html页面：

注释：遮罩层的大小取决于   ＝＝＝》layerwidth/layerheight = largewidth/largeheight 

enlarge.js页面

/*
jquery.enlarge.js v1.1
by AbelYao, USKY.ORG
*/

(function($)
{
var defaults =
{
shadecolor: "#FFD24D",
shadeborder: "#FF8000",
shadeopacity: 0.5,
cursor: "move",

layerwidth: 400,
layerheight: 300,
layerborder: "#DDD",
fade: true,

largewidth: 1280,
largeheight: 960
}

var enlarge = function(option)
{
option = $.extend({}, defaults, option);

$(this).each(function()
{
var self = $(this).css("position", "relative");
var img = self.children().first();

var ratio =
{
x: img.width() / option.largewidth,
y: img.height() / option.largeheight
}

var size =
{
shade:
{
width: option.layerwidth * ratio.x ,
height: option.layerheight * ratio.y
}
}

var shade = $("<div>").css(
{
"position": "absolute",
"left": "0px",
"top": "0px",
"background-color": option.shadecolor,
"border": "1px solid " + option.shadeborder,
"width": size.shade.width,
"height": size.shade.height,
"opacity": option.shadeopacity,
"cursor": option.cursor
});
shade.hide().appendTo(self);

var large = $("<img>").css(
{
"position": "absolute",
"display": "block",
"width": option.largewidth,
"height": option.largeheight
});
var layer = $("<div>").css(
{
"position": "absolute",
"left": self.width() + 5,
"top": 0,
"background-color": "#111",
"overflow": "hidden",
"width": option.layerwidth,
"height": option.layerheight,
"border": "1px solid " + option.layerborder
});
large.attr("src", self.attr("href"));
large.appendTo(layer);
layer.hide().appendTo(self);

var half =
{
x: size.shade.width / 2,
y: size.shade.height / 2
}

var area =
{
width: self.innerWidth() - shade.outerWidth(),
height: self.innerHeight() - shade.outerHeight()
}

var offset = self.offset();

var show = function()
{
offset = self.offset();
shade.show();
if(option.fade) layer.stop().fadeIn(300);
else layer.show();
}

var hide = function()
{
shade.hide();
layer.hide();
}

self.mousemove(function(e)
{
var x = e.pageX - offset.left;
var y = e.pageY - offset.top;

if(x < 0 || x > self.innerWidth()) return hide();
if(y < 0 || y > self.innerHeight()) return hide();

x = x - half.x;
y = y - half.y;

if(x < 0) x = 0;
if(y < 0) y = 0;
if(x > area.width) x = area.width;
if(y > area.height) y = area.height;

shade.css(
{
left: x,
top: y
});

large.css(
{
left: (0 - x / ratio.x),
top: (0 - y / ratio.y)
});
})
.mouseenter(show)
.mouseleave(hide);
});
}

$.fn.extend(
{
enlarge: enlarge
});
})(jQuery)

参考网址：https://github.com/abelyao/enlarge.js

放大镜效果 －－－ enlarge.js