I think you gave the answer in your last comment.
if (get_field('alt_image_f')){
$post_thumbnail_attr = array('alt' => get_field('alt_image_f') );
the_post_thumbnail( 'full', $post_thumbnail_attr );
}
else the_post_thumbnail();
Cleaning up your code a bit, the following should work
$custom_image_alt = get_field('alt_image_f');
$post_thumbnail_attr = array();
if ($custom_image_alt) {
$post_thumbnail_attr['alt'] = $custom_image_alt;
}
the_post_thumbnail('full', $post_thumbnail_attr);