if(!get_field('afbeelding_onder'))
{
$prev_post = get_previous_post();
echo '<p>' . get_field('afbeelding_onder', $prev_post->ID) . '</p>';
}
else
{
echo '<p>' . get_field('afbeelding_onder') . '</p>';
}
not tested. you might want to include a while loop, if the previous post does also not have that value set?