- [Narrator] Whatever display you're working with,
there's going to be a maximum brightness for each pixel
and a minimum brightness, this minimum isn't just of,
it's the lowest brightness the pixel can have
while still showing detail, bellow that
it's like underexposed film that doesn't have any texture.
Above the maximum brightness, it's equivalent
to overexposed film, again with no texture.
When viewing your image in a waveform,
you'll see the maximum and minimum brightness
currently in your image and there are lots of ways
to make adjustments to this brightness range
and a Levels adjustment is one of the quickest and easiest.
You can achieve similar results using a Curves Control,
but this is even quicker in my view.
Not all Levels controls give you access
to the Output Levels as well, but let's look at that first.
Here, I've got the minimum output and maximum output,
which in this eight bit control zero to 255,
just as we'd expect for a regular RGB image.
Take a look at the lesson on the difference
between eight bit, 10 bit, 12 bit and beyond,
for more info on why this is normal.
We can change these outputs if we want
and this is important if you're going to
output for broadcast television.
The YUV color system used for broadcast TV
goes from 16 to 235 on this scale.
It's a weird kind of mathematics, I know,
but if you viewed this footage with a minimum level of 16
on the RGB scale on a computer monitor,
the shadows would look rather gray.
View it on a TV monitor and they'll look properly black,
because that's where the scale begins on a YUV monitor.
The difference between the two types of display
is one of the reasons you need your final display
attached to your system, it's why you need
to print an image to be sure it'll look right,
even if you're calibrating and color matching
your computer monitor.
Now let's say I want to deliver this image for the Web,
so I'm happy to keep the zero to 255 range.
It's time to look at the other control,
simply this changes the relationship
between the darkest part of the image
and the darkest part of the output,
or the brightest part of the image
and the brightest part of the output.
Here, I'm raising the darkest part of the image,
which means anything darker than this
is now completely black and the range between
the two controls is now spread between
my darkest and lightest levels for output.
So if I increase the black level of my image to 50,
I'm now spreading the range from 50 to 255
between my full output of zero to 255.
The result is pretty clear, especially
if we look in the waveform, the contrast increases.
Lowering the white level has the same effect
but from the upper end, our eyes are most sensitive
to mid range levels and there is a control
to adjust those right here in the middle.
So once you've set your upper and lower limits,
usually to make sure they line up reasonably well
with the top and bottom of your waveform for output,
you'll want to tune the mid tones to please the eye.
Our eyes don't see brightness in a linear way,
it's a logarithmic curve, it's not a straight line.
When we adjust the range of brightness in the image,
we need to compensate for this and adjust the midterms too.
Like much of the image adjustment technology we have,
a lot of work goes into compensating
for the way human vision is relative
rather than objectively linear, which is how cameras,
computers and displays would work by default
and the way light works most of the time in fact.