Accommodation

This is the change in focal length of the eyes as we focus on objects at different depths. Objects closer to us require the lens in the eye to be more curved and so we tighten muscles around the eye ball to make this happen. We subconsciously feel this and feed it into our interpretation of what we are looking at. Often, in middle age, these muscles are not as strong as they used to be and so many of us need glasses to see things that are close to us.

Convergence

This is the inward rotation of the eyes as we look at an object that is close to us.

Binocular Disparity

This is the small difference we see between the left image and the right image due to the different positions of each eye. Look through one eye and then try rapidly closing one eye and opening the other. See how objects closer to you shift with respect to an object further away. This is the main tick we will be exploiting with our photographic techniques.

Motion Parallax

If we move our head from side to side we will get depth clues as objects move relative to each other. This can produce weird sensations when looking at stereoscopic images, as we shall see later.

Linear Perspective

Objects further away occupy a smaller portion of our field of vision than objects close up.

Shading and Shadowing

Shadows cast by one object onto another gives clues about depth and shape where as an object close to a light source will be brighter than one further away.

Aerial Perspective

Objects further away tend to be hazy and a little bluer as blue light is more penetrating that other visible colours.

Interposition

If one object obscures or partially overlaps another, we assume that object is closer to us that the one being obscured.

Texture Gradients

We can see detail more easily in objects that are close to us than those further away.

Colour

The fluids in the eye refract different colours to a different degree so objects of some colours appear slightly displaced to objects of anther colour. This gives the illusion they are at different depths and is known as chromostereopsis.