Source size → hard vs. soft light
How the size of a light source controls edge softness and shadow transitions.
What it is
Source size means how big the light appears from the subject's point of view, not its wattage. A bare bulb close to a figure is a “large” source; the same bulb far away is a “small” source. The larger the source, the softer the light-to-shadow transition. The smaller the source, the sharper it becomes.
In practice you watch edges. Hard light (small source, or far away) gives you crisp cast shadows, sharp terminators — the line where light turns into shadow on a rounded form — and strong highlights. Think noon sun on a clear day, or a spotlight on stage: eyelashes cast distinct lines, and the shadow edge under a nose is tight and clear.
Soft light (large source, or close) gives you blurry cast shadows, wide penumbra — the fuzzy zone around a shadow edge — and gentle form turns. Think overcast daylight, or a figure facing a big window or softbox: light and shadow are both present, but no single hard edge separates them.
When you paint, choose one behavior and apply it consistently. If the scene is lit by a small, distant source, cast shadows should have firm, clean edges that stay readable far from the object casting them. If you choose a broad source, edges soften as they travel, contact shadows break up, and even strong contrasts blur at their boundaries.
Why it matters
If you ignore source size, your lighting looks confused: a sharp shadow under the chin, a foggy blur under the nose, and a mix of hard and soft edges in the same cast shadow, as if five different suns are shining at once. Forms turn mushy when a hard-light scene gets soft, gradual transitions, and look flat or cut-out when a soft-light scene gets hard, abrupt ones. Tie shadow edges and form transitions to one clear source size, and the whole scene locks together: silhouettes read as lit by the same environment, materials sit in space believably, and you can push contrast or mood without breaking realism.
You can check it when
- ✓You can look at a reference and describe the light as “small and hard” or “large and soft” just from the edge quality of cast shadows and form turns.
- ✓You can keep shadow hardness consistent within a painting, so all cast shadows and form edges agree on whether the light is hard or soft.
- ✓You can change a study from hard light to soft (or the reverse) by adjusting only edge softness and transition width, and the lighting still reads clearly.
- ✓Your cast shadows don't mix razor-sharp and blurry edges without a clear reason in the setup, such as distance from the source or overlapping sources.