What is vignette?
A black part like a black tunnel which occurs when using an incompatible lens with image ssensors. It looks like it peeps in the tunnel.
See the dark side in the 4 corners of the frame
Cause of VIGNETTE
For example, in the case of a borescope,
As the sensor size of the camera connected to the borescope becomes larger, the vignetting area becomes wider. On the other hand, the image seen directly with the naked eye becomes closer to the image observed through the borescope.
In the case of a microscope
Typically, microscope cameras are designed not to have vignette (shadows in the four corners). Designers make allowances to prevent vignette, but the degree varies depending on the manufacturer.