原理：
A:放大镜 B：小图片
C：大图片可视区域
D：大图片
鼠标的位置应该在放大镜的中央，所以鼠标位置为：clientX=A.offsetLeft()+B.offsetLeft+1/2*A.offsetWidth;
clientY=A.offsetTop()+B.offsetTop+1/2*A.offsetHeight;
鼠标移动过程中:放大镜A和大图D是一起随鼠标成比例运动的，因为当放大镜A的右边框移动到与小图B的右边框重合时，大图D也应该移动到了右边框与C的右边框重合的地方
所以，他们的移动比例是：(D.offsetWidth-C.offsetWidth)/(B.offsetWidth-A.offsetWidth)=b/a
HTML部分：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"> <head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>放大镜效果</title> <style> *{
margin:0;
padding:0;
} #demo{
position: relative;
margin:30px 50px;
width: 1000px;
height: 600px;
border: 1px solid #000;
} #zhezhao{
position: absolute;
z-index:2;
background:red;
width:402px;
height:402px;
left: 20px;
top:20px;
opacity: 0;
} #small{
position: absolute;
width:402px;
height:402px;
left: 20px;
top:20px;
border: 1px solid #000;
z-index: 1;
} #small img{
position: absolute; } #big{
position: relative;
top: 20px;
left: 460px;
width:500px;
height:500px;
border: 1px solid #000;
overflow: hidden;
display: none;
z-index: 1;
} #big img{
position: absolute; } #glass{
position: absolute;
width:100px;
height: 100px;
opacity: 0.3;
background:orange;
display: none;
} </style> </head> <body>
<div id='demo'>
<div id='zhezhao'> </div>
<!-- 在ie浏览器中，当鼠标在放大镜上是，浏览器并不认为鼠标同样在小图的div上，所以加个遮罩层 兼容ie--> <div id='small'>
<img src='images/small.png' alt=''>
<div id='glass'></div>
</div>
<div id='big'>
<img src='images/big.jpg' alt='' > </div> </div> </body> </html>
js部分：
<script>
window.onload=function(){
var demo =document.getElementById('demo');
var small =document.getElementById('small');
var big =document.getElementById('big');
var glass =document.getElementById('glass')
var image =document.getElementById('big').getElementsByTagName('img')[0];
var zhezhao=document.getElementById('zhezhao');
zhezhao.onmouseover=function(){
glass.style.display='block'
big.style.display='block'
}
zhezhao.onmouseout=function(){
glass.style.display='none'
big.style.display='none'
}
//弄清楚clientX，offsetLeft，left的关系，注意区分
zhezhao.onmousemove=function(ev){
var event=ev
var left=event.clientX-demo.offsetLeft-small.offsetLeft-glass.offsetWidth/2;
var top =event.clientY-demo.offsetTop -small.offsetTop -glass.offsetHeight/2;
if(left<0){
left=0;
}else if(left>(small.offsetWidth-glass.offsetWidth)){
left=small.offsetWidth-glass.offsetWidth
}
if(top<0){
top=0;
}else if(top>(small.offsetHeight- glass.offsetHeight)){
top=small.offsetHeight- glass.offsetHeight
}
glass.style.left =left+'px';
glass.style.top =top+'px';
var percent=(image.offsetWidth-big.offsetWidth)/(small.offsetWidth-glass.offsetWidth)
image.style.left=-percent*left+'px'
image.style.top =-percent*top+'px'
}
}
</script>