Internal focusing telescopes: focusing is achieved by moving which internal optical element?

Introduction / Context:
Many modern surveying telescopes use internal focusing to keep the external length fixed and to protect against dust/moisture. Understanding which lens is moved clarifies why such telescopes have stable collimation and reduced backlash.

Given Data / Assumptions:

• Telescope is of the internal focusing type.
• Objective remains fixed relative to the tube ends.
• Focusing alters the position of the real image formed by the objective.

Concept / Approach:

Internal focusing designs commonly employ a movable negative (concave) lens between the objective and eyepiece. Translating this lens shifts the focal position without moving the objective, allowing focus over a wide range with minimal external movement and preserved sealing.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturers’ optical diagrams and adjustment manuals confirm the use of a movable negative lens for internal focusing.

Why Other Options Are Wrong:

Moving the objective is characteristic of external focusing; the eyepiece is adjusted for reticle clarity, not target focus; naming a specific “plano-convex only” is too restrictive.

Common Pitfalls:

Confusing eyepiece focusing (reticle clarity) with target focusing; assuming objective movement in sealed internal-focus designs.

Final Answer:

An internal concave lens (negative focusing lens)