Sigma AF 20mm f/1.8 EX - Review / Test Report
Lens Reviews - Canon EOS (APS-C)

Review by Klaus Schroiff, published October 2005


The Sigma AF 20mm f/1.8 EX owns one of the industries records - with its max. aperture of f/1.8 it is the very fastest 20mm mass production lens around! As of today the lens is available for the classic systems as well as in native Sigma mount. Sigma has recently released a DG version of the lens featuring extra coating on the rear elements for better flare performance but other than that the principal design remained unaltered so most of the findings should also be valid for this new variant.
In the scope of this report the lens was tested using an APS-C DSLR resulting in a full frame equivalent of 32mm.

Thanks to the ultra-large aperture and the corresponding need for large diameter glass elements the lens is rather massive with a weight of 520g and a size of 89x90mm. The filter size is a whopping 82mm and it is a good idea to use one to protect the super-sized front element. As illustrated in the product images above the package also includes a dedicated petal-type hood. The optical construction is made of 13 elements in 11 groups with two aspherical elements and 9 aperture blades. The min. focus distance is 0.20m resulting in a surprisingly large max. object magnification of 1:4.

The lens incorporates a Dual-Focus (DF) mechanism which allows you to decouple the focus ring from the focusing gear by pulling/pushing it back and forth. The idea behind this remains somewhat mysterious to me because you still have to switch the lens from AF to manual focusing - independent from the dual-focus mechanism that is. So with two switches (DF & AF/MF) you've four different behaviors:

FRONTAFAF with rotating focus ring
BACKAFAF with non-rotating focus ring
FRONTMFmanual focusing via focus ring
BACKMFneither manual nor auto-focusing

To me it seems as if Sigma is trying to market a mechanical workaround as a feature. Most better quality lenses don't have a rotating focus ring in AF mode anyway so what's the advantage of DF compared to convention focusing mechanism ? Tokina's one-touch focus-clutch mechanism used in some of their AT-X Pro lenses is similar but it combines DF with a change of the focusing mode which makes more sense to me.

Apart from DF the construction quality is decent using the typical EX ("Excellence") finish (crinkle style). The very broad, rubberized focus ring has a smooth action but unfortunately it feels a little wobbly when decoupled using the DF mechanism.

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