protected function getSelectableDimensions()
 {
     if (isset($this->app->memcache)) {
         $cache_key = 'PinholeBrowserDetailsPage.getSelectableDimensions.' . $this->photo->id;
         $dimensions = $this->app->memcache->getNs('photos', $cache_key);
         if ($dimensions !== false) {
             return $dimensions;
         }
     }
     $dimensions = clone $this->photo->getSelectableDimensions();
     if (isset($this->app->memcache)) {
         $this->app->memcache->setNs('photos', $cache_key, $dimensions);
     }
     return $dimensions;
 }