Custom Field from Page ID

So I have the following code.

function field_func($atts) {
  global $post;
  $name = $atts['name'];
  if (empty($name)) return;
  return get_post_meta($post->ID, $name, true);
}
add_shortcode('field', 'field_func');

This lets me use the shortcode, [field name=’fieldname’] to display the value for that custom field of the current page. However, how would I display the custom field of another post/page on this page with an ID? What code needs to be added so that I can pass [field name=’fieldname’] a second parameter of ~ id=’pageid’ ~?