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Wide Angle Alternatives

The region of around 28mm/30mm equivalent focal length is somewhat of a transition area on dSLRs. To go shorter than 17...20mm you almost always will have to use APS-C lenses. Longer than that, you can find reasonably small and capable full-frame lenses that work nicely on dSLRs.
To see what lenses that are wide or even ultra-wide on film can do on APS-C CCD, I ventured out with four lenses:

Tamron SP Adaptall 17mm 3.5 The shortest focal length I have and well-regarded on film
Sigma 18-125mm 3.5-5.6 DC APS-C only, lazy zoom
Cosina 20mm 3.8 The cheapest ultra wide I know, also rather small
Carl Zeiss Jena Flektogon 20mm 2.8 A legendary lens - but criticized by many as being hyped

I now reshot all photos wide open, plus at f/8.0. I tested the Flektogon also at f/3.5, as this is the aperture that the other lenses are roughly wide open. I shot from a tripod, took three or four images with each lens and aperture, manual focus each time from new, picked the best by file size. None of the images have been sharpened or softened or levels adjusted, the crops are the utmost corners. I did not vary the distance, it was 2m for all lenses (so roughly 100x focal length).

I chose this rather bad page design (horizontal&vertical scrolling) because I believe it makes comparing easier.

full frame	Centre	upper left corner	upper right corner	lower right corner	lower left corner
Tamron SP Adaptall 17mm 3.5 @ f/3.5	Tamron	Tamron	Tamron	Tamron	Tamron
Sigma 18-125mm 3.5-5.6 DC (@18mm) @ f/3.5	Sigma	Sigma	Sigma	Sigma	Sigma
Cosina 20mm 3.8 @ f/3.8	Cosina	Cosina	Cosina	Cosina	Cosina
Carl Zeiss Jena Flektogon 20mm 2.8 @ f/2.8	Flektogon	Flektogon	Flektogon	Flektogon	Flektogon
Carl Zeiss Jena Flektogon 20mm 2.8 @ f/3.5	Flektogon	Flektogon	Flektogon	Flektogon	Flektogon
Now, all lenses at f/8
Tamron SP Adaptall 17mm 3.5 @ f/8.0	Tamron	Tamron	Tamron	Tamron	Tamron
Sigma 18-125mm 3.5-5.6 DC (@18mm) @ f/8.0	Sigma	Sigma	Sigma	Sigma	Sigma
Cosina 20mm 3.8 @ f/8.0	Cosina	Cosina	Cosina	Cosina	Cosina
Carl Zeiss Jena Flektogon 20mm 2.8 @ f/8.0	Flektogon	Flektogon	Flektogon	Flektogon	Flektogon

Conclusion:
Ok, I admit I focused them manually without a viewfinder magnifier. Still, by focusing three or four times and picking the best, I hope some conclusions can be drawn.
I think it is obvious that at f/3.5 the Sigma lazy-zoom wins clearly, the Tamron has the most coverage and paying twice as much for a used Flektogon than for the cheap Cosina is not a good deal unless you really need that extra 1.5 stops. Also, the 20mm Cosina is wider than the 20mm Flektogon. Distortion on the Sigma is very weird, it is a mix of barrel and pincushion distortion, called gullwing or moustache distortion. The Flektogon has the best distortion correction, IMO.
At f/8.0 the film lenses are closer to the Sigma lazy-zoom, but still no match.

I am willing to extrapolate: If you are looking for an affordable ~18mm wideangle lens to use on your APS-C dSLR, consider dedicated "digital" wideangles lenses in favour of old film "ultrawides".

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