Sharpness of the 12mm Voigtlander, f/5.6

Whilst looking at lens tests on the photozone web All Tests / Reviews site three tests came to my attention, the Panasonic kit 14-45, f/3.5-5.6 lens, the Olympus kit 14-45, f/3.5-5.6 lens and the Voigtlander 12mm, f/5.6 tested on a Sony Nex.

The sharpness figures for the centre and extreme edges are very interesting. Looking at f/5.6, f/8 and f/11 the Voigtlander is sharper in the centre than either of the two kit lenses and at the edge is sharper than the Olympus at both f/8 and f/11 and as sharp as the Panasonic at f/11.

Where it suffers is in vignetting in the corners equivalent to 2 stops.

The photozone site sums up the Voigtlander lens thus :-

The lens is extremely sharp in the image center and still good in the image borders/corners. Distortions as well as lateral CAs are very well controlled for a lens in this class. The primary weakness of the Heliar is the heavy amount of light fall-off combined with a slight color shift in the image corners. These two issues are mostly correctable but this requires a manual step - e.g. either in Photoshop or via CornerFix. However, if you're not willing to care it will be probably a better idea to stay clear. The build quality of the Voigtlander lens is very good and a joy to use in the field. Regarding the slow max. aperture and the rather long min. focus distance its scope is mostly limited to maximum depth-of-field applications but this is probably fine for most users anyway. The pricing is quite steep for a non-dedicated lens but on the other hand it doesn't get any smaller and more light-weight (for the time being).

The report then goes on to detail a way of correcting the colour shift.
Vignetting and Color Shift
"Classic" rangefinder lenses used on digital camera tend to produce a color shift in the image corners. This is a major problem on the Leica M8/M9 due to their bigger (APS-H/full-format) but the issue isn't quite as pronounced on the APS-C sensor of the NEX cameras - at least for the Heliar 12mm. Nonetheless the shift can be noticed at times but it can be resolved fairly easily (along with the light-falloff) during post-processing e.g. in Photoshop ... if that's an option for you. All it takes is an image mask - from here on the solution is just a mouse-click away.
How to create a correction mask:
1. look for a uniform white or gray plane and apply manual WB on this plane
2. take an image with ~1 f-stop exposure compensation
3. create a 2nd image and fill it with the gray value from the very center of the first image
4. overlay both images using "divide" overlay mode - that's your mask - it's basically white in the center and dark in the corners. You may save it for future use.
5. overlay your real world image with this mask and choose "divide" again.
6. done.
Alternatively you may try "CornerFix" over at SourceForge.org.

I hope this information might be of interest to anyone contemplating the purchase of this lens. I took some time to make my mind up about it but having used it for some time now it's beginning to grow on me.

Barrie