How Do We See Volumes?

Before going any further, let us share some basic information about human vision.

In particular, since humans have two eyes positioned on the front of the face, they can see two slightly different images of the same scene, which the brain analyzes to reconstruct a single image with volume (3 dimensions).

Diagram illustrating how binocular vision enables three-dimensional depth perception through dual-eye image processing.

However, while our visual field often covers more than 180° from right to left, the useful portion for the binocular vision described above is significantly more limited.

By convention, in Fine Arts, clear vision is considered to be limited to 60 degrees in all directions, forming a cone extending from the center of the eye. Beyond this angle, the risks of drawing distortion become real. However, one can observe that a wide-angle camera lens allows us to bypass this limitation without excessive distortion.

Nevertheless, it is not recommended to do so in a drawing unless you truly master the concepts of perspective.

From 3D to 2D

Representing two different images on a single sheet of paper (or a single canvas) being hardly feasible, the artist therefore represents a reality corresponding to only one eye of the observer.

The following two photographs show the difference in vision between the two eyes. This is particularly visible on the lateral sides of the photos.

Comparison of left and right eye perspectives demonstrating parallax differences crucial for observational drawing.

If you decide to draw from observation (that is, to draw what you see rather than what appears in a photograph), you will need to choose which eye you work with.

This may seem ridiculous to some, but as we have seen, our eyes see two slightly different scenes, and in most people, the brain favors one eye over the other.

Let us see how to determine what is called the "dominant eye".

1. Take a sheet of paper and make a small hole in the middle.
2. Hold the sheet in front of you at arm's length.
3. Look at an object in your environment through the hole, with both eyes open.
4. Do the same while blinking one eye, then the other.
5. The eye that allows you to continue seeing the chosen object is your dominant eye.

The Cone of Vision

The cone of vision, or visual cone, is therefore this 60° cone extending from the center of the observer's retina, with its axis perpendicular to that retina. Put more simply: an eye sees directly in front of it!

Technical diagram of the 60° cone of vision showing the visual field extending from the observer's eye center.

However, the cone of vision can move in three different ways:

• When the observer moves their body (by walking, sitting down, etc.)
• When the observer moves their head (looking right or left, up or down, etc.)
• When the observer moves their eyes (for example, by looking out of the corner of their eye)

One can draw a cross in the middle of the vision, which we will discuss again later.

Crosshair marker indicating the center of the cone of vision for precise perspective alignment in drawing.

Before Perspective

For a long time, reality was represented through conventions:

For example, the Egyptians favored the profile view, but the torso of figures was always shown from the front, and the relationship between sizes was not respected...

Example of ancient Egyptian artistic convention combining profile and frontal views in figure representation.

For their part, the Aztecs used symbolic forms.

Aztec artwork demonstrating symbolic and pictographic artistic conventions.

As for the Europeans of the Middle Ages, they mixed different viewpoints within the same scene. Here, the table is seen from above while the figures are seen from the front.

Observing the World

How were the rules of classical perspective defined?

Through the observation of several easily verifiable phenomena:

• Objects diminish with distance (at least our perception of them).

Perspective drawing illustrating the principle of diminishing size with distance.

• Parallel lines appear to converge until they merge into a single point as they extend "to infinity."

Perspective drawing demonstrating how parallel lines converge toward a single vanishing point on the horizon.

• If one draws several groups of parallel lines, without the groups being parallel to each other, each converges toward a particular point, the vanishing point, but all points are placed on the horizon line.

Two-point perspective architectural drawing showing how different sets of parallel lines converge to separate vanishing points on the horizon line.