Plano-Concave lens have a negative focal length. They are commonly used for light diverging and image reduction . These lens also form virtual images. 
  Specifications
      | Diameter  |  Surface Quality  |  Diameter Tolerance
  |  Center Thickness Tolerance
  |  Edge Thickness Tolerance |  Centering Tolerance
  | 
  |   |    |    |    |    |    | 
  | 3.00 - 5.00 |  40-20 |  +0/-0.05 |  +/-0.2 |  Reference |  3-5arc mins | 
  | 5.01 - 25.40 |  40-20 |  +0/-0.1 |  +/-0.2 |  Reference |  3-5arc mins | 
  | 25.41 - 40.00 |  60-40 |  +0/-0.1 |  +/-0.2 |  Reference |  3-5arc mins | 
  | 40.01 - 75.00 |  80-50 |  +0/-0.1 |  +/-0.2 |  Reference |  3-5arc mins |