Full-frame sensors and even APS-C sensors used with rangefinder lenses on cameras like the Sony A7R and Sony NEX can exhibit degraded performance off-center, including a hit to sharpness, but also a color cast. This behavior is documented extensively in Guide to Leica in Ray angle and Color Shading With M Lenses.
The off-center color cast is not limited to rangefinder lenses and in fact can occur on DSLRs as well, particularly with designs having less telecentricity (and hence a more acute ray angle).
The Zeiss 18mm f/3.5 Distagon is one such lens with a slight off-center color cast (at lest on some DSLRs). A few years ago I documented how to correct an off-center color cast in Correcting Color Shading Using Photoshop Masks. The technique is generally applicable to any camera.
When the Sony A7R arrives, I might take up this topic again and add a generalized form to the Workflow area in DAP. depending on whether I feel the results are satisfactory: while a color cast can be corrected, there is no way to compensate for the loss of sharpness in the same area; that is an optical blurring through the sensor filter pack.
Toggle the image below to see the corrected vs uncorrected result.
Steven K writes:
Have you tried Adobe Labs DNG Flat Field Plugin for Lightroom for correcting color casts? See https://labs.adobe.com/technologies/Lightroom plugins/
It’s much more convenient than mucking about with Photoshop layers and produces good results with my Leica M9P and wide angle lenses. I am vexed that Leica’s profiles for the M9 series are not good enough at correcting color for critical work for any of my lenses (24mm/3.8 ASPH, 28mm/2 and 28mm/2.8 ASPH and 35mm/2.5 Summarit (not to mention the Zeiss 21mm/2.8 and 25mm/2.8 ZMs), but Adobe’s software fix usually does the job of correcting color (or color + vignetting) quite well, even on snow. I use a credit card sized piece of opaque white plexiglass plastic held in front of the lens for making my white reference files at each aperture.
The main drawback is that you have to use DNG files, so if you want to process .NEF files, you have to make a DNG copy of those first. Also the file takes the original scene file and reference file and calculates and saves a third DNG file (adding the ending ff onto the original file name). But since the whole process is in RAW in LR, not Photoshop, the files are much smaller than they would be using the layers approach in Photoshop for color cast correction.
DIGLLOYD: first, note that the name is badly chosen: “flat field” refers to field curvature and the plugin has nothing to do field curvature. It ought to be named “Adobe Color Shading Correction Plugin”.
The Adobe Lightroom plugin looks like a good solution to correcting a color cast for Lightroom users. But until and unless it is supported in Adobe Camera Raw in Photoshop, it’s a non-starter for me personally. Also, correction is aperture-dependent and that is tricky, since the camera cannot know or record the true aperture used—it makes a guess and records that.
With regards to Leica M, it is disappointing that after 4 years, Leica has not made the effort to provide a correction-customization tool for particular lens samples (which could be installed into the camera); there is enough variation with M lenses that the generic correction is worthwhile, but far from perfect with many lenses.