How do i set the Access Strategy in the mapping class to point to the base _photos field?

public class Content
{
  private IList<Photo> _photos;
  public Content()
  {
     _photos = new List<Photo>();
  }
  public virtual IEnumerable<Photo> Photos
  {
    get
    {
      return _photos;
    }
  }

  public virtual void AddPhoto() {...}
}

public class Article : Content
{
  public string Body {get; set;}

}

I am currently using the following to try and locate the backing field but an exception is thrown as it cannot be found.

public class ArticleMap : ClassMap<Article>
{
   HasManyToMany(x => x.Photos)    
   .Access.CamelCaseField(Prefix.Underscore)  //_photos
   //...
}

i tried moving the backing field _photos directly into the class and the access works. So how can i access the backing field of the base class?