SIGMA 20mm f1.8 EX DG Aspherical RF
SIGMA 20mm f1.8 EX DG Aspherical RF is the first 20mm super wide-angle lens in the world with a large aperture of F1.8 and angle of view 94.5� (58.7 on a 1.6 crop DSLR). It has Macro focusing capability with a minimum focusing distance of 20cm/7.9inches (reproduction ratio 1:4). The iris diaphragm has 9 diaphragm blades to obtain beautiful out of focus image. It incorporates an aspherical lens element in the front as well as rear lens groups, to minimize distortion, spherical aberration and astigmatism. The lens has non-vignetting optical construction, in order to obtain adequate peripheral brightness with open aperture. This is especially desirable for digital cameras. The lens incorporates a rear focus system eliminating front lens rotation, thus allowing the use of a Perfect Hood and easy use of polarizing filters. The lens also incorporates dual-focus mechanism. It is easy to hold the lens, since the focusing ring does not rotate during auto-focus, yet it provides adequate focusing torque of the focusing ring during manual focusing of the lens. The lens materials used in this new lens are lead and arsenic free ecological glass.
The Sigma 20 quickly became one of my most used lenses. I find it sharp, fast and with fairly neutral colour rendition (slightly warmer than Canon). CA is very well controlled and barrel distortion within reasonable limits for a 20mm lens. It is useful for working in poor light and even at f/1.8 is reasonably sharp (at centre). At f/2.8 it becomes much sharper (reasonable at corners) and for landscape work at f/8 - f/11 it is sharp form corner to corner on a Canon 10D.
For landscape work I usually pre focus manually with the infinity mark just touching the right side of the dof scale centre mark (illustrated). I find this gives me more reliable depth of field and sharpness rather than letting the AF decide where to focus (or trying to remember hyperfocal distances which are unreadable at at this focal length scale anyway).
The odd dual focus mechanism takes a little getting used to. However, I find that with manual pre-focus, having the main switch set to manual and having set the focus point - pushing the focus ring forward to the AF position - locks focus - which is convenient.
The only slight downside to the Sigma 20mm for me is that - with my adapted hood - the f/1.8 element makes it a BIG lens.