Introduction

Sigma 20mm f/1.8 EX DG ASPH RF is the fastest 20mm prime lens currently available on the market. Sigma, which is known for trying to push the envelope when it comes down to lens designs (think of the 50-500mm BigMa) has tried to strike a balance between capabilities, features and price here as well - despite it's ultra-fast aperture, the lens remains quite affordable at about US$370 (as of October 2007), making it a viable choice for amateur and professionals alike.

The optical construction of the lens consists of 13 elements in 11 groups, including 2 aspherical glass elements designed to compensate for distortion and reduce spherical aberration. The build construction of the lens is pretty good, as is common with Sigma's EX line of lenses - the barrel is made of hardened plastic with nice crinkle finish, while the focus ring is fully rubberized and is very smooth to operate. The lens feels quite sturdy if not a bit bulky - it weighs 520g (18.3oz) and measures ~89 x 89mm (3.5 x 3.5in). The lens sports an AF alongside with a full time manual focusing system which can be controlled by an AF/MF switch on the side of the barrel. Sigma included a so called Dual Focus system in this lens, which prevents the focus ring from rotating when the lens is in AF mode. This Dual Focus system reminds me of Tokina's Clutch Focusing where the focus ring needs to be pushed forward/backward to switch the focusing mode. The minimum focusing distance is 20cm (7.9in), giving a maximum object magnification of 1:4 and the minimum supported aperture is f/22 (aperture is electronically controlled from the camera, so there's no dedicated aperture ring). The lens accepts 82mm screw-in type filters.

Image

Sigma 20mm f/1.8 EX DG ASPH RF is designed for full frame cameras and on APS-C type bodies with 1.6x crop factor the field of view of the lens resembles that of a 32mm lens on a full frame camera. The lens tested in the review was a Canon EF mount lens, but SIgma also offers Nikon, Sigma, Pentax and Sony variants. The factory box includes Sigma 20mm f/1.8 EX DG ASPH RF lens, front and rear caps, LH875 lens hood, vinyl case, manual and registration card.

 

Summary
Lens Composition 13 elements in 11 groups
Angular Field ~94 degrees
Minimum Focus 20cm/7.9in
Focusing Action AF/MF
f-stop Scale f/1.8-f/22, camera-controlled
Filter Size 82mm
Lens Hood LH875-02 (included)
Weight 520g/18.3oz
Dimensions 88.6x89.5mm/3.5x3.5"
Lens Case Vinyl (included)

 

Field Tests

Image quality was all over the map with this lens. The lens showed pretty dismal performance with wide open apertures, where both image center and borders were visibly soft. This was the case with both APS-C and FF cameras. Quality in general improved with stopped down aperture and Sigma 20mm f/1.8 EX DG seemed to produce more balanced results in the f/5.6-f/11 range, where both center and border quality was quite decent.

One major disappointment with the lens was the AF system, which was noisy and quite slow to focus. AF also hunted pretty much at all aperture levels. Another issue is this Dual Focusing system which requires you to pull the focusing ring forward to be able to focus while shooting in the MF mode. Guess what's gonna happen if you forgot to pull the ring? The lens won't focus no matter how much you rotate the ring. Left me guessing a couple of times before I got used to it. Naturally, this is not going to be a problem for those of you who plan to shoot in AF all the time.

 

Vignetting @ f/1.8 - full frame vs 1.6x crop
Vignetting @ f/1.8 - full frame vs 1.6x crop

 

The lens showed noticeable vignetting on a full frame Canon 5D with wide open aperture, which is reduced with stopped down aperture and by f/4 becomes quite insignificant. On an APS-C body with a reduced image sensor, the lens did not exhibit any noticeable vignetting throughout the aperture range. Color reproduction was more or less accurate, but since images were quite soft at wide apertures, images lacked contrast and left an impression of being somewhat washed out. The lens showed some barrel distortion which can be considered relatively small for such a wide angle lens as well as fell prey to flare, predominantly at the widest aperture setting. The lens also exhibited color fringing around borders, which is typical for wide angle lenses.

 

ISO 100, 1/4000, f/1.8, 20mm
ISO 100, 1/4000, f/1.8, 20mm
Sigma 20mm f/1.8 EX DG image gallery...

 

Lab Tests

Please note that MTF50 results for APS-C and Full-Frame cameras are not cross-comparable despite the same normalized [0:1] range used to report results for both types of cameras.

 

Canon APS-C: Sigma 20mm f/1.8 EX DG showed pretty mediocre results on an APS-C type Digital Rebel XTi with wide open apertures. At f/1.8 both image center as well as borders were dismally soft. Both center and border performance improved with stopped down aperture and by f/2.8 center quality is pretty solid, with border quality falling a little bit behind. From f/4 and on, the lens performance is very satisfactory with excellent performance throughout the frame. At its peak around f/5.6 the lens is capable of producing outstanding 16 and decent 24in prints. Conclusion? A decent f/2.8, but mediocre f/1.8 lens. If you require fast wide angle, be aware of the above-mentioned limitations.

 

MTF50 (Line Width/Inch on the Print) @ 20mm
MTF50 (Line Width/Inch on the Print) @ 20mm

 

Normalized raw MTF50 @ 20mm
Normalized raw MTF50 @ 20mm

 

Chromatic aberration on an APS-C camera was somewhat on a higher end with wide apertures, reaching ~0.9px in the center and ~1.2px around borders at f/1.8. CA in the center quickly drops to quite low levels with stopped down aperture and averages ~0.3px in the f/2.8-f/11 range. CA around borders also drops, albeit at a slower pace, averaging ~0.7px from f/2.8 through f/11.

 

Image borders @ 20mm (100% crop): f/1.8 vs f/8
Image borders @ 20mm (100% crop): f/1.8 vs f/8

 

Canon FF: The lens gave up some ground on a full frame camera. This is especially noticeable around borders, where the lens struggled the most. But everything in due order. At f/1.8, performance in the center is as mediocre as with an APS-C camera, and border performance is even worse. While image quality in the center improves quickly with stopped down aperture, reaching very solid levels by f/2.8, border quality does not reach decent levels until f/5.6. Performance around borders as well as in the center peaks in the f/5.6-f/11 range, which seems to be the sweet spot for this lens. Conclusion? Performance at wide aperture levels is quite dismal, but at least quality improves to quite decent level with stopped down aperture. Some of the alternative wide angle primes are not capable of even that.

 

Normalized raw MTF50 @ 20mm
Normalized raw MTF50 @ 20mm

CA remained more or less under control on a full frame body, averaging ~0.8px across the frame at f/1.8. By f/4, CA drops to a more manageable 0.4px in the center and ~0.6px around borders.

 

Image borders @ 20mm (100% crop): f/1.8 vs f/8
Image borders @ 20mm (100% crop): f/1.8 vs f/8

 

Alternatives

Sigma currently manufactures two other super fast wide angle lenses, which are worth taking a look at, assuming you're willing to go for a slightly longer focal length. The two are 24mm EX DG ASPH Macro and 28mm f/1.8 EX DG ASPH Macro. Don't get fooled by the 'Macro' designation - neither of the two provide 1:1 life-size macro, it's just Sigma likes giving 'Macro' name to all lenses capable of producing 1:3 or larger magnification. Canon's EF 20mm f/2.8 USM (review) is another wide angle alternative for the EF mount, however, this lens does not offer much benefit over Sigma's lens, considering it is slightly slower, slightly more expensive and and a bit worse performer. You might also want to consider Canon's 24mm f/1.4L USM (review), which offers a slightly faster aperture, but also costs about 3 times more.

 

Recommendation

I can only admire Sigma's determination to produce fastest, meanest lens glass at affordable price. Sometimes the company manages to create an instant classic, other times it gives us a complete dud. Sigma 20mm f/1.8 EX DG lies somewhere in between an excellent and a completely mediocre lens. It sports an ultra fast (for a wide angle) aperture, but does not deliver good quality at such wide apertures. Nevertheless, quality improves with stopped down aperture and can be considered quite good in the f/5.6-f11 range. Build quality is another pro for the lens, AF performance is obviously con. But overall I would say that this is a lens that is worse your consideration, especially if you don't need very fast apertures.