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Wide Angle Alternatives


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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:
  1. Tamron SP Adaptall 17mm 3.5 The shortest focal length I have and well-regarded on film
  2. Sigma 18-125mm 3.5-5.6 DC APS-C only, lazy zoom
  3. Cosina 20mm 3.8 The cheapest ultra wide I know, also rather small
  4. 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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