Handling 3D scene by hand can be daunting. Camera and light are what users must set themselves to get a render they are happy with, and anything confusing or slow makes that harder. These tools had to be understandable, quick and efficient, so users always feel in control.
1. Adjusting the focal length
Choosing a focal length had to be quick.
2. Operating the camera
Moving the view had to feel intuitive.
3. Managing the lighting
Lighting controls had to be understandable.