This kind of lens has positive focal lengths. They work best when producing real images at near infinite conjugates or where one of the conjugates is virtual. Specifications - Dimensions Tolerance: +0.0/-0.15mm - Paraxial Focal Length: ±2% - Centration: 3 arc minutes - Clear Aperture: 90% - Surface Quality: 60/40 - Bevel: 0.25mm at 45 deg