We have used all the lenses mentioned. The Sigma 12-24mm is an exceptional lens if you are willing to carry something much heavier and lose a little of that wide-angle end; also, it will be 'ready' for first full frame digital when you can afford one (and one day, you will be able to afford one). The Tamron/KM/Sony 11-18mm are all the same optical unit. The slowest focusing, and noisiest, is the Tamron design. KM's version has a much smoother focus drive and the same goes for the Sony, which also seems to have rather smoother zooming.
The distortion corrections of the Sigma 10-20mm and the 11-18mm are very different. Sigma has chosen a slightly spherical projection which is almost pure. This creates curved lines at all points, along both horizontal and vertical sides of the image, and there is no way of composing architectural shots with the Sigma to avoid the slight curve. However, by using PTLens, DxO Optics Pro or Adobe's Lens Distortion filter the Sigma is easily corrected. The 11-18mm design uses an optimised balanced distortion, where the projection changes from spherical (barrel) in the central field through a phase of almost rectilinear rendering around 10mm from the axis, to pincushion beyond this point.
Here is a comparison between the two which I made using b/w film and a full frame body:
You can see that the Sigma has more curve visible beyond the edge of the frame, but the progression is smooth. It bunches up the slats of the blind, where the Tamron type has isometric projection (it retains an equal spacing visually).
If you shoot interiors in landscape shape (horizontal) then the Tamron can be used with vertical wall edges or corners aligned with the short side of the frame, slightly in from the edge, and they will look dead straight without any processing:
This was actually shot on a Canon 300D, and this camera has a smaller sensor than the Sony models (1.6X factor instead of 1.5X) which makes the lens equal to a 12 to 20mm on Canon, relative to its 11 to 18mm on Sony. You will see that at the right hand end, a very slight barrel on the WiFi sign changes to a hardly visible pincushion at the extreme of the shot. On the Sony, this pincushion becomes more visible. However, the 95% viewfinder crop means that you get a very straight rendering just next to the viewfinder edge.
The Tam/KM/Sony is not so good for vertical compositions or sea horizons, as at the 7-8mm zone from axis, there is some barrel (centre of the long side) but beyond 12mm it becomes pincushion. This means that vertical walls placed near the frame edge in upright shots appear to have a slight wavy curve, and same applies to straight horizons - they bulge up in the middle, dip, then go up again to the corners. This is called 'moustache' distortion. The Sigma tends just to produce a curved horizon which looks half way to being a fisheye shot.
The 11-18mm design is fully corrected by DxO Optics Pro, when used as a processor for the raw or JPEG images. DxO contains a map for the lens, and removes the complex distortion. It also corrects the Sigma, but it's easier the correct the simple distortion of the Sigma using ordinary tools in Photoshop. See:
http://www.photoclubalpha.com/2007/05/3 ... ke-an-eos/
This article covers the earlier version of DxO Optics Pro.
Here is the rollover image page which shows how the program corrects illumination, sharpness, aberrations and geometry when using the 11-18mm - just move your cursor over the original image, and it changes to show the DxO processed version:
http://www.photoclubalpha.com/dxorollov ... lover.html
My preference is for the Tamron design, as it also offers better illumination and a larger image circle. The Sigma was designed for Canon 1.6X and Sigma's own 1.7X formats; it's pushing its own quality in the frame corners on 1.5X. The Tamron design as the b/w example shows has much better field coverage. In fact, you can set the Tamron/Sony design to 14mm, and it will cover full frame sharply with excellent results if stopped down to f/11.
David
