Hi @raziel79, thanks for the post. There you can set your opacity using triggers, which include scroll into view and scroll out of view. You can setup different triggers on different sections on your page to adjust the opacity as you wish.
Can you please update your post with some more information so we can help you faster? Things like your read-only link, and screenshots of the elements in question really speed things up.
Posting guidelines can be found here: Posting Guidelines for the Design Help Category - General - Forum | Webflow
Thanks in advance! 
Cheers,
Dave