Question about lighting quality and the Inverse Square Law (3 replies and 2 comments)

6 months ago
thancha 6 months ago


I have been struggling to understand about lighting quality and its relation to the Inverse Square Law, so I hope someone can enlighten me about it.
From my understanding (please correct me if I’m wrong), when we move a light source further away, they become ‘harsher/harder’ because the source itself gets smaller (for example.. the sun), thus less wraparound.
And according to the Inverse Square Law (again, correct me if I'm wrong), when we double the distance between the light source and the subject, the light level reduces by 1/4, but the area being lit increases by 4 times. 
What I'm quite confused about is... if the area being lit increases when we move a source further away, does that not turn it into a softer source because the light is illuminating a bigger portion of the space?
Since moving the light source away from the subject leads to smoother falloff between the light and the shadow areas, it seems to go against the idea that smaller light source (in this case, light that has been moved further away) creates harsher light.
I hope it makes sense.
Thank you in advance!
Roger Deakins
6 months ago
Roger Deakins 6 months ago

When you move a light further away it becomes 'smaller' relative to the area it is lighting. There is no 'smoother falloff between the light and the shadow areas'. Think of moonlight, which is, for us, a huge 'point source'.

6 months ago
dmullenasc 6 months ago

You're confusing two separate issues. The size of the source relative to the subject determines the softness.  Whether the fall-off from the source is fast or slow depending on distance to the subject is a separate issue.

For a moment, think of a soft light not as one light but as multiple lights so that the sharp shadow edge created by a single hard source is blurred by the next shadow created by another light next to that light, and so on and so on, each source creating one more shadow edge until the effect is not a single shadow but a very blurred graduated shadow.  Imagine all those multiple hard lights in a grid pattern, let's say in a 20'20' grid, four across and four down, so 16 hard lights creating 16 little shadows.  Now imagine instead a grid if 10 lights across and 10 down in the same 20'x20' grid, so 100 hard lights creating 100 little shadow edges on the subject.  Now imagine 500 lights across and 500 down, for 250,000 hard lights and hard shadows.  At this point, considering how close each light would be next to each other, the shadow pattern would look like a single soft shadow because you couldn't make out the 250,000 individual edges of the shadows.

The daylight overcast sky, for example, produces soft shadows because the subject is being lit from many directions all at once, so no single hard shadow can be created, rays are hitting the subject from many directions. 

None of this has anything to do with intensity or fall-off speed or spread though.  A light that illuminates a larger area is not softer just because it has a lot of spread, the rays are still hitting the subject from one narrow angle and the shadow created is still sharply defined.  You can't get much bigger than the sun in the sky in terms of spread, and it's a hard source.  But the fall-off over distance is very gradual because the sun is so far away.

6 months ago
DEAN DODOS 6 months ago

Allow me to expand a little on what both Roger and David mentioned above.

I think you are confusing the term "falloff" with the term "shadow edge transfer", which a lot of people do.

The term "falloff" (in photography) should be used to describe the physical property of light loss between 2 relative objects away from a light source, not to describe the inverse square law or the highlight to shadow transition on a subject.

For instance, if you had the light source 1 meter away from the subject that was standing 1 meter away from a wall that is behind them, and then you moved the light 2 meters away from the subject, the "falloff" between the subject and the wall would decrease. However if you moved the light source closer, eg; only 1/2 meter away from the subject, the "falloff" between the subject and the wall would increase.

Put simply, if you have a fixed distance between a subject and a wall and wanted to make the wall brighter relative to the subject(falloff decrease), you would have to move the light source farther away and correct the exposure loss due to distance increase, conversely, if you wanted to make the wall darker relative to the subject(falloff increase) you would bring the light source closer to the subject and correct the exposure increase due subject-to-source distance decrease. Note, your shadow edge transfer will change in both of these cases.

Falloff is independent of the size or shape of the light source, it is directly affected by the inverse square law between 2 objects, in this case it is "falloff" between the subject and the wall.

Whereas "shadow edge transfer" describes the transition between shadow and highlights on the illuminated subject (what i think you were actually referring to as falloff)

Hence the farther away one moves the light source from the subject, the more abrupt the "shadow edge transfer" will be on both the "subject shadow" and also on the "cast shadow", since there will be less light wrap due to the source decreasing in size relative to the increase in distance, this is why the light is said to get "harder" as you move it away from the illuminated subject.

Shadow edge transfer is affected by; light source to subject distance, the shape and also the size of a light source.

The term "Inverse square law" should only be used to describe the loss the of energy intensity due to increase in distance between source and subject and "falloff" to describe the loss of light between 2 relative objects.

I hope this doesn't add to your confusion as it is quite a technical subject, but if you physically apply these theories you may have an easier understanding of these two different properties of light.

6 months ago

Well I use the more general term fall-off to describe the rate of decrease in intensity over distance for a soft light because technically the inverse square law applies to point sources though I suspect the rate is similar for a soft source.

6 months ago

You suspect correctly.
A point source is used as scientific constant for calculation purposes, as it is difficult to measure exact figures for comparison with a collimated light source or diffused light source.
Even though a lot of people say, for example, a PAR will fall off quicker than a soft diffused source, i suspect the are referring to it's shadow gradient being more abrupt(faster fall-off) and not the actual intensity fall-off over distance.
Thank you for taking the time to discuss.
Much appreciated.

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