Generating soft shadows in real-time is difficult. Exact methods (such as ray tracing, and multiple light source simulation) are too slow, while approximate methods often over-estimate the umbra regions. In this paper, we introduce a new algorithm based on the shadow map method to quickly and accurately render soft shadows produced by a light source. Our method builds inner and outer translucent fins on objects to represent the penumbra area inside and outside hard shadows respectively. The fins are traced into multi-layered light space maps to store illuminance adjustment to shadows. The viewing space illuminance buffer is then calculated using those maps. Finally, by blending illuminance and shading, a scene with highly accurate soft shadow effects is produced. Our method does not suffer from umbra over-estimation. Physical relations between light, objects, and shadows demonstrate the soundness of our approach.