CSS中如何防止模糊效果扩展到子元素？

<code class="html"><div class="content">
    <div class="overlay"></div>
    <div class="opacity">
        <div class="image">
            <img src="images/zwemmen.png" alt="" />
        </div>
        <div class="info">
             a div wih all sort of information
        </div>
    </div>
</div></code>

<code class="css">.content {
    float: left;
    width: 100%;
}

.content .overlay {
    background-image: url('images/zwemmen.png');
    height: 501px;
    -webkit-filter: blur(3px);
    -moz-filter: blur(3px);
    -o-filter: blur(3px);
    -ms-filter: blur(3px);
    filter: blur(3px);
    z-index: 0;
}

.opacity {
    background-color: rgba(5, 98, 127, 0.9);
    height: 100%;
    overflow: hidden;
    position: relative;
    z-index: 10;
}</code>