Theory:

For this first theorical part of these shadow tutorials I will strictly be using a 3D software, as it produces perfect shadows that will serve as perfect examples. I will display various light settings and though I won't explain how they work, I will analyze them.

Shadow can be defined by the absence of light caused by the interposition of an object between the source of light and the surface that receives the light. Shadow must not be perceived as a projected darkness but as the absence of light. Objects do not cast shadows but their opacity prevents light from being diffused. We often think of shadows as a dark counterpart of an object which is only a mind created illusion. In reality shadows are not part of a dualistic system of light/darkness but only the absence of light. Although this definition might not change the way you graphically perceive shadows, it is important for the theorical aspect we're dealing with here.

Therefore shadows can be also seen these ways:

Their shape and contours are defined both by the way the object retains light (its opacity, its specularity, its own contours, its texture and bumps) and by the way light bounces off that object (defined by both the object's charcteristics and by the light's nature). For instance, in this first 3D render of a sphere the shadow is defined both by the sphere's shape and by the light source placement (height, angle, diffusion cone width and length, intensity, falloff). In this example, that can remind us of the sun on a planet, it is obvious that the shadowed part of the scene is just the unlit part of that scene.

Why it is important: Most artists, from painters to photographers, let their brain see shadows in a more complicated way than it really is.

Now as a an exercise you could consider stopping painting shadows with brushes, and start creating shadows by erasing light from a dedicated layer that contains it. Even though it might prove to be a real pain in the back for you retouchers, it will surely improve both the way you perceive shadows and therefore the quality in the long term.

I'm not going to explain and depict every possible type of shadow as it would be pretty useless for the average photo retoucher; instead I will explain a few main types and illustrate them with 3D renders to provide you with a visual reference.

Let's as a first step analyze this random render that provides us with a good illustration of shadows complexity. Although there are only two objects, I can count 11 different shadows. Let's see them:

1. Inner shadow produced by the curvature of the box's edges.
2. Large soft shadow produced by the whole object.
3. Inner shadow induced by the unperfect junction of the box's parts.
4. Inner shade produced by the outside box and drawer like part's edges.
5. Inner shade produced only by the outside box which is therefore lighter than number #4.
6. Shade produced by the central position of the light source above the box, the edge is therefore shaded on sides just because it receives less light than the center.
7. Inner shadow because light doesn't penetrate that tight interstice.
8. Soft outside shadow produced under the box's edges fillet.
9. Same as number #3, lighter as it receives more light.
10. Large soft shade on the box's edge induced by its placement relatively to the light source.
11. Shadow produced by the match's head. Notice that this shadow is tinted by the red glow that is itself produced by the actually very low specularity and reflection rate of the red part.

That scene is rendered with global illumination which means the light comes evenly from a surrounding sphere, kind of like a sky with no visible sun.

Here is an exercise for you to work on. Simply try to recreate the shadows you see in this 3D render accurately in Photoshop; we provide you with a shadowless render to work on.

Shadowed render:

Shadowless render:

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