Distortion & Conclusion
Distortion/Vignetting at f8 (full frame)
Rollover Sigma 12-24mm image to show Voigtlander 15mm
You can’t miss the vignetting at all apertures, which according to some reports is also a feature of its performance on the rangefinder cameras for which it was designed.
What you may also have noticed progressing through the 100% crops is the fact that, contrary to expectations, it is the Sigma 12-24mm that displays the best geometric correction, whereas the Voigtlander has some very subtle waveform distortion.
It should be noted that correcting for this vignetting is a piece of (non-deleterious) cake. A five second Quick Mask/Gradient/Curves action on each image would harmlessly fix the problem and give a better result than the Sigma every time. The Voigtlander at least gets enough information to the sensor to allow corrections to be made, like this:
So, we can pluck a rangefinder lens out of a Leica; we can adapt the mount and, with a little trepidation, we can sweep aside the mirror and force it down the gullet of our Canon digital cameras. But should we?
The Voigtlander 15mm Super Wide Heliar is sharper across the frame at pretty much all apertures. It’s corner performance at f5.6 and f8 is much better than the Sigma 12-24mm. It is certainly amazingly compact and portable on the camera, and makes a fabulous focus and forget hip shooter out of your 5D, with superb wide aperture performance… but every shot requires vignetting correction, the viewfinder isn’t the last word in utility and you do have to go the extra mile in forgoing even aperture priority metering.
Then again, the Sigma, for all its good behaviour, is no angel. This, the fourth sample I’ve tested, shows major decentring at the long end, making it unusable from 20mm up. But if you can find a sample that works, the combination of convenience and acceptable (if not stunning) performance of the Sigma 12-24mm makes it a tough act to beat if you’re prepared to stop down to the tiny apertures required for it to perform.
In other words, it’s hard to recommend either lens unreservedly. Much will depend on your priorities, subject matter and shooting style.