Hi I’m Sareesh Sudhakaran and in this video
I’ll try to explain – as simply as I can
– how ISO works, and why you should use
it in moderation.
I’m assuming you have already watched the
video on the exposure triangle which tells
you why we have an ISO and what it does for
exposure. If you haven’t, please watch it
first for a better understanding of its significance.
ISO is one of those things that are easy to
use but is in fact extremely complicated in
its implementation. Unless you believe in
magic, you’ll know that having the magical
property to boost a sensor’s sensitivity
must come at a price. We have these modern
cameras like the Sony a7S and a7S II that
boasts of insane ISOs to shoot at low light,
but they do ask for a pound of flesh in return.
Every sensor has pixels or sites that collect
light. This is called a signal. When there’s
not enough light, the pixel is black. However,
there’s a small boundary where if it is
hit by stray light, it causes the pixel to
randomly light up. This is what we see as
noise. If the light is only in one channel,
R, G or B, and if the response is still random,
what we see is color noise. I’m drastically
simplifying the working of a sensor here.
It’s a lot more complicated than this.
The range of capability of the pixel, from
the maximum light it can take to the least
light it can take, is measured by the signal-to-noise
ratio, what we call dynamic range.
Now how does this relate to ISO? Every sensor
is designed to operate best at one ISO. This
ISO is called the native or base ISO. It is
only at this ISO that you get the maximum
dynamic range from a sensor. Anything lower
or higher than this will reduce the dynamic
range. E.g., here are the native ISOs of some
modern cameras. You can see how each one is
different because they use different sensors
and electronics.
If you go below the native ISO, say go to
ISO 400 on the Arri Alexa, or if you boost
the ISO above the native ISO, say go to ISO
6400 on a Sony a7S, you will reduce dynamic
range.
When you boost an electric signal, you introduce
more noise, and this reduces the dynamic range.
There’s nothing you can do about this, it’s
how nature works. Unless someone comes up
with a miracle sensor, this is how things
are going to be.
But if you bump up the ISO it’s not disaster.
There’s a certain threshold of ISOs that
you can go up to and still get acceptably
good results. E.g., you can shoot with the
Arri Alexa at ISO 400 and the difference might
be too small to notice on most shoots. On
the other hand, if you raise the ISO to 1600
the difference might be unacceptable. There’s
no formula you can apply. Just because you
can crank up the ISO on the Sony a7S II to
a million doesn’t mean you should. If you
do, you’ll lose a whole lot of dynamic range.
And it gets worse. In addition to dynamic
range, you also lose another thing – color.
When you raise the ISO to insane levels, you
lose color information. That’s why companies
can sell high ISO cameras at low prices. When
you boost the ISO you can shoot in low light,
but not much better than a CCTV camera. The
dynamic range and color at these ISOs are
unacceptably bad, and will not match shots
that were shot with the native ISO. This color
performance is what separates the higher end
cameras to the lower end ones. One camera
that does extremely well at most of its ISO
range is the Panasonic Varicam.
So I hope you’ve understood that ISO works
like a complicated signal amplifier. You do
get the freedom to play around with exposure,
but always at a cost. The more you deviate
from the base ISO, the worse it gets. You
can test your cameras right now for dynamic
range and color at different ISOs and see
for yourself!
If you have any questions, please feel free
to ask me in the comments below. If you liked
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Bye now.
