FPS and refresh rate

Imagine yourself watching movie of an unbelievably slow fog. You don't see edges and
sharp borders. Now play the movie with 10fps. It will look fluid. Why? Because
the difference from one frame to the other is very low. The extreme would be a
totally unmoving wall: Then 1 fps would equal 1000 fps.

Now take your hand and move it slowly in front of your face. Then move it
faster until it's blurry. How many frames per second do you see? It must be
little, because you see only a blurred hand without being able to distinguish
every change per millisecond, but it must be many, because you
see a fluid motion without any interruption or jump. So this is the eye's trick in both
examples: Blurring simulates fluidity, sharpness simulates
stuttering. (It's similar to
"rotation simulates gravity".)

Motion blur example1: Capture from a live performance of The
Corrs "What can I do" at MTV Unpluged

Motion blur example2: Capture from "Basic Instinct",
where you see a woman plunging an ice pick into a man's body while sitting on
him.

The fact
is that the human eye perceives the typical cinema film motion as being fluid at about
18fps, because of its blurring.

If
you could see your moving hand very clear and crisp, then your eye needed to
make more snapshots of it to make it look fluid. If you had a movie with 50 very
sharp and crisp images per second, your eye would make out lots of details from
time to time and you had the feeling, that the movie is stuttering.

Also 25fps but without motion blur: Footage from BBC's story
about Ed Gein, the murderer, who's case inspired Hitchcock to make "Psycho"
and Jonathan Demme to make "Silence of the Lambs". The music is
from CNN's "Marketmakers" (0.52 MB).

Just think
of modern games: Have you ever played Quake with 18fps? There is no motion blur
in those games, thus you need a lot of frames per second more.

However, you see the spots and the dirt of single frames in a cinema film,
don't you? And those movies are played at 24fps. So there is a difference
between seeing motions fluid and seeing that there's something (dirt) at all.
Read on.

Imagine you look at a shining white wall. Now this wall turns totally black
for 1/25th of a second. Would you notice it? You surely would. 1/50th of a
second, well maybe harder. 1/100th of a second? Very difficult. Think of your
100Hz TV sets. They are called flickerfree, because at flicker rates of 100
times per second you
stop to notice the blackness of the TV screen, though the TV screen isn't
shining all the time, but pulsating 100 times per second. Brightness eats darkness.

Take again "Test 1: Smoothness of motion". You have a fluid film
with 24 fps. The film roll has to roll thru the projector. To not see
it rolling you have to make the picture black while the film rolls on.
You would have to blacken the screen 24 times per second. But 24 black
moments are too visible. Thus you have smooth motions but flicker.
The solution is: Show each frame 3 times and make the screen black 3
times per frame. This makes the black moments shorter and more
frequent: "Triple the refresh rate". So you see about 72fps in the
cinema, where 3 consecutive frames are the same. Strange solution?
Solution of an analog world. And an example how "Brightness eats
darkness".

Let's do the opposite test to "Sensitivity to darkness". Let's talk about, how sensitive the eye is to
brightness.

Imagine yourself in a very dark room. You have been there for hours and it's
totally black. Now light flashes right in front of you. Let's say as bright as
the sun. Would you see it, when it's only 1/25th of a second? You surely would.
1/100th of a second? Yes. 1/200th of a second? Yes. Tests with Air force pilots have
shown, that they could identify the plane on a flashed picture that was flashed only for 1/220th
of a second.

That is identifying. So it's pretty safe to say, that recognizing,
that SOME light was there is possible with 1/300th of a second. Now if you take
into consideration, that you have two eyes with different angles and different
areas of sensitivity (you probably know, that you see TV flickering best, when
you don't look directly into the TV screen, but with the sides of your eyes) and you can move/rotate/shake your head and
your eyes to a different position, you probably needed flashes as short as 1/500th of second to make sure, nobody
sees them in any case.

Now, what happens if I flashed you 1/500th of a second once in a second for 365 days
directly into your eye? Would you feel something strange? Would it feel
different than without it? Would you notice that something is wrong?

So, we should add a security value, to make sure nobody sees ANYTHING even unconsciously
and feels comfortable about it.

Maybe the industry didn't add enough security factor to CDs and that's why many
people still feel that analog is sometimes better. It's like in a room full of
neon lights. You just know that something isn't right.