For anyone with the same issue I managed to get this working by altering my code as below:
<?php
$images = get_field('images_test_field');
if( $images ): ?>
<ul>
<?php foreach( $images as $image ): ?>
<?php $photographer_name = get_field('photographer_name', $image['id']); ?>
<li>
<a href="<?php echo $image['url']; ?>" rel="lightbox">
<img src="<?php echo $image['sizes']['soldier-additional-photo']; ?>" alt="<?php echo $image['alt']; ?>" />
</a>
<p><?php echo $image['alt']; ?></p>
<p><?php echo $image['caption']; ?></p>
<p><?php echo $photographer_name; ?></p>
</li>
<?php endforeach; ?>
</ul>
<?php endif; ?>
