took some images of some desktop items at 24mm setting,one foot from desktop items at f8.0 etc. in jpeg and Raw dng. The jpeg image came out fine. But the raw images on each corner have circular black areas(vignettes) which i presume is the lens? Why is this happening and how can this be corrected. Also i shot the images at f 8.0,but when downloaded the jpeg image showed shot at f8, but the raw images taken showed taken at f 7.5. Why the differential? All six of the raw images although shot at different shutter speeds showed shot at f7.5? Attached are the two images.