All lenses produce a vignette due to light taking longer to reach the pixels at the edge of the sensor. A vignette is where the pixels at the edges of the sensor are darker than they should be. The wider the lens angle of view the more likely the wavelengths in a narrow band-pass filter will also be shifted outside of the incident angle, resulting in a vignette.
To correct the lens vignette a series of flat field images are captured in various lighting conditions.
You can apply vignette corrections using a parameter during the conversion of RAW(.mapir) to TIFF using the "Convert RAW(.mapir) to TIFF" script.