Linear perspective uses lines and vanishing points to determine how much an object’s apparent size changes with distance. Atmospheric perspective deals with how the appearance of an object is affected by the space or atmosphere between it and the viewer.

Aerial perspective, also called atmospheric perspective, method of creating the illusion of depth, or recession, in a painting or drawing by modulating colour to simulate changes effected by the atmosphere on the colours of things seen at a distance.

Atmospheric perspective is achieved by using less intense, more neutral color as you move back through the painting, the sky color is mixed into the color of the object. Also, notice the use of stronger value contrast in the foreground.

15 Proven Ways to Draw the Illusion of Depth in Art

1. Volume. Unlike flat objects, 3D objects appear to have volume and therefore indicate depth in the painting.
2. Size.
3. Overlap.
4. Position/Height.
5. Contrast & Details.
6. Edges.
7. Horizon.
8. Perspective.

Shading is used traditionally in drawing for depicting a range of darkness by applying media more densely or with a darker shade for darker areas, and less densely or with a lighter shade for lighter areas. Light patterns, such as objects having light and shaded areas, help when creating the illusion of depth on paper.

A tortillon (The French name for /tɔːrˈtiːjɒn/; or blending stump) is a cylindrical drawing tool, tapered at the end and usually made of rolled paper, used by artists to smudge or blend marks made with charcoal, Conté crayon, pencil or other drawing utensils.

With that being said, shading is much easier. You simply need to put down each color and that’s it. Blending takes much more time and while mistakes are less visible and there is much more freedom to do what you like, it takes far more practice to perfect blending as opposed to shading. They are not separate entities.

While blending tools can be used with just about any medium, certain drawing media are better suited for their use. Graphite, charcoal, and soft pastel are the most popular drawing mediums in which blending tools are used, although some artists use them other media such as conté and colored pencils.

When the tip of your blending stump becomes too dull or dirty, you can sharpen it using a sand paper sharpener, which usually comes with the stump if you buy it in a pack. I recommend having dedicated stumps for dark, medium and light shades to avoid cleaning your stump multiple times for one portrait.

Some of the Procreate standard brushes that can be used for example are the Gouache (under Artistic brushes), Bonobo Chalk (under Sketching brushes) and the Stucco brush (under Artistic brushes). The Gouache gives a smooth blend, while the Bonobo Chalk and Stucco brush give a more textured look.