Programming rookie back again.

I have been studying the `add_action('save_post','')` but I am having difficulty getting it to work for my needs. The concept is simple - update the database information for an attachment's meta info.

Here's where I'm at. I have my `add_meta_box` working great with an input box that is pre-populated with the Camera information from image meta.  

If I make an edit in this Camera field and update the post, no matter what variation of `update_post_meta` I try, it's failing. For example:

    $camera = $imgmeta['image_meta']['camera'];
    $update = $_POST['exif_field_camera'];
    update_post_meta($post_id, $camera, $update);

I am fearing a horrible rookie mistake here, since it's not working. 

The code I am using to retrieve the information and using it as part of my `add_meta_box` is:


    $args = get_posts( array(
    'post_type' => 'attachment',
    'numberposts' => null,
    'post_status' => null,
    'post_parent' => $post->ID
    ) );
    foreach ($args as $attachment) {

    $imgID = $attachment->ID;
    $imgmeta = wp_get_attachment_metadata($imgID);
    $camera = $imgmeta['image_meta']['camera'];
    }

What would the code be to write the new Camera input box information?