Little snippet that fetches an image attachment's caption in WordPress.

Get-Image-Caption-ShortCode-WP.php

<?php
function get_caption_directly($atts = array()) {

	// set up default parameters
	extract(shortcode_atts(array(
		'id' => '52',
	), $atts));

	return wp_get_attachment_caption( $id );
}

add_shortcode('get_caption', 'get_caption_directly');
?>