These are small, but those are far away

Or... how do you gauge depth in a flat cinema image?

For this experiment, you will need to close one eye.

This is because we primarily (so we are told) gauge depth via our hawk-like binocular vision. Which does not mean you need binoculars, but that you need two eyes. So - throw one away (but keep it - you'll need it again later)..

Try to think of as many ways as possible (with your one good eye) of deciding which of two objects is closer. Essentially, for every one of these parameters, apart from the two-eyes thing, a flat cinema image is indistinguishable from 'reality'.
Therefore, by knowing these ways of accentuating depth, and working them into your filmmaking, you will liberate your images from flatness - if you want to, that is... because maybe you don't...

OK here goes:

SIZE
For identical objects, the closer one seems larger. The correct explanation is that its angular size increases (i.e. the angle it occupies to the eye)

BLOCKING
A closer object passes in front of a more distant object.

PARALLAX
Closer objects seem to move faster when you move from side to side. More distant objects seem more relatively static (hold a finger at arm's length, then focus on the moon. Move your head from side to side. Which 'moves' more?

DETAIL
Closer objects have more apparent detail.(Try reading a book in your hand, then from across the room).

CONTRAST
(for landscapes - except under unusual lighting conditions)
More distant objects have less contrast due to haze.. Check out the catskills one sunset to test this.

KNOWLEDGE OF THE UNIVERSE
We know how big most objects are (e.g. a person) so, by gauging their apparent size, we can approximately work out their distance from us. This breaks down with very large or very small objects, and of course with objects we have never encountered.

So: all of the above are seamlessly blended into our eye/brain perception of depth.
Now, open up your other eye (hope you didn't throw it away) and see how the binocular vision does not add an easily definable difference. the objects are still the same, but somehow we now KNOW better how far away they are. This is because with two eyes, the eye/brain synthesis now kicks in. The images are richer, more alive, more engaging.

Now, your task is to involve all of these factors in your staging and camerawork to increase the involvement and engagement with your films.

Think of how to employ each of the above factors.

A clue: movement helps.

Eye / Brain

I hope this never happens to you. Still, it's instructive.
The eye is only half of the apparatus we have for apprehending the universe. How do we make sense of what we see? It's not only a series of shaky blurs and swathes of color, but an actual space we can understand and move in, and react to. Credit for this goes to the brain, which synthesizes a 3D map of what we see and it is this map - not the glimpses and fragments we actually see at any given instant - which creates our act of seeing.
Whew... that's pretty heady. Specifically, we all know the adage that the 24 frames per second of film, or the 24 drawings of an animation 'blur' into motion when we watch - therefore we can experience the 'magic' of cinema motion from a series of stills due to the short time lag that the retina (see above) emits a signal after light hits a cell. This afterglow is easily noticeable if you hold your hand up by a lamp (at night) then turn off the lamp. You will see the outline of your fingers for a surprisingly long while...
But as you might guess from the above paragraphs, it's a wee bit more complex.
Even from still images, we build up a mental map and use this to synthesize the cinematic space. We assimilate this to 'natural motion' and smooth out the judders.
In cinema (not in video, though) there's also the question of flicker: though there are indeed 24 frames per second, these images are separated by shorter sections of black (due to the shutter closing and blocking out the light while the next image is lined up), so in an actual movie theater, we are watching an alternating flickering of image and darkness.
I find this quite romantic..
Tomorrow, we will look at the various factors we use, looking out of just ONE EYE - to gauge distance, hence depth. See how many you can think of.
Clue: remember Father Ted and the sheep...

Incipit

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