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Does the focal length change? Or does it remain constant and only the brightness of the image is reduced?

enter image description here

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    $\begingroup$ The shape of the bokeh will change to match the new shape of the lens aperture. Assuming the lens is cut exactly through the center, the bokeh will become semicircles instead of full circles. $\endgroup$ – user205719 Sep 14 '18 at 15:38
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    $\begingroup$ Is the cut off portion of the lens discarded or displaced like in the picture? $\endgroup$ – Mark H Sep 14 '18 at 15:59
  • $\begingroup$ @MarkH its displaced $\endgroup$ – Aura Sartori Sep 14 '18 at 16:07
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you will end up with 2 lenses at different locations and some light will go to 1 lens, some to the other and some to both resulting in 3 images in 3 different planes - so yes the focal length will change for the 2 new images.

Each lens piece will have a lower aperture than the full lens and therefore a lower image quality.

In microscopy we often hide part of the incident light (see oblique illumination)- which amounts to the same as using a section of the lens- for increasing contrast at the expense of definition.

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