Understanding WordPress image sizes and responsive images

by 

function adjust_image_sizes_attr( $sizes, $size ) {
   $sizes="(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px";
   return $sizes;
}
add_filter( 'wp_calculate_image_sizes', 'adjust_image_sizes_attr', 10 , 2 );

As outlined here:
https://www.smashingmagazine.com/2015/12/responsive-images-in-wordpress-core/