Shortcode But Without The Equals Sign?

Try this:

function font_fam($atts, $content = null) {

    extract(shortcode_atts(array(
            'f' => isset($atts[0]) ? $atts[0] : '' ,
            ), $atts));

    return '<span style="font-family:' . $f . '">' . $content . '</span>';
}

add_shortcode ('f','font_fam');