WP FocusLock is a WordPress plugin for adding responsive focal points to images. It's essentially a wrapper for https://github.com/jonom/jquery-focuspoint with an admin interface for setting where the focal point is.

For a better idea of what this does check out this demo from the jQuery FocusPoint repo: http://jonom.github.io/jquery-focuspoint/demos/helper/index.html

[focuslock_image id="<attachment_id>" size="large" classes="feature-image rounded" width="100%" height="500px"] 

For a permanent fixture in a theme:

<?php focuslock_image(135, 'large', 'guy', '100%', '500px'); ?>

An example of using an attachment ID from an ACF photo field:

<?php focuslock_image(get_field('photo'), 'large', 'guy'); ?>

An example of using the post's featured image:

<?php focuslock_image(get_post_thumbnail_id(), 'large', 'gal'); ?>

Focused images will naturally adopt a style with a width of 100% and a height of 450px. This default was chosen because a width/height is needed for anything to be displayed at all and this makes for a good example. You can use the width/height attributes of the shortcode or function to change this. You can also set an extra class and use CSS to set the dimensions (among other things).