[рони]

lybovy12,
https://jsfiddle.net/jp98deb5/

var magnify = function(e) {
    var ratio = native_width / cur_img.width() > 1.2 ? 1 : cur_img.width() / native_width * 2.7;
    var rx = Math.round(mouse.x / cur_img.width() * native_width * ratio - ui.glass.width() / 2) * -1;
    var ry = Math.round(mouse.y / cur_img.height() * native_height * ratio - ui.glass.height() / 2) * -1;
    var bg_pos = rx + "px " + ry + "px";
    var glass_left = e.pageX - ui.glass.width() / 2;
    var glass_top = e.pageY - ui.glass.height() / 2;
    ui.glass.css({
        left: glass_left,
        top: glass_top,
        backgroundPosition: bg_pos,
        backgroundSize: native_width * ratio + "px " + native_height * ratio + "px"
    });
    return
};