shadowBlur

var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");

ctx.shadowOffsetX = 4;  
ctx.shadowOffsetY = 4;  
ctx.shadowBlur = 4;  
ctx.shadowColor = "rgba(0, 0, 0, 0.5)";
ctx.fillStyle = "red";
ctx.fillRect(50,50,200,100);