Is there any way to hover over one element and affect a different element?

I want it to be as simple as this, but I know it isn't:

img {
  opacity: 0.4;
  filter: alpha(opacity=40);
}

img:hover {
  #thisElement {
    opacity: 0.3;
    filter: alpha(opacity=30);
  }
  opacity:1;
  filter:alpha(opacity=100);
}

So when you hover over img, it changes the opacity of #thisElement to 30% and changes the opacity of the image to 100%. Is there a way to actually do this using only css?

So this is the HTML

<!DOCTYPE html>
<html>
<head>
<script type="text/javascript" src="C:\Users\Shikamaru\Documents\Contwined Coding\LearningToCode\Learning jQuery\js\jquery-1.6.2.min.js"></script> 
<script type="text/javascript" src="briefcase.js"></script>
<link rel="stylesheet" type="text/css" href="taskbar.css"/>
<link rel="stylesheet" type="text/css" href="briefcase.css" /> 
<title>Briefcase</title> 
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> 
</head> 

<body> 

<div class="mask"></div>
<div class="float">
  <div id="album1">Album Title</div>
  <img class="left" src="bradBeachHeart.JPG" alt="Brad at the Lake" />

  <img class="left" src="mariaNavi.jpg" alt="Making Maria Na'vi" />

  <img class="left" src="mattWaterRun.jpg" alt="Photoshopped Matt" />
</div>

<div class="gradientTop"></div>
<div class="gradientBottom"></div>


</body> 
</html>

And this is the CSS:

body {
  font: normal small/3em helvetica, sans-serif;
  text-align: left;
  letter-spacing: 2px;
  font-size: 16px;
  margin: 0;
  padding: 0;
}

div.gradientTop {
  position: absolute;
  margin-top: 5px;
  z-index: 2;
  width: 206px;
  height: 30px;
  float: left;
  background: -webkit-linear-gradient(rgba(255, 255, 255, 2), rgba(255, 255, 255, 0))
}

div.gradientBottom {
  position: absolute;
  margin-bottom: 5px;
  z-index: 2;
  width: 206px;
  height: 120px;
  float: left;
  bottom: -210px;
  background: -webkit-linear-gradient(rgba(255, 255, 255, 0), rgba(255, 255, 255, 1))
}

div.float {
  border-right: 1px solid orange;
  position: absolute;
  z-index: 2;
  margin-left: 5px;
  margin-top: 5px;
  float: left;
  width: 200px;
}

div.mask {
  position: relative;
  z-index: 1;
  margin-top: 5px;
  float: left;
  width: 206px;
  height: 805px;
  background-color: white;
}

img.left {
  z-index: inherit;
  margin-bottom: 3px;
  float: left;
  width: 200px;
  min-height: 200px;
  /* for modern browsers */
  height: auto !important;
  /* for modern browsers */
  height: 200px;
  /* for IE5.x and IE6 */
  opacity: 0.4;
  filter: alpha(opacity=40)
}

img.left:hover + #album1 {
  opacity: .4;
}

img.left:hover {
  opacity: 1.0;
}

#album1 {
  z-index: 2;
  width: 200px;
  color: white;
  text-align: center;
  position: absolute;
  background: orange;
  top: 70px;
}

Solution

The only way to do this with CSS is if the element to affect is either a descendent or an adjacent sibling.

In the case of a descendent:

#parent_element:hover #child_element, /* or */
#parent_element:hover > #child_element {
    opacity: 0.3;
}

Which will apply to elements such as:

<div id="parent_element">
    <div id="child_element">Content</div>
</div>

For adjacent siblings:

#first_sibling:hover + #second_sibling {
    opacity: 0.3;
}

Which works for mark-up such as:

<div id="first_sibling">Some content in the first sibling</div> <div id="second_sibling">and now in the second</div>

In both cases the latter element in the selector is the one chosen.

Given your pseudo-code example, you probably want something like:

img:hover + img {
    opacity: 0.3;
    color: red;
}

JS Fiddle demo.