Advanced custom fields with Wordpress 4.4 SRCSET

front-page.php

<div class="product__images">
  <?php
    $image = get_field('billboard_featured_image');
    if( !empty( $image ) ) {?>
      <img <?php acf_srcset( $image['id'], 'medium', '480px' ); ?> alt="<?php echo $image['alt']; ?>" />
    <?php
    } else if( has_post_thumbnail() ) {
      the_post_thumbnail( 'medium' );
    }
  ?>
</div>

functions.php

<?php

/**
 * Responsive Image Helper Function
 *
 * @param string $image_id the id of the image (from ACF or similar)
 * @param string $image_size the size of the thumbnail image or custom image size
 * @param string $max_width the max width this image will be shown to build the sizes attribute
 */

function acf_srcset($image_id,$image_size,$max_width){
  // check the image ID is not blank
  if($image_id != '') {
    // set the default src image size
    $image_src = wp_get_attachment_image_url( $image_id, $image_size );
    // set the srcset with various image sizes
    $image_srcset = wp_get_attachment_image_srcset( $image_id, $image_size );
    // generate the markup for the responsive image
    echo 'src="'.$image_src.'" srcset="'.$image_srcset.'" sizes="(max-width: '.$max_width.') 100vw, '.$max_width.'"';
  }
}