Hi @kandb
Consider using get_attachment_link(...) which returns the URI of the attachment field. Use the following code:
get_attachment_link($image['ID']);
Change your code to the following:
<?php
$images = get_field('photos');
if( $images ): ?>
<div id="gallery">
<h3>Photos</h3>
<?php foreach( $images as $image ): ?>
<a class="tooltip" href="<?php echo get_attachment_link($image['ID']); ?>">
<span>
<img src="<?php echo $image['sizes']['medium']; ?>" alt="<?php echo $image['alt']; ?>" />
</span>
</a>
<?php endforeach; ?>
</div>
<?php endif; ?>
