Change image class if meta field available

imagelens-image-class.php

<?php

add_filter('get_image_tag_class', 'add_imagelens_frames', 0, 4);

function add_imagelens_frames($classes, $id) {
	
	$singlelens = get_post_meta( $id, '_enable_lens', true );
	if ($singlelens === 'imagelens-single') {
	return $classes . ' ' . $singlelens;
	} else {
	return $classes;
	}
}