How does the sharpness of a dark nebula's edge relate to its viewing geometry?

Answer

A closer, moderately dense cloud creates a sharper edge against a busy background

The perceived contrast and sharpness of the edge are influenced by the geometry; a closer cloud blocks a higher percentage of background light, resulting in a sharper delineation against a bright, busy field.

How does the sharpness of a dark nebula's edge relate to its viewing geometry?

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