File:Lens rayfan.gif

Ray fan with aperture in place for the parallax situation. Extreme rays as limited by the aperture stop and center ray are shown:

• 0₁ and 0₂: Object points 1 and 2
• I₁ and I₂: Image points 1 and 2
• F and F': Focal points 1 and 2
• P and P': Principal planes (both in identical position)
• The object distances o₁ and o₂ are the distances from the object positions 0₁ and 0₂ to the principal plane P (not shown in image)
• The image distances i₁ and i₂ are the distances from the image positions I₁ and I₂ to the principal plane P' (not shown in image).
• The focal length f and f' are the distances from the focal points F and F' to the principal plane P (not shown in image). Both focal length are identical if the medium on both sides of the lens is identical (air, in our case).