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Sigma 20mm f/1.8 EX DG Review

The 20 f/1.8 is an unique sigma lens: it is one of the brightest 20mm lenses even manufactured, and it has a very affordable price. I've become interested about 20 f/1.8 after having seen some spectacular Milky Way photos taken with this lens (Tessi's photos are those that inspired me most!); I was very doubtful about buying it because I had read only negative reviews, and I live in a light polluted area, so I don't have many occasion to take photos of the starry sky. In october 2008 I've finally bought it...

Lens Specifications

 Focal lenght 20 mm
 Lens construction 13 elements in 11 groups
 Number of Diaphragm Blades 9 blades
 Angle of view 94.50º (on 35mm camera), 71.25º (on APS-C camera)
 Max. Magnification 0.25x (1:4)
 Maximum Aperture f/1.8
 Minimum Aperture f/22
 Image stabilization No
 Autofocus Inner AF motor
 Full Time Manual Focus No
 Closest Focusing Distance 0.20 meters
 Filter size 82 mm
 Dimensions (Diameter x Length) 88 x 89 mm
 Weight 520 g
 Weather sealing No
 Price $ 460
 Announced 1998(?)
 Accessories Rear cap, front cap, lens hood, lens case.



The 20mm is relatively small - considering the extreme brighness, I expected a much larger lens, instead the 20mm is not too big and it is quite light. It is just a little larger than the Canon 20 f/2.8 and smaller than the Canon 20mm f/1.4. The built quality is good, even though it is not as good as Canon L lenses.  
It has a very large focus ring, that allows to fine tune the (manual) focus. At f/1.8 the depth of field is pretty shallow, so you have to focus carefull - whenever possible, I recommend to use live view for a precise focus. Of course, you can even use autofocus: this lens does not have HSM, nor full time manual focus, but the AF speed is acceptable. That said, it has the most noisy AF that I have ever heard! It sound like the scream of a velociraptor (if you are curios, click here to download a small audio file with the Sigma 20 f/1.8 autofocus noise).  
The lens is sold with a small hood and with a nice lens case. The only similar lens is the Canon 24mm f/1.4 L. The Canon lens is a bit less wide but it is is 0.5 stop brighter, and it is has much better image quality, even wide open. On the other hand, the Canon is three times as much expensive - it is difficult to justify the purchase of the 24 f/1.4, unless low light photography is a very important component of your work.

Image Quality

Remember that you can easily adjust color, distortion, chromatic aberration and vignetting with Photoshop; even though I list every aspect of image quality I don't consider these aspects as important as the other three that can not be corrected with Photoshop (sharpness, contrast and flare). You can enhance a little sharpness and contrast with PS, but the results are not as good as a photo taken with a lens that is sharp and contrasted.  
The image samples has been taken in RAW format and converted with Photoshop CS3; I processed each photo to optimize the image quality (contrast, color, sharpness), and I saved them as high-quality JPEG for web display. You can download both the processed JPEG and the original, untouched RAW files. Click on the image to download the full size JPEG sample, or use the RAW link to download the untouched RAW (large file!). Please respect the copyright! This image can be printed only to evaluate the quality of the camera, for personal purposes. All other usages are prohibited.

Mountains and stupid writings - Sigma 20mm f/1.8, ISO 100, 1/40 f/1.8 [ JPEG / RAW ] - even the mountains are not out of reach for human stupidity (someone thought that it was funny to create offensive writings with rocks)

 Sharpness the sharpness is ok in the center, but it is poor or very poor in the corners.
 Contrast the contrast is average in the center and poor in the corners.
 Color the colors are perfect, without any lens-introduced color cast.
 Flare if there are strong light sources in the photo it flares easily.
 Distortion there is some barrel distortion.
 Chromatic aberration it has a little of chromatic aberration, but it can be easily fixed with PS.
 Vignetting it has a strong vignetting (it is on par with the Canon 24-105 at 24mm wide open)

Overall, the image quality is poor; the corners are so soft that they look out of focus...indeed, I think that this lens is not able to project a flat image on the sensor; instead it has a curved plane of focus, so when the center is in focus the corners are slightly out of focus. Stopping down improved a lot the image quality in the center, but even at f/5.6 or f/8 the corners are barely acceptable.

Other Reviews

. Sigma 20mm f/1.8 EX DG Review by The-Digital-Picture.com 
. Sigma 20mm f/1.8 EX DG Review by Petteri Sulonen 
. Sigma 20mm f/1.8 EX DG vs Canon 20-35 f/2.8 by Yaron Kidron 
. Sigma 20mm f/1.8 EX DG User Reviews on Fred Miranda Reviews


The Sigma 20 f/1.8 is the unique 20 f/1.8 of the world, so if you want such lens, there is not any alternative. That said, this lens has many optical flaws - lack of sharpness, lack of contrast, distortion, vignetting, coma, flare... I not recommend the Sigma 20 f/1.8, unless you want to take night photos (with a FF camera) - but even in this case, be ready to trade up a lot of image quality: it is the price to pay to get that unique super bright apertue on a 20mm, $460 lens. I have not decided yet if I'll keep this lens or if I'll sell it; for sure I am going to bring it with me in my next trips, and with time I'll evaluate if, for my personal needs, the trade up in image quality is justified by the results (currently I am quite doubtful).

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