different thumbnail sizes

Can someone help with creating thumbnails for the gallery AND post main images.

Each post is to have a main image, the size for this is 450×300 (set as featured image)
We have an image gallery page, the thumbnails on this page should be 180×100 each.

In the functions.php file I have

// Add support for Featured Images
if (function_exists('add_theme_support')) {
    add_theme_support( 'post-thumbnails', array( 'post' ) );
				set_post_thumbnail_size( 450, 300 );
}

and in the wordpress admin (settings > media) I have set

Thumbnail size: 180×100 and ticked the box ‘crop thumbnail to exact dimensions’

When I add images to the gallery the 180×100 thumbs are working great, but the main post image (set as featured image) is also going in at 180×100 instead of 450×300.

I guess I am confilcting setting the sizes in functions and in the wordpress settings.

Can someone help, what do I do to make the thumbnails the correct size for each area?

Thanks