Using custom field to change alt text for a featured image

Question

I am trying to see if I can change an alt text of a featured image on only specific blog posts.

Whenever I upload an image, it allows me to add a default title, caption, alt text and description.

However, I sometimes need to use a different alt text for the same image. The problem is I am only able to use the default alt text I set upon uploading the image whenever I add a featured image.

I have created a custom field via advanced custom fields plugin. Custom field value can be accessed via using the code:

<?php the_field('myvalue'); ?>

Now, I am trying to see if I can come up with some code that allows me to add value from custom field and update the featured image

I am trying to see how i can edit the following code so that the alt attribute can be changed to whatever value in custom field "myvalue"

$title_attribute = the_title_attribute( array( 'echo' => FALSE ) );
the_post_thumbnail(
    'full', 
     array(
        'alt'   => $title_attribute, 
        'title' => $title_attribute 
    )
);

Answer 1

I think you gave the answer in your last comment.

if (get_field('alt_image_f')){
    $post_thumbnail_attr = array('alt' => get_field('alt_image_f') ); 
    the_post_thumbnail( 'full', $post_thumbnail_attr );
}
else the_post_thumbnail();

Cleaning up your code a bit, the following should work

$custom_image_alt = get_field('alt_image_f');
$post_thumbnail_attr = array();
if ($custom_image_alt) {
    $post_thumbnail_attr['alt'] = $custom_image_alt;
}
the_post_thumbnail('full', $post_thumbnail_attr);