模糊滤镜聚焦模糊（高斯）

<svg width="800px" height="600px">
   <defs>
  <filter id="focus-blur" >
    <feDiffuseLighting result = "diffOut" diffuseConstant = "1" lighting-color="white">
       <feSpotLight id="spotlight" x = "500" y = "100" z = "150" pointsAtX = "500" pointsAtY = "100" pointsAtZ = "0" specularExponent ="12" limitingConeAngle="70"/>
    </feDiffuseLighting>

     <feColorMatrix in="diffOut" result="alphaMap" type="luminanceToAlpha"/>
     <feComponentTransfer in="alphaMap" result="invertlight">
        <feFuncA type="table" tableValues="1 0 0"/>
     </feComponentTransfer>
          
     <feGaussianBlur in="invertlight" result="featherspot" stdDeviation="5"/>
     <feComposite operator="xor" result="infocus" in2="SourceGraphic" in="featherspot"/>
     <feGaussianBlur in="SourceGraphic" result="outfocus" stdDeviation="2"/>
     <feComposite operator="over" in="infocus" in2="outfocus"/> 
    <feComposite operator="in" in2="SourceGraphic"/>       
</filter>
   </defs>
        
   <image xlink:href="https://upload.wikimedia.org/wikipedia/commons/a/af/Fruit_Stall_in_Barcelona_Market.jpg" x="20px" y="20px" width="300px" height="200px" preserveAspectRatio="xMinYMin meet" />
   <image filter="url(#focus-blur)" xlink:href="https://upload.wikimedia.org/wikipedia/commons/a/af/Fruit_Stall_in_Barcelona_Market.jpg" x="340px" y="20px" width="300px" height="200px" preserveAspectRatio="xMinYMin meet"/>
</svg>

（ 源图像通过 Daderot 在维基共享资源）