An excellent lens at all apertures, albeit one that takes its time to reach full resolution as the aperture is narrowed. Apparently this is be deliberate design decision by Zeiss -- the lens prioritizes an even performance across the frame when fully open over sharpness at the centre. Image quality is outstanding, that is why you but the lens after all. The construction is excellent. Auto-focusing is pretty quick on my a7RII, slowing a bit in poor light. It is less quick on the A7R. Manual focusing, should you require, it is a delight to use. One slight concern is the slight amount of longitudinal chromatic aberration that is visible at times from maximum aperture through to f/3.5 (its pretty well gone by f/4.0). This manifests itself as a cyan colour cast for objects behind the plane of focus (particularly at bright edge transitions -- twigs against the sky, that sort of thing) and magenta for objects nearer than the plane of focus. Not a huge problem and it can usually be mitigated in good image editing programs. Note, this is not the chromatic aberration (lateral) that shows up as coloured edges -- green on one side and red on the other which is usually quite easy to remove and may be done in-camera for JPGs.