The inverse-square law, why do we care/talk about it?

[sk66]

IMO, as a basic/general concept it has a *minor* benefit for understanding. But the fact is it doesn't really apply to "lighting" significantly.

At ~ 5x distance the inverse-square law *is* accurate. But that's because at ~ 5x distance the light is "small" enough that the light reaching the subject effectively resembles a point source (the rays are parallel and the light is hard). Beyond that distance the light doesn't get "harder" really. And the fact that it takes more power to throw longer distances is fairly obvious and almost irrelevant.

But with lighting what we are (should be) more concerned with is short (relative) distances and using/manipulating the fact that we are *not using point light sources*. And at short distances the inverse-square law can be very wrong... (it's more like 1/2 inverse-square or inverse-double).

So where's the benefit and why do we go on about it? (BTW, I'm probably about as guilty as anyone)

 

[HoppyUK]

In another thread Steven, you were asking for a sanity check. Well, you're certainly in need of one here.

The ISL is massively significant and applies to every studio image, influencing both brightness and shadows. So what if it's only approximately right at close distance with a softbox? It's still having a big effect.

I don't think arguments about when and how it strictly applies are helpful. The point is, with artificial studio light, distance has a much bigger impact than most people expect (probably because it doesn't behave like the sun). Use that to your advantage, but ignore at your peril

 

[sk66]

I'm not sure I would consider a 30-50% error "approximately right." And it doesn't behave like the sun specifically because it's not a point light source (as the sun effectively is).

I agree that distance is massively important. But it's not really due to size increasing/decreasing at Sq/SqRt (it's due to angles), nor is it due to power increasing at Sq/SqRt (because it's not).
Why aren't the simple concepts of "the less power you use, the less distance it will travel" and "the light needs an angle to reach the area" enough? Why do we involve math that doesn't actually apply?

I will admit that the inverse-square law does apply at longer distances (power only, not "quality") and it applies to things like power to fill a modifier or bouncing/OnCF (which is TTL's world). But is it really of benefit for us to make it "technical?" Especially when it is quite often "technically wrong?" Because I have to say, it wasn't until I started to think of lighting in terms of angles instead of size/distance that I really started to understand lighting.

 

[Kodiak Qc]

The ISL is massively significant. Use that to your advantage, but ignore at your peril

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+100… no way out!

 

[david357]

It's useful to know purely to pre-judge the effect of the light source not only on the subject but also on the background.

Being aware of its effect allows you to have a white background rendering anywhere between white and black.

Does for me anyway!

 

[sk66]

It's useful to know purely to pre-judge the effect of the light source not only on the subject but also on the background.

Being aware of its effect allows you to have a white background rendering anywhere between white and black.

Does for me anyway!

To me that's as simple as a direct correlation to DOF... if you have less (use less power) it takes less distance for the BG to be out of it (i.e. black).

If you actually use it to say "I'm going to use this 3ft modifier from 2ft and I want the BG two stops under mid (i.e. black, assuming a mid exposure) so the BG needs to be at 4ft," well, it doesn't work.

 

[Peter B]

As someone once said, "Ye cannae change the laws of physics, Cap'n!" https://en.wikipedia.org/wiki/Inverse-square_law

 

[Cistron]

At ~ 5x distance the inverse-square law *is* accurate. But that's because at ~ 5x distance the light is "small" enough that the light reaching the subject effectively resembles a point source (the rays are parallel and the light is hard).

If they were parallel there wouldn't be any fall-off. And I'm not sure why you are so concerned of the law being erroneous at close distances. I've never seen anyone grabbing a calculator (possibly when planning film sets). edit: Thinking about it, the 'error' must surely to be the same at all distances.

Sure, there's more than one point source at the front of a softbox, but the fall-off still is exponential - which becomes very apparent in your shadow transitions and background lighting. That's the important point to take home.

 

[Garry Edwards]

Yes, the ISL is flawed, in the sense that in photography we don't actually use point sources of light, we don't operate in free space (a vacuum), we have light bouncing around from reflective surfaces and we direct our light in specific directions, and this affects the results when using large light sources in small spaces which reflect light.

And, when large light sources such as softboxes are involved, there are untold millions of separate but adjoining point sources of light, we can't actually see them, just as we can't actually see the millions of adjoining pixels on our digital camera sensor, but we know that they're there. What makes the ISL SEEM to be unreliable with things like softboxes is that the light is travelling different distances from different point sources when used close, for example light from the centre may be travelling 2', light from the extreme edges may be travelling 6' and the light from each individual point source will be travelling a different distance, directly relative to its position. The fact that we can't be bothered to calculate the different distances doesn't mean that they aren't there.

People who have seen me working may have noticed that I set lights quickly, basically I just throw the light around, and normally get distance, angle and intensity right first time, or with a minimum of adjustment. That will be partly down to experience, but, whether I know it or not, I've got the ISL chiselled onto my brain and I use it constantly, which avoids most of the experimentation and error.

 

[HoppyUK]

If they were parallel there wouldn't be any fall-off. And I'm not sure why you are so concerned of the law being erroneous at close distances. I've never seen anyone grabbing a calculator (possibly when planning film sets). edit: Thinking about it, the 'error' must surely to be the same at all distances.

Sure, there's more than one point source at the front of a softbox, but the fall-off still is exponential - which becomes very apparent in your shadow transitions and background lighting. That's the important point to take home.

Maybe not these days, but pre auto-metering flash we all used Guide Numbers to estimate flash exposure, with a table of numbers printed on the gun or a little rotary calculator. They're still around, and based strictly on the ISL.

Lots of examples here https://www.google.co.uk/search?q=v...0Q_AUIBygC#tbm=isch&q=guide+number+calculator

 

[sk66]

If they were parallel there wouldn't be any fall-off. And I'm not sure why you are so concerned of the law being erroneous at close distances. I've never seen anyone grabbing a calculator (possibly when planning film sets). edit: Thinking about it, the 'error' must surely to be the same at all distances.

Sure, there's more than one point source at the front of a softbox, but the fall-off still is exponential - which becomes very apparent in your shadow transitions and background lighting. That's the important point to take home.

Actually the ISL is based upon light radiating out from a point light source and the fact that the light reaching the subject (area) is only the rays going "straight" towards it. i.e. effectively "parallel" at the subject w/ no "buildup" from other "sources."
The error in the ISL is exponential. Beyond ~5x it's negligible. And that's because at longer distances the light that reaches the subject is again "parallel" and "hard" (there's no real point to using the modifier). At very short distances the falloff is not exponential (but it's certainly still a significant factor). And that's specifically because it's not a point light source and there is "buildup" from "other sources."

 

[sk66]

Yes, the ISL is flawed, in the sense that in photography we don't actually use point sources of light,....

And, when large light sources such as softboxes are involved, there are untold millions of separate but adjoining point sources of light, we can't actually see them, just as we can't actually see the millions of adjoining pixels on our digital camera sensor, but we know that they're there. What makes the ISL SEEM to be unreliable with things like softboxes is that the light is travelling different distances from different point sources when used close, for example light from the centre may be travelling 2', light from the extreme edges may be travelling 6' and the light from each individual point source will be travelling a different distance, directly relative to its position.

That's kind of my point... it's not a point light source, it doesn't act like a point light source, and the ISL doesn't actually work with accuracy.

People who have seen me working may have noticed that I set lights quickly, basically I just throw the light around, and normally get distance, angle and intensity right first time, or with a minimum of adjustment. That will be partly down to experience, but, whether I know it or not, I've got the ISL chiselled onto my brain and I use it constantly, which avoids most of the experimentation and error.

I would argue that it's simply the knowledge that the less power you use (distance) the less distance the light will travel (falloff), and experience/familiarity. Basically, you're "guessing" with greater accuracy because you've done it a thousand times before.

Edit: Would you take a softbox at 4ft, move it to 2ft and drop the power 2 stops? I wouldn't...

 

[Garry Edwards]

That's kind of my point... it's not a point light source, it doesn't act like a point light source, and the ISL doesn't actually work with accuracy.


I would argue that it's simply the knowledge that the less power you use (distance) the less distance the light will travel (falloff), and experience/familiarity. Basically, you're "guessing" with greater accuracy because you've done it a thousand times before.

Edit: Would you take a softbox at 4ft, move it to 2ft and drop the power 2 stops? I wouldn't...

Well, probably a thousand times every year.... I take your point, but although experience helps, knowledge helps even more, and is vital.
To sum it up, I sometimes get it right and never get it wrong.
When I get it right, I'm probably relying on experience, which can shorten the process quite a lot.
Not getting it wrong is down to science and knowledge, i.e. I know what NOT to do because I know when science tells me not to bother trying to do something that the laws of physics tell me can't be done... Einstein was fond of saying that the definition of insanity is doing the same thing over and over again and expecting different results - and he had a point - people who don't understand the principles won't know why something didn't work and will try the same thing time and time again, often with very minor tweaks that couldn't possibly make any real difference.

An example of where (even unconciously) making use of the ISL just makes it easy, or shows that it can't be done, is when I need to skim a honeycombed light across the front of a subject to reveal the texture. Without an understanding of this most basic of physics principles, someone might place the honeycombed light 6" and expect the light to be realitively even from one side to the other, someone who understands the principles would place the light as far away as practical, and wouldn't even try if there wasn't enough space. And the person who understands the principles will usually get the exposure right, or nearly right, first time.

So, although I'm well aware of the fact that external factors can affect the actual figures, understanding and making use of the ISL is absolutely fundamental to lighting..

 

[sk66]

The Inverse-square Law is very specific and uses/gives specific numbers/results. And we tend to throw those numbers around in our explanations... I'm as guilty as anyone.

But, in the example I gave, I know that if I move a light from 4ft to 2ft it's going to be closer to 1 stop... since nothing is "accurate" (I'm not using tape measures) that's about where I would start. If I went from 4ft to 3ft I would probably only change it 1/3 stop before checking. And that's nothing like what the ISL says I should do.

The physics we are actually using are much simpler/more generic. It's as simple as power -vs- distance. The farther away the light is, the more power you have to use, and therefore it will travel farther beyond the subject (first surface)... we could just call it the DOF(L). And angles of course (size, placement/feather, grid, etc, etc.). IMHO, the rest really is just guesstimation based on experience.

Is there really a difference between "experience" and "knowledge?" Not unless you make a distinction between "theoretical" and "practical." And we all know which type of knowledge is better...

In your example (surface texture) my first reaction would be to use a bare light flagged, from a (relatively) shorter distance...

 

[Garry Edwards]

There's no real difference between theoretical and practical knowledge, if the "knowledge" doesn't include an understanding of the science behind it then it's just experience, and people can be very experienced at doing things consistently inefficiently...

In your example (surface texture) my first reaction would be to use a bare light flagged, from a (relatively) shorter distance...

Well, a honeycombed light is likely to be much better controlled, but no matter, a bare light, flagged, is an option.
BUT the ISL clearly tells us that distance matters. If the light is traversing a distance of say 12", from a distance of 12", then at the nearest point (if the acceptable exposure is f/8) then the furthest post will only get 1/4 or the light, or f/16 - is it OK to have 1 end 4x brighter than the other?
Increase the distance to 24" and increase the power x4 and it will still be f/8 at the nearest point but will be f/11 at the furthest point, double the distance again and increase the power setting again and the fall off of brightness will start to become acceptable. Double the distance again, if you have enough power, and the problem has gone away to the point where it still has a theoretically uneven exposure but in practical terms the uneven exposure won't be visible.
That's a very basic example of a practical application of a theoretical concept.
So, what's best - use experience to put the light a long way away without knowing why (or experiment with different distances/positions without knowing why), or use the ISL to work out the lower level of acceptable distance?

 

[sk66]

There's no real difference between theoretical and practical knowledge, if the "knowledge" doesn't include an understanding of the science behind it then it's just experience, and people can be very experienced at doing things consistently inefficiently...

LOL!
As the (correct) saying goes... "perfect practice makes perfect."
I know my "bare light flagged" isn't any better or worse necessarily... I wasn't trying to be argumentative (I added a smiley face). I just thought it kind of funny that my first instinct was to go a completely different direction.

If the light is traversing a distance of say 12", from a distance of 12", then at the nearest point (if the acceptable exposure is f/8) then the furthest post will only get 1/4 or the light, or f/16 - is it OK to have 1 end 4x brighter than the other?
Increase the distance to 24" and increase the power x4 and it will still be f/8 at the nearest point but will be f/11 at the furthest point,

I would have to verify before I state it as a fact, but I'm fairly certain these numbers are quite a ways off if we are talking about a modified light source (soft box). I believe it would be much more like f/8 and f/11 to start with. And an increase of distance to 24" will require more like a 2x (1 stop) power requirement, not 4x/2 stops. If we are talking bare bulb/hard light, then ok... close enough, but that's not really the point is it?

So, what's best - use experience to put the light a long way away without knowing why..., or use the ISL to work out the lower level of acceptable distance?

IMHO, experience is better 99.9% of the time. I taught rotary wing aerodynamic theory as a military instructor pilot for years, and I *like* the subject. But pilots really only need to know "if you pull on this houses get smaller"... the "why" is pretty irrelevant.
Do you need to know all of the specifics of what makes your car's motor run or how much horsepower it puts out at a given throttle setting? Nope, and most don't.
Experience will (probably) tell you approximately what the minimum acceptable distance is if you've done it more than once...

 

[HoppyUK]

Well I just measured moving an 80cm softbox from 2ft to 4ft (using a tape measure!) and the difference was exactly 1.5 stops. It's all very well saying "It's as simple as power -vs- distance. The farther away the light is, the more power you have to use" and that's logical enough. But how much power? And say what you like, the main driver of the change is the ISL and that fundamental causes much more of a shift than newcomers to studio photography realise. They also need to know that while moving a light from 2ft to 4ft causes a drop of 1.5 stops, moving it from 10ft to 12ft makes hardly any difference, and the ISL also explains why that is, too.

Nobody is saying the ISL always applies exactly, but if you need a rough guide, it will usually get things pretty close. The most common area of error is the one you cite - a softbox used close - but even at just 2ft (and that's a lot closer than typical) the ISL is still working as a decent working guide IMHO.

What about if you move the softbox further away, from 4ft to 8ft? What would exposure change to then? Well, since I've just measured that too, it's either 1.9 stops or 1.6 stops - the first is measured outdoors with virtually no reflected light coming back from spill/surroundings, whereas the second is indoors in a normal room with white ceiling. There are always additional factors to take into account that only experience can anticipate.

 

[Garry Edwards]

We could continue this conversation for ever, but let's not...

I know my "bare light flagged" isn't any better or worse necessarily... I wasn't trying to be argumentative (I added a smiley face). I just thought it kind of funny that my first instinct was to go a completely different direction.

That's because The Force (AKA the ISL) wasn't with you

I would have to verify before I state it as a fact, but I'm fairly certain these numbers are quite a ways off if we are talking about a modified light source (soft box). I believe it would be much more like f/8 and f/11 to start with. And an increase of distance to 24" will require more like a 2x (1 stop) power requirement, not 4x/2 stops. If we are talking bare bulb/hard light, then ok... close enough, but that's not really the point is it?

No, my figures are near enough correct, they're based on the ISL. I'm not talking about a softbox, which (as the ISL tells us) would spread the light over a large area, I'm talking about a standard reflector fitted with a honeycomb. But, reading back, I see that I didn't actually mention that the honeycomb would be fitted to a standard reflector, so my apologies for not stating the obvious. Actually I normally use a 5 degree honeycomb for this, I picked it up when I was working in NY NY and it's almost worth going back there to get another one... If I didn't have that rather rare and very expensive 5 degree honeycomb then I would use a 10 degree one and stick on a bit of Cinefoil to stop the light going where it isn't wanted, and this would be based on experience not physics - but I know from the physics that this would not affect the fall off of light, so I wouldn't need to faff around trying various different levels of lighting power...

IMHO, experience is better 99.9% of the time. I taught rotary wing aerodynamic theory as a military instructor pilot for years, and I *like* the subject. But pilots really only need to know "if you pull on this houses get smaller"... the "why" is pretty irrelevant.
Do you need to know all of the specifics of what makes your car's motor run or how much horsepower it puts out at a given throttle setting? Nope, and most don't.
Experience will (probably) tell you approximately what the minimum acceptable distance is if you've done it more than once...

In many situations, I would agree with you, sometimes it's a mistake to over-think things. But flying aircraft involves pretty complex physics, and although I know nothing about it I'm guessing that it makes sense just to teach pilots the minimum theory that they need, although I may be wrong because I remember that the famous Captain Eric Brown said that the main reason for his survival was that he put so much time into study, research and "trying to understand what makes the bloody things work".
But lighting isn't complex physics, it's schoolboy physics.

 

[mike weeks]

We could continue this conversation for ever, but let's not..

 

[sk66]

but even at just 2ft (and that's a lot closer than typical) the ISL is still working as a decent working guide IMHO.

Thanks for doing exact measurements and even comparing indoors/outdoors!

I agree that the ISL explains the basic theory behind what is going on... and I'm not saying that not knowing the general principles of what is going on would be better. I'm just saying we put to much emphasis on the specific numbers the ISL dictates.

But, reading back, I see that I didn't actually mention that the honeycomb would be fitted to a standard reflector, so my apologies for not stating the obvious.

Ha! I don't own a reflector grid (except for my BD)... to me it was "obvious" that the grid would be on a softbox.
(maybe I should rectify that)

 

[Sejanus Aelianus]

If God meant me to use the inverse square law, She'd never have let me buy my Sekonic L308!