Hi John Hess from Filmmaker IQ - today I’m going to address a common misperception - that
distance plays a role how you expose: that you need more light to light a far away distant
object than you do a near one.
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Well from exposing social and community problems to exposing for the camera - let’s get back
to why distance doesn’t after exposure.
Now some of you might be scratching your heads thinking, well I never thought that would!
Well good for you.
But when I was coming of age as a filmmaker I remember watching the Citizen Kane commentary
by none other than the great Roger Ebert where he said this:
Everything from the foreground to the background in focus.
The foreground backlit to give us this effective silhouette and all the way in the background
the actress Dorothy Commongore and Gregg Toland used double arc lamps to put all that light
in the background because the further you are away from the camera, the more light there
must be on you in order for you to show up, it takes less light in the foreground than
in the background, and so he used big lamps that were used for Technicolor because Technicolor
needs a whole lot of light, more than black and white...
Hone in on that right there: The further you are from the camera, the more light must be
on you in order for you to show up.
It takes less light in the foreground than in the background.
Well I stored this infonugget in my brain and when it came time to develop ideas for
Filmmaker IQ videos, along with deep focus and the hyperfocal distance and I thought
it might be cool to demonstrate this concept...
But the problem is…
Roger Ebert was completely wrong.
Now let’s clear the air so to speak… there are atmospheric effects called aerial perspective
that give distant objects a desaturated look - this is caused by particulates in the air
adding up over distance... that’s not what we’re talking about here.
What we’re talking about is the misapplication of the inverse square law for light.
The inverse square law says that for an isotropic point light - as you increase the distance,
the light received will fall off by the square of that change in distance.
So it would make sense that as you increase the distance from the camera to the subject,
less light will make it onto the camera’s sensor, therefore the exposure will be less.
Makes sense right?
Well let’s set up quick experiment.
Here is my camera shooting an exposure pop up chart.
We are shooting from 2 feet away.
Now without changing any camera settings I’m just going to dolly that camera back to 8
feet.
We’ve increased our distance by a factor of 4- therefore the light received back here
should only be 1/16th of that from the original distance - that’s one over four squared.
But when we compare the shots, the exposure looks pretty much the same - even when we
zoom in digitally, yet we are only receiving 1/16th of the light at 8 feet than we did
at 2 feet.
Why isn't the further shot 4 stops darker?
Is the inverse square law broken?
Well not at these distances, we’ll cover where it doesn’t quite work right in a future
video - but at this distance it is accurate with an error factor of less than 1%.
The answer lies in the basics of what exposure actually is.
Exposure is not about the total amount of light gathered, it’s about the concentration
of light in a given area.
If you ever played with a light meter in spot metering mode you’ll know that the spot
meter measures light in candelas per meter squared.
And that’s the giveaway - it’s candelas, a measure of light intensity PER Meter Squared
- that’s area.
The next natural question is how do we measure area from a vantage point?
Well, let's go back to my video on the Math and Science of Forced Perspective.
In that lesson we imagined our viewpoint as an angle and derived this formula for calculating
the angle of view it occupies.
Then later in the video, I showed that the formula can actually be approximated this
simpler formula, width divided by distance.
But that’s only one dimension.
If we want to compare the surface area from our viewpoint called, the solid angle, we
need to have two dimensions: width times height - set both width and height equal to 1 and
now watch what happens… the surface area of an object of in our field of view diminishes
by the inverse of the square of our distance.
Look familiar?
The inverse square law.
So when we increase the distance from our camera to a subject, the solid angle - the
surface area - diminishes by the inverse of the square of the distance which is exactly
the same as the same amount of actual light we recieve..
So the concept that governs diminishing light received is the exact concept that governs
the diminishing perceived size.
In case the concept is still a bit undevlopped, let’s attach some real numbers in a thought
experiment.
Let’s say we have a square white piece of paper and a camera.
At say 1 meter, the light coming off the square is enough to cover 100 pixels square or 10,000
pixels.
The light also fills each one of those pixels precisely to 100% capacity.
Now let’s just say it takes 1 lidget to fill a pixel on our sensor.
I completely made up that unit because it doesn’t matter precisely what it is, but
it’s some number of lux.
We know that at 1 meter, our sensor is getting 10,000 lidgets from this bright square.
Now let’s increase the distance from 1 meter to 10 meters - that’s a ten fold increase…
Now where that square covered 10,000 pixels, it will now only cover 100 pixels - 10x10.
It still takes 1 lidget to fill a pixel to capacity - so at 10 meters, the camera is
only receiving 100 lidgets - which is exactly as the inverse square law for light suggests.
So now how did Ebert completely blow it on the Citizen Kane commentary?
Truth is Citizen Kane cinematographer Gregg Toland did have to use big lights to accomplish
some of the deep focus shots but it’s not the distance from the camera - it’s the
distance from the light source.
When the camera is very close to the subject, you can put light fairly close because it
will be out of frame.
But if the camera is very far away and you want the light out of the shot, you have to
move the light much further away from the subject.
And if you move the light further away, the amount of light landing on the subject will
be reduced by the inverse square law.
The scene that Ebert used in his example is probably not the one that used big arc lights.
You could have easily hidden some lights pretty close behind the foreground walls.
A shot like this one where you need to reach all the way to the background would require
the bigger powerful arc light to deliver reach all the way to the far background.
And so there you have it, I spent about 10 years with this myth in my head because the
most revered modern critic doesn’t understand how photography actually works.
Thanks Ebert.
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with the medium of motion pictures.
Thank you -I’m John Hess and I’ll see you at Filmmaker IQ.