So let me set the tone for these series of posts by saying - using the right adapter you can attach any full frame or medium format lens created in the last 60-70 years to the Sony E-mount cameras. It makes these cameras very versatile. This is not to take away from other mirrorless cameras all of which can support the same lenses, but only a7 family packs the full frame sensor... ...for now.
Ability to fit such a variety of lenses is due to mirrorless cameras having a short flange focal distance, the distance between the lens and the sensor. Short FFD allows fitting many 3rd party lenses through adapters that place the lens at the right distance from the sensor - matching the FFD of camera mount the lens was manufactured for. This is true for all mirrorless cameras.
Unfortunetly, this is also a drawback. Mirrorless camera mounts were originally developed with a smaller sensors in mind - either Micro 4/3 or crop 1.5/1.6. Hence in two cameras that have the same FFD, the angle at which the light hits the sensor edges is shallower for a full frame camera than it is for a smaller APS-C sensor camera - causing light falloff and astigmatism. Of course Sony could've avoided this problem by introducing a new mount or changing the FFD, but then none of the E-mount lenses would work on the a7 family and all the adapters released for E-mount would be useless. So Sony and the folks at Zeiss decided to stick to the current mount design and deal with the challenges. Both a7 and a7R have offset microlens arrays at the edges of the sensor to compensate for this problem, however when using some optics this is not enough.
Sony a7 family are not the only cameras that have to deal with challenges of short flange focal distance and full frame sensor. Leica M has similar issues, however Leicas M mount FFD is 9.8mm longer than the Sony, which is a huge difference for precision optics, and it changes the dynamics quite a bit. Nevertheless, Leica M has to do some corrections in software to ensure image quality across its whole lens range. Sony does it as well, but not for 3rd party lenses.
Using some 3rd party lenses on the Sony results in image quality issues. This is true mostly for wide angle lenses. Any lens wider than 35mm, that doesn't have aspherical design will exhibit significant image quality issues. Lenses of focal lengths 35mm and up are less affected. Even aspherical lenses, such as Zeiss Distagon, experience some of the issues mentioned below.
In technical terms, wide angle spherical optics design positions the focal point of the lens too close to the sensor, causing increased light falloff and astigmatism. As a result, image quality suffers.
Wide angle issues include:
- Smudging and loss of detail towards the corners, mostly due to astigmatism
- Heavy vignetting due to light falloff
- Magenta casts in the corners, inline with the vignetting
- Yellow/red colour casts along the sides of the frame (less apparent)
- Increased chromatic aberration
Besides #1, all the issues above can be solved by using exposure compensation and using flat field correction in post processing. Exposure compensation brightens up areas of heavy vignetting, reducing noise levels in post processing, and flat field correction takes care of the rest. For #1, smudging and loss of detail, there is no solution in post processing, but stopping the lens down to f8 or f11 helps. There is some residual distortion in the corners that cannot be fixed. I don't think its a big problem, especially if you use vignette to hide the defects.
I have tested several wide angle and ultra wide lenses, read numerous reports, and have seen photos shot on a7 and a7R with every wide angle lens possible (look for links to parts 2 and 3 of this report below). There is no 3rd party lens below 35mm that doesn't exhibit any of the aforementioned issues besides the ones of aspherical design, and even those cause minor issues in most cases. The safest wide angle options are modern 24mm DSLR primes that were designed with full frame sensor in mind. Some modern DSLR lenses wider than 24mm as well as all older SLR lenses wider than 35mm exhibit noticeable issues in the corners. All rangefinder lenses exhibit the aforementioned issues below the 35mm focal length.