Shapeoptics telephoto lens
  • Imaging Techniques

Telephoto Lens Design Principles and Practical Tips

A telephoto lens provides long effective focal length while maintaining a shorter physical length than its focal length. This allows distant subjects to be magnified without requiring a physically long optical system—an essential advantage in SLR and DSLR camera design, surveillance optics, and long-range imaging systems.

Telephoto lenses are widely used when:

  • Space and weight must be minimized
  • Long focal length is required
  • Back focal distance must accommodate a camera mirror or sensor package

 

Basic Telephoto Lens Structure

A classical telephoto lens consists of:

  • A front positive group with focal length Fa
  • A rear negative group with focal length Fb
  • A separation distance T between the two groups

This configuration shortens the overall lens length while preserving long focal length.

 

Telephoto Ratio Definition

The telephoto ratio is defined as:

Telephoto Ratio = L/F

Where:

  • L = overall physical length of the lens
  • F = effective focal length

A telephoto ratio:

  • < 1 → telephoto design
  • Smaller values → more compact lens

This definition follows classical lens theory (Cooke, 1965).

 

 

Solving the above equations gives

 

Under the thin-lens assumption, the combined focal length of the two groups and their separation can be expressed analytically.

A key design result: The absolute value of the rear negative focal length (|Fb|) is maximized when T=L/2​

This provides an important guideline for balancing:

  • Compactness
  • Aberration correction
  • Mechanical feasibility

 

Example 1: f/2.8 Telephoto Lens for SLR Camera

The figure below (referenced design) shows an f/2.8 telephoto lens designed for a single-lens reflex (SLR) camera, with:

  • Film diagonal: 1.703 in
  • Compact overall length
  • Adequate back focal distance for mirror clearance

 

The full lens prescription is shown below:

This example illustrates how a telephoto configuration enables fast aperture performance without excessive lens length.

 

 

Example 2: 1000 mm f/11 Telephoto Lens Design

A second example demonstrates a 1000 mm f/11 telephoto lens, suitable for SLR cameras.

 

Key Parameters

  • Image diameter: 1.704 in
  • Overall length: 20.00 in
  • Telephoto ratio: L/F=0.508

 

Mechanical and Optical Characteristics

  • Front triplet includes a thick middle element, making the lens front-heavy
  • Focusing is achieved by moving the rear cemented doublet
  • Aperture diameter remains fixed during focusing
  • Overall lens length is held constant, a common requirement in telephoto systems

The detailed focusing movement data is provided in the accompanying table and design file.

 

References

  1. Laikin, Milton. Lens Design. CRC Press, 2007.
  2. https://www.zemax.com/
  3. The design file used in this article is attached. Single-lens reflex (SLR) / SLR camera