How light source size affects shadow sharpness and light behavior

Knowing how the size of your source of light affects the sharpness of your subject’s shadow is one of the most fundamental things to know about lighting, and yet I see many amateur photographers and beginners completely unaware of this concept. So if even photographers sometimes don’t know how light behaves, I can only imagine what’s it like for beginning CGI artists to understand lighting.

If I could summarize the most important thing I’ve learned in college, it was this: big light source = soft shadows. small light source = sharp shadows. Done. Now you don’t need to go to art school for photography anymore (in reality, nobody does). I just saved you 80k or more in art school tuition.

To illustrate this concept I made an animation in 3Ds Max and rendered it with V-Ray. You’ll see that as the light source gets bigger, the light disperses around the subject, making the resulting shadow softer. Watch below:

It is important to note that the exposure value in the camera was also animated to compensante for the change of exposure caused by the increasing size of the light source.

“But Thomas, what about the Sun? It is the largest source of light in our solar system, and yet it casts a very sharp shadow on clear days”

you say. And you’re right. But the sun casts sharp shadows because, even though it is so big, it is so far away from us that as a light source its size is but a little dot in the sky. That’s why shadows are soft and almost imperceptible on cloudy days. The clouds disperse the sun’s light around, turning the sky into a huge softbox (which is great for portraits).

Hope this helps you better understand how light behaves and how to achieve softer or sharper shadows, both in the real world of photography and in the world of computer-generated imagery. Feel free to leave any questions or suggestions in the comments!

The 3D dude is a free model from Renderpeople (I’m not affiliated or anything)