspective exaggeration in the 10mm shot with the near tiles, which a visually stretched into rectangles.
Overall, the Sigma 10-20 produces fairly consistent detail rendering across the frame, even at 10mm. Peak sharpness occurs around f/8. Naturally, image sharpness is higher in the central portion of the image than it is at the edges, but corner performance is hardly a weakness. That said, there's sharpness and then there's the subjective appearance of sharpness. The Sigma 10-20 is sharp, but it lacks that extra bit of bite that a more contrasty lens would have.
|Sigma 10-20mm, close focus|
As an added bonus, the minimum focus distance for this lens is only 24cm (from the shutter plane). This allows you to bring the lens in close for some dramatic spatial exaggerations, as with the 4" tall figurine of the Terracotta Warrior above.
Sigma 10-20mm f/4-5.6 EX DC HSM versus Sigma 10-20mm f/3.5mm EX DC HSM
The f/4.5-5.6 and f/3.5 versions of this lens are similar, but with differences. However, the difference between the variable aperture lens and the constant aperture lens lens follows an all too familiar photographic trend; for a modicum of improvement, you have to pay a substantial increase in price. The f/3.5 lens has a simpler geometric distortion pattern but is weaker in the extreme corners at 10mm compared to its less expensive sibling. If the f/3.5 lens could be summed up, its that for the most part it does the same thing as the f/4-5.6 version, but with the benefit of faster apertures.
The benefit of the constant aperture isn't readily apparent, as the depth of field of ultra-wide focal lengths makes out-of-focus backgrounds impossible and because the minimum safe shutter speeds are significantly lower than for "normal" focal lengths. However, the 10-20mm lenses do have a measure of flexibility; they do not necessarily need to be used at their widest all the time, and the ability to pull the lenses back to 20mm make them useful for photographing people and social situations. Theoretically, the f/3.5 lens would be able to eek out some low light situations where the variable-aperture lens would struggle. However, this begs the question; why buy the Sigma 10-20mm f/3.5 when you could have the Tokina 11-16mm f/2.8 AT-X 116 Pro for roughly the same price. The Tokina is optically better than either Sigma lens, but it's only significant drawback is that it is only 16mm at its longest focal length, which limits its usefulness for general purpose photography.
|Left: Sigma 10-20mm f/4-5.6 Right: Sigma 10-20mm f/3.5|
Sigma 10-20mm f/4-5.6 EX DC HSM vs. Nikon 10-24mm f/3.5-4.5G ED AF-S DX and Canon EF-S 10-22mm f/3.5-4.5 USM
Compared to the first-party lenses sold by Canon and Nikon, the Sigma 10-20mm f/4-5.6 is the better value, as it sells at roughly 2/3 the price of the mainline lenses. It doesn't lag behind the Canon by very much in terms of optical performance, though the Nikon is sharper, albeit with more pronounced barrel distortion at 10mm. As a general rule of thumb, Sigma lenses aren't as flare and ghosting resistant as those made by Canon and Nikon, but given the price difference, most shooters won't mind.
|Left: Sigma 10-20mm Right: Nikon 10-24mm|
Often, the impetus for acquiring an ultra-wide zoom lens is an impending trip to a scenic location. To that end, the Sigma 10-20mm f/4-5.6 EX DC HSM fits the bill, as shooting below 17mm on APS-C is a relatively infrequent occurrence for most people, and it makes little sense to overpay for a lens that doesn't see constant use. However, if you do use ultra-wide focal lengths frequently (e.g., landscape enthusiasts)... especially the wide end of the lens....there are (slightly) better choices, but none are as inexpensive as the Sigma. If you already happy with your crop sensor DSLR kit but want to try something new, then this is an affordable way to explore a difference style of photography.
With thanks to Broadway Camera.