- [Instructor] When working with digital video,
there's a relationship between the brightest
part of the subject and the darkest part.
A lot of complex technical discussion
goes into the way these two extremes are measured.
But the principle is a very simple one.
There's a darkest part of the picture and a brightest part.
As a film viewer, you can be forgiven for thinking
the brightest part would be something like
a white shirt or a white wall.
But actually, that's usually said to be
well below the maximum brightness captured by the camera.
That level is saved for specular highlights.
That's gleaming car windscreens
or chrome catching the sunlight.
Reflections in glass of all kinds,
any kind of subject that focuses light
in such a way, as to make it significantly
brighter than the rest of the image.
Usually, it's considered acceptable
for these parts of the picture to burn out.
That means to have no detail in them,
because they're brighter than the camera
can register with its particular setup.
In any case, however we measure it
and however bright it is, there's going to be
a darkest pixel in the image that's stored
and a brightest pixel.
More precisely, there's a darkest level
the image could go to and a brightest level it could go to.
The brightest part of the image
could be fairly dark in reality.
It might be an indoor bulb or even
street lighting if your camera sensor is sensitive enough.
The darkest part might not be that dark.
The brightest part might be incredibly bright.
Maybe a sunny day on a mountain
will produce much brighter subjects
than candle light, obviously.
The full range of potential brightness or darkness,
is defined by the camera you use
and the settings you give it.
Luckily, human perception of light is relative
and our brains adapt quickly, perhaps in
just a couple of seconds to the levels we see in an image,
so that we're convinced towards seeing shadows,
even if the image isn't really
that dark in the darkest areas.
We have a challenge though, displays vary
quite a lot when they show images.
Some displays maybe 10 times as bright as others.
How do we make sure the image acquired by the camera
is accurately represented when the viewer sees the image.
Luckily, the fact that our vision is relative
makes this fairly straight forward.
As long as the relative brightness levels
of the image match the original, it will look fine.
Our perception of brightness isn't linear as reality is.
So, cameras, video editing software and displays
all have to compensate for the relative perception
of darker or lighter areas of the picture.
But manufacturers know this and in many cases,
the process is largely automated.
Or at least, works acceptably with default settings.
Still, if we're going to convert photons
hitting a camera sensor into numbers,
we need some universal way of using those numbers
to represent things like brightness.
There's a complex approach to this problem and a simple one.
Here's the simple one, the lightest a pixel can be,
is 100% and the darkness is 0%.
No matter what system you're using,
how the information is stored,
which language it's in or how it'll be displayed
a hundred percent, is a hundred percent.
In practice, colorists tend to think of a pixel
as being somewhere between zero and one.
Which, I always found rather strange and confusing.
Of course, you can have as many decimal points as you like
to make the steps between zero and one very, very subtle.
Personally, I've always found it easier
to think of these two ends of the range
as fully off and fully on.
All the numbers you'll deal with
that control the journey from off to on,
will always fit inside that range.
Right now I'm talking about brightness,
but I could just as easily be speaking about redness.
But we'll look at color channels in another lesson.