To my understanding the softness of the light and its consequent shadows can be linked back to:

  1. the angular size of lighting source
  2. the presence (type, thickness, etc.) of a diffusion material

If I’m not wrong the reason the size and/or distance of the source matters, is because at a, for example, infinite distance the rays of light would appear/project virtually parallel, which leads to very hard shadows.

Do the size of the object and distance of the object have separate effects or do they both explain the reasoning I just gave?

Secondly, from what I have read, the diffusion material seems to scatter the light by consecutive partial reflections of the beams of light inside the medium. Ultimately this results in scattered light of primary rays, secondary rays and so on.

So I would assume that this would mean, that if I added a perfectly molding piece of diffusion over a lightbulb, which is virtually keeping the size of the source the same, the light would still be softer due to the diffusion?

Lastly, from what i have noticed, it still seems that when I put the diffusion further away from the source, the light seems to be even softer. Why is this the case?

To me the source of light would still be the same size, as adding the diffusion at location x, shouldn’t make a difference seeing as, when I am imagining the flux lines traveling from the source. The area of the incident flux lines at the location of x, would have occupied that same size regardless of the presence of a diffusor? So although the answer might be that the relative source of the light is now much bigger, that reasoning doesn’t quite make sense to me.

I added a very rough drawing below, to hopefully better support my incoherently structured question.

Thank you for your time

enter image description here

  • \$\begingroup\$ Try to keep keep posts to one question at a time, it seems like you have a few related but different questions. \$\endgroup\$
    – Topcode
    Nov 15, 2023 at 21:34

1 Answer 1


The diffuser re-emits the light it receives. The light it receives decreases like the square of the distance to the source. If you have a small source and the diffuser is (roughly) a plane, the center of the diffuser(*) is closer to the source and receives significantly more light than the edges, so the center of the diffuser re-emits more light (in other words, diffusion isn't completely uniform). The farther the source, the less noticeable this becomes because the relative difference of distance to the source between center and edges is reduced (assuming that the diffuser size remains the same)

(*) technically, the part of the diffuser for which the light from the source is perpendicular to the diffuser

  • \$\begingroup\$ So what you are saying is that the reason that the diffusion at a greater distance seems to diffuse the light more than at a short distance is be cause the diffusion is more even? So wouldn’t this mean that if the diffusion at a close distance perfectly resembled the arc of the radiation of the flux lines and the shape of the diffusion at the farther distance also perfectly resembled that arc, then the distribution should be even and the diffusion would be the same at both ? \$\endgroup\$
    – vannira
    Nov 16, 2023 at 8:16
  • \$\begingroup\$ @vannira Yes. If the diffuser is part of a sphere and the light source is at the center, then the diffusion is uniform. However, the re-emitted light isn't itself completely anisotropic, so you would likely observe something akin to limb darkening, and it could be not that much better than a flat diffuser while being more difficult to build. \$\endgroup\$
    – xenoid
    Nov 16, 2023 at 8:30
  • \$\begingroup\$ I see interesting, but so does this mean that practically, if I placed a diffusion 1 meter away from a subject (covering the whole subject) and 5 meters away from the subject (covering the whole subject), the softness of the light (the sun) would be pretty much the same ? As you mentioned the reason the diffusion looks softer further away from the light source is due to the uneven distribution, but an object like a sun which is so far away, the distance of 1 and 5 meters or even 20 meters as long as it covers the subject should be Negligible, or is this incorrect? \$\endgroup\$
    – vannira
    Nov 16, 2023 at 9:10
  • \$\begingroup\$ Or if you were for example only wanted to frame the face of your subject. If you used a diffusion a that was bit larger than the face of the person vs a diffusion that could cover a whole house, should be irrelevant right, as the softness in both cases on the face should be the same ? \$\endgroup\$
    – vannira
    Nov 16, 2023 at 9:12
  • \$\begingroup\$ @Vannira Yes, yes, and yes. \$\endgroup\$
    – xenoid
    Nov 16, 2023 at 9:32

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