Soft Light and it’s link to distance and diffusion materials

[silvan schnelli]

I asked the same question on photography stack exchange, but I am curious on what answers people would give me on this forum: https://photo.stackexchange.com/questions/132889/soft-light-and-it-s-link-to-diffusion-and-distance#

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?

However, 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

 

[silvan schnelli]

I was miss-assuming that diffused light and soft shadows are one and the same. I also kept imagining the light as a singular point source. 

[Nicolas POISSON]

A diffuse light is not the same as, nor is contrary to,  a directional light.

Soft shadows are the result of a light source being big compared to the distance to the object. This is just basic geometry. It does not matter if it is a medium size source close to the object or a bigger source a bit farther away. A 60cm square source at 1m will give the same shadows as a 1.2m square source at 2m, or a 1.8m square source at 3m.

Diffusion very close to the source will not make shadows softer, it will spread the light. Adding diffusion on a narrow beam source will light a greater area. Some areas that were not in the beam will now receive light. The trade-off is that areas that were previously in the intense beam now receive less light.

In theory, there is no benefit adding diffusion close to a point source that emits at 360°: the beam is already as wide as it can be. There is also no strong benefit adding diffusion close to a source that is already huge and with a wide angle (like a LED flat panel that emits at 120°, a kinoflow, or a COB LED with a softbox). Adding diffusion right on a softbox because you find the shadows still too hard does not help. You have to bring that softbox closer, or use a bigger softbox, or add diffusion at some distance - provided the softbox enlighten the whole surface of this additional diffusion.

Well, in practice... one often use additional diffusion close to flat panels with wide beam because they are composed of multiple small sources that create cross-hatched shadows. And of course one can use diffusion close to flat panels that have a narrow beam just to make it wider.

Also, adding diffusion close to a narrow beam source might spread the light to other reflecting surfaces like walls or ceiling, all of these acting like a fill light, reducing the contrast of shadows.

And of course, if the diffusion is light, the beam at the output can be a mix of diffuse and directional light.

Edited November 17, 2023 by Nicolas POISSON

[silvan schnelli]

@Nicolas POISSON

Thank you for the great response. I definitely was just focused on diffused light, falsely believing that it would lead to soft shadows, without considering the geometry of the light and the key requirement, which is for the light to be able to “wrap” around the subject.

I now understand that diffusion has much more to do with the quality of the light and is often used to achieve a more harmonious light quality. For example, as you mention, to prevent any specular or uneven lighting from the discrete light sources that make up the lighting unit.

I think this is also an important point to note that lighting units are often not just a single large point source that project light but are a constitute of many lighting units/LEDS. I think this was important for my understanding as in theory a huge point source of light projecting beams in a sphere like direction, no matter how big would still lead to hard shadows. Or for example if we considered a lighting unit that strictly projected parallel beams no matter how big, would need a diffusion material to produce softer shadows, but of course this doesn’t really happen in practice.

However, I do think it is important to discuss soft shadows in terms of the lights ability to wrap around the subject, as in theory the directionality of the light still does play a big role and simply saying that a larger source leads to softer shadows isn’t entirely true.

At least this is my understanding of the subject, although it may not be entirely correct.

[Nicolas POISSON]

I think there may be a question of wording here. In physics, the concept of "diffuse reflection" means a beam of light made of parallel rays will be scattered in every direction. However my understanding is that in the film-making world, "diffuse light" refers to a source that is both large and with a rather wide beam. That may not be scientifically correct, but I think this is the way it is commonly understood.

Although it is possible to create a big source emitting strictly perpendicularly to its surface, such a device will rarely be encountered in practice. LED panels emit with a certain angle. An object close to these panels will have soft shadows, because it still receives light from a great surface. Even panels with very directive LEDs will create rather soft shadows at proximity. The angle of each individual LED is wide enough to blend with a significant amount of neighbours. Soft-boxes with honeycombs will act the same.

You can find exceptions, such as this DIY parabolic reflector that is both huge AND creates hard shadows.

 

[David Mullen ASC]

Softness is due to the size of the source (the soft light) relative to the subject. All the heaviness of the diffusion material does is make the soft source more even in intensity edge-to-edge but once the frame is filled equally with light corner-to-corner, it cannot get any softer unless it gets bigger relative to the subject (i.e. you make the frame larger or move the subject closer to it.)