Make index.php do what single.php does?

My specific problem: if I put a <?php comments_template(); ?> in my index.php, it shows nothing. If I put excactly the same into single.php it does show comments. How can I either get index.php to show comments, or how can I tell WP to use single.php as the home page?

Background: I’m making a photoblog. Each entry will be only one photo, a short description, and comments under that, i.e. all pages are exactly the same.

Extra credit question: I only want one entry per page, but of course I want *more* than one entry shown on archive pages. But when I set “Show at most 1 post” under Options > Writing, it also reduces my archives to one entry and paginates. How do I get 1 entry only photo pages, with many entry archives?