This effect is called vignetting and is caused by using a lens attachment that is not properly designed for use with the camera.
To prevent this issue, ensure you are using a compatible lens attachment . Alternatively, using a higher f-stop number to make the aperture of the lens smaller can minimize, and sometimes eliminate, the vignetting effect.
IMPORTANT: Vignetting will occur in the viewfinder when using a DT Lens on a full-frame digital SLR camera when shooting wide-angle pictures. DT lenses are not recommended for use with full-frame digital SLR cameras .