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The quintessential window-lit portrait has a soft glow about it. A photo may be sharp, but it's perceived sharpness is on a different level from a photo taken using a single light utilizing a snoot or honeycomb grid.

To what extent and how does the light source affect this difference in perceived sharpness?

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    Adding some examples to illustration your point might help. I think you might be confusing sharpness with something else.
    – Caleb
    Jan 5, 2018 at 19:39

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The light doesn't affect sharpness. However, too much light in the wrong place can appear to reduce sharpness due to bleeding.

If you take a picture of someone inside a room against a window looking out onto a sunlit scene, then the overwhelming brightness from the window will bleed into the foreground a bit. This will be especially apparent at the edges. That isn't technically less sharp, but can appear that way.

Otherwise, diffuse light can hide small variations and textures, while a point light accentuates them. The more smoothed out details resulting from diffuse lighting can again appear like less sharpness, due to the lower contrast. Again, though, the lens is still doing the same job and the diffuse-lit picture doesn't really have less sharpness. It does have lower contrast because the scene itself has lower contrast. That can appear as, or be misinterpreted as, lower sharpness.

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  • I note perceived sharpness and maybe I should have said edge sharpness, micro contrast, or acutance. The lens captures a relatively small amount of any of the photons being reflected by a subject, so how is it that diffuse/scattered light - still being captured at the same angle as those being reflected by a hard light, creates a drastically different photograph?
    – OnBreak.
    Jan 5, 2018 at 18:13
  • @Corey: For the same reason that diffuse light creates a drastically different scene.
    – Olin Lathrop
    Jan 5, 2018 at 19:21
  • I understand the what...I'm asking about the why...
    – OnBreak.
    Jan 5, 2018 at 19:34
  • @Corey: You changed your original question quite a bit. However, diffuse light comes more more angles, and therefore more evenly illuminates surfaces at different orientations. This makes surface detail less evident, which can make photographs appear less sharp.
    – Olin Lathrop
    Jan 5, 2018 at 20:27
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From the Wikipedia article on acutance:

In photography, the term "acutance" describes a subjective perception of sharpness that is related to the edge contrast of an image. Acutance is related to the amplitude of the derivative of brightness with respect to space. Due to the nature of the human visual system, an image with higher acutance appears sharper even though an increase in acutance does not increase real resolution.

The human visual system is more sensitive to differences in contrast than to differences in absolute luminance.

Soft light reduces contrast, particularly the edge contrast between darker and lighter areas. Hard light increases contrast, particularly the edge contrast between darker and lighter areas. Our eye-brain vision interprets the increased edge contrast as "sharper."

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