Just a quick note about color shading. And a camera that allows you to correct unwanted color shading for up to 10 individual lenses.

this is an image with no unwanted color shading. The Sigma 65mm and the Leica SL2 are both members of the L mount Alliance and lens information is automatically transferred to the camera. Corrections for most major lens aberrations are made at the time of exposure.

Some of us who like to dive into unneeded complexity in our photo lives seem hellbent on adapting old lenses (which are often called "legacy" lenses), or inappropriate lenses, to whatever camera we feel like shooting in the moment. If we were rational we'd just select the newest lens from the same camera company and same generation as our cameras and take advantage of the built-in corrections for distortion, fall off, vignetting, chromatic aberrations and COLOR SHADING.

Color Shading is the tendency for non-telecentric lens designs to have color shifts across their frames. It usually shows up as a shift in color at the corners, or corners and edges, of the frame and sometimes coincides with vignetting. But sometimes color shading comes alone...

The biggest culprit isn't really some sinister, planned thing, it's more of a mismatch between lenses  designed for use with film cameras which are pressed into service onto digital cameras. Although just about any sort of lens can exhibit some color shading which, if left uncorrected, can be an issue. This color shading shows up more often in wider angle lenses and is usually absent as lenses crest 50mm focal lengths and longer (for the 35mm format). 

I won't get into the physics cuz I'm just a photographer but it has to do with designing lenses for digital tech where all the light rays need to be perpendicular to the sensor plane versus the way they designed lenses for film. Since film didn't have "depth" (pixel wells) wide angle lenses for non-SLR cameras (Leica and Zeiss rangefinder lenses) could be designed to sit much closer to the film plane and let the rays that needed to hit the edges (corners and sides of the frame) arrive at the furthest areas at a shallower angle instead of head on. The angle of the light rays from lenses designed this way are attenuated because not all the light is used efficiently at the edges of the sensors. To combat this in current Leica rangefinder (M Series) cameras the camera makers use much thinner filter stacks in front of the digital sensors. They can also try to design progressive "light pipes" across the frame to better deliver the angled light rays to their peripheral targets. But it's always a bit of a compromise. If you buy Leica M cameras and use them exclusively with Leica M lenses then, in addition to the customize filter stacks, there is a catalog of profiles and corrections in the camera's memory that allows the system to fine tune the response of the camera and lens together to correct most faults. It's a software augmentation.

Where it all falls apart is when you use "designed for film" rangefinder lenses on mainstream digital bodies from other makers. Or third party "designed for film" lenses on Leica SL cameras. These cameras do have a fairly extensive catalog of Leica M series lens settings but nothing for third party lenses; like the Zeiss lenses I'm using...

So, when I first shot the 28mm and 35mm ZM lenses on a Leica SL2 body I got colors that shifted in the corners. The shift was also non-linear. I could work on removing it in PhotoShop but it was almost impossible in images where important and extensive detail was placed near the sides or corners of the frames. 

I could get closer if I "cheated' and used a Leica M lens profile from the menu that was a near enough match for the ZM lens I was using but it was hit and miss. 

A few weeks ago I touched on the fact that the Sigma fp camera, and I'm pretty sure the fpL also, has a feature that allows you to go into the lens compensation menu, choose: "color shading" and make a correction for up to ten individual lenses. You set up the camera and lens to face a monochrome target ( I used a big, white foamcore sheet tacked to the wall. Light it so it's uniform from side to side and corner to corner. They you push the "AEL" button on the camera while in the color shading sub-sub menu and it makes a measurement and writes it to the camera. You can select a slot for the info for each of your lenses and label them. I started with the 28mm. It's lens #1. The feature also allows you to describe the lens at each slot and label it. My top slot reads: "28mm f2.8", and in smaller type: "ZM 28mm f2.8." 

I compared the before and after tests and the feature in the camera works very, very well. The corners on all my tests were as neutral and shift free as I could ask for. Being set up already I decided it would be a good time to test and calibrate all my favorite wide lenses that featured older, film-oriented designs. I did the 28, the 35mm, the 40mm and even tossed in the CZ 50mm f1.4 ZF just for grins. I don't know of another camera that offers this degrees of lens optimization but I'm happy to have it in the Sigma fp. 

Now that I've optimized the camera to work well with these lenses I'll probably end  up using it a lot more for day to day work. It's small, light, has amazing color and now boasts the highest degree of optimization between these interesting lenses and the sensor of any camera I own. 

It took all of five minutes to "fix" four different lenses. And I still have six more slots to fill up. Fun with cameras.