More specifically: optical obscuring of the content at the horizontal edges.
Or: optical obscuring of the content at the upper horizontal edge.
Alternative: optical aberration of the content at the horizontal edges.
Or: optical aberration of the content at the upper horizontal edge.
Ref.:
https://amostech.com/TechnicalPapers/2018/Optical-Systems_In...
"The term aperture efficiency is not normally found in journals and texts on optical theory. Here
aperture efficiency is defined as the product of the aperture diameter divided by the diameter
of the largest optical element, squared, times the integrated field dependent vignetting function
and obscuration function as seen in equation 1."

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Note added at 5 hrs (2022-04-19 20:31:05 GMT)
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Synonym of optical obscuring: optical obscuration (as in the Reference).