Optical and physical vignetting

Vignetting can be caused for various reasons and results in the darkening of the corners of an image compared to the center. 

Optical vignetting

This can happen with wide-angle lenses when using a wide-open aperture setting. The darkest area of the vignetting is the corner of the image with a gradation inwards towards the center of the image, sometimes this effect is hardly noticeable. 

Light entering the front of the lens at acute angles (not immediately in front of the lens) is partially blocked and does not cover the whole open aperture area with the same intensity, thus the light at the edge of the image frame is weaker. When the aperture is stopped down, light entering at acute angles will cover the smaller area aperture with the same light intensity and thus the light across the image frame is consistent. 

Another reason for optical vignetting is that light further away from the optical axis has to travel a longer distance which causes it to be weaker and thus cause the vignetting. 

optical vignetting

Vignetting can also be reduced using the vignette control option in the camera’s shooting menu. This can be set to High, Normal, Low and Off to suit your preference and is applied to images until it is turned off. 

vignetting removed in software

Physical vignetting

Vignetting can be caused by the use of non-compatible accessories, which may block light from entering the lens. The effect of the vignette is stronger on the image than optical vignetting as light is completely blocked from reaching the image sensor and can be a small corner vignette as shown in the image or cover a larger area of the image depending on the circumstance.
A common cause of this is the use of a non-compatible lens hood that will physically fit the lens, but the shape and size do not match the lens optics and the hood will physically reduce the light reaching the sensor at the edges of the frame.

Physical vignetting can also occur when using a flash or a shadow cast on images when using a built-in Speedlight.

physical vignetting

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