Here’s a quick, rough comparison of 3 APS-C ultra-wide angles for the Sony Alpha mount! All shots handheld using the A900 in full-frame mode, then cropped to APS-C region and midtones of Levels adjusted from 1.0 to 1.5. Pardon the exposure variance.
Photoshop’s Polygonal Lasso Tool was used to plot the corners of each straight line; a screenshot of that was then taken to capture the dotted outlines.
Left to right: Tamron 10-24mm F3.5-4.5 DiII, Sigma 10-20mm F4-5.6 EX DC, Konica Minolta 11-18mm F4.5-5.6 DT.
The following six pictures can be clicked on for a larger view:
The Tamron gives full-frame coverage at 24mm. It also does so from 15mm onwards. (This is the same shot as D) above.)
The Konica Minolta gives full-frame coverage at 18mm. It also does so from 15mm onwards. (This is the same shot as F) above.)
The A850 and A900 switch to APS-C mode automatically when a Konica Minolta DT or Sony DT lens is attached. However we can override this by pressing the lens release button and turning the lens slightly to disconnect the pins. Further turning will close down the aperture blades.
The Tamron at 24mm F4.5 at its Minimum Focus Distance of 24cm gives the shallowest depth of field on full-frame. Bokeh is not all that clean, though.
Honestly, I can’t tell if there is complex distortion, but all of these lenses have barrel distortion at the widest focal length (where the wall connects to the ceiling.)
As expected, none of the lenses distort lines coming from the center outwards (the table is straight.) Heck, my Peleng 8mm F3.5 circular fisheye doesn’t do so, either!
I do find the Tamron sufficiently sharp at the focus point, even wide open. However you might be fussier than I am – in which I’d recommend you stop down to F8-F16 as what people who shoot landscapes and interiors do.
24mm on the Tamron, is a lot longer than 20mm and 18mm on the Sigma and Konica Minolta respectively. It is very obvious (between D) and F) for example). Personally, I like my ultra-wides to reach 24mm on APS-C or 35mm on full-frame at least, so the Tamron can look at least somewhat normal and not ultra-wide if I needed to get a less-distorted shot. Full-frame ultrawides usually reach 35mm, anyway!
My personal choice would be the Tamron 10-24mm F3.5-4.5 DiII, if I had an APS-C camera.
Bonus comparison! Left: Sony Alpha 850 with Minolta crossed-XX 50mm F1.7 lens; right: Sony Alpha 900 with Minolta 50mm F1.4 lens. Notice that the surface is different; the A850 has a matte finish which looks more gray while the A900 has a speckled black finish.