Nginx的location if 太奇怪了

需要请求的图片，其中$station_id是变量

http://host/thumb/200x/$station_id.jpg

http://host/thumb/160x/$station_id.jpg

为了容错，提供了两张默认图片，当$station_id对应的图片找不到的时候，用默认图片替代

http://host/thumb/notfound_160x.jpg

http://host/thumb/notfound_200x.jpg

为此在Nginx里面配置：

location ^~ /thumb/ {

       set $notfound "/thumb/notfound_160x.jpg";

       if ( $uri ~ "/200x/" )  {

          set $notfound "/thumb/notfound_200x.jpg";

       }

       try_files $uri $notfound =404;

       add_header Cache-Control max-age=100;

       log_not_found off;

       #access_log /usr/local/nginx/logs/a1.log access_format;

       access_log  off;

    }