Which of the following term is not associated with a lens?

Options

A. Aperture
B. Focal length
C. Principal focus
D. Efficiency

Explanation

To determine which term is not associated with a lens, we will analyze each of the provided terms one by one.

### Step-by-Step Solution:

1. **Identify the Terms**: The terms provided are Aperture, Focal Length, Principal Focus, and Efficiency.

2. **Define Aperture**:
- The aperture of a lens is the total area of the lens's front face. It determines how much light can enter the lens.
- **Conclusion**: Aperture is associated with a lens.

3. **Define Focal Length**:
- The focal length of a lens is the distance between the optical center of the lens and its focal point (the point where parallel rays of light converge).
- **Conclusion**: Focal length is associated with a lens.

4. **Define Principal Focus**:
- The principal focus is the point where light rays parallel to the optical axis converge after passing through the lens.
- **Conclusion**: Principal focus is associated with a lens.

5. **Define Efficiency**:
- Efficiency, in the context of optics, generally refers to how well a lens transmits light or how effectively it focuses light. However, it is not a standard term used in the basic definitions and characteristics of lenses.
- **Conclusion**: Efficiency is not typically associated with a lens.

6. **Final Answer**: The term that is not associated with a lens is **Efficiency**.