MapServer / OpenLayers: Is there a limit of 16 zoom levels?

Question

I set up a MapServer (WMS) with a raster basemap like this (Mapfile):

MAP
  PROJECTION
   "init=epsg:900913"
  END
  WEB
    METADATA
      "wms_title" "mymap"
      "wms_srs" "epsg:900913"
    END
  END
  LAYER
    METADATA
      "wms_title" "myraster"
      "wms_srs" "epsg:900913"
      "wms_enable_request" "*"
    END
    PROJECTION
     "init=epsg:900913"
    END
    NAME         myraster
    DATA         "/path/to/myraster.tif"
    STATUS       ON
    TYPE         RASTER
    PROCESSING   "BANDS=1,2,3"
    OFFSITE      0 0 0
  END 
END

Now I display this map with OpenLayers:

var mylayer = new OpenLayers.Layer.WMS( "My Map", "http://127.0.0.1/cgi-bin/mapserv?map=/path/to/mymapfile.map", {layers:'myraster'} );

That works fine so far. However, there seems to be a limit of 16 zoom levels and I would like to be able to zoom in deeper than that. I tried to add the option numZoomLevels:22 but that did not have any effect. My raster image should be large enough - opened in an image viewer the resolution is much higher than displayed with OpenLayers (I also tried using a larger raster image). Is there a way to enhance the maximum number of zoom levels?

EDIT - added OpenLayers code:

var wgs84 = new OpenLayers.Projection("EPSG:4326");
var sphericalMercator = new OpenLayers.Projection("EPSG:900913");

map = new OpenLayers.Map( 'map', { projection: sphericalMercator });

layer = new OpenLayers.Layer.WMS( "OpenLayers WMS",
        "http://127.0.0.1/cgi-bin/mapserv?map=/path/to/mapfile.map",
        { layers: 'myraster' } );

map.addLayer(layer);
map.setCenter(new OpenLayers.LonLat(7.7, 48).transform(wgs84,sphericalMercator), 13);