“In the quiet geometry of a catadioptric lens, light does not simply blur, it returns to itself, forming perfect rings like whispered promises, each circle a reflection of a moment we almost touched, yet chose to admire from afar.” – MJ Martin
Introduction
To say that I love mirror lenses is an understatement. To me, they create the most amazing images. They are often very long lenses, typically available as 500mm or 1000mm focal lengths. So most would assume that they are used for photographing birds or airshows, and they would be correct.
But some of the best street photography or even personal portraiture photography have been shot with these long catadioptric lens. The bokeh is extraordinary. The diffused background isolates the subject in the foreground. This bokeh and isolating aspect make these lenses unique. The internal mirrors generate defused circles or rings which are very pleasing.
Catadioptric lenses occupy a unique niche in optical engineering, combining refractive and reflective elements into a single imaging system. Unlike conventional lenses that rely solely on glass elements to bend light, catadioptric designs employ mirrors to fold the optical path, enabling long focal lengths within compact physical dimensions. These lenses, often referred to as mirror lenses, were once a compelling solution for photographers seeking super-telephoto reach without the size and weight penalties of large refractive optics. Despite their elegant engineering, they have largely receded from mainstream photographic use.
Optical Principles and Design
At the core of a catadioptric lens is a hybrid optical system that uses both lenses and mirrors to form an image. The most common configuration in photography is derived from the Schmidt Cassegrain design. Incoming light first passes through a corrector plate, which is a thin refractive element that mitigates spherical aberration. The light then reflects off a concave primary mirror at the rear of the lens and is directed forward toward a convex secondary mirror mounted near the front. This secondary mirror reflects the light back through a central aperture in the primary mirror and onto the image sensor.
This folded optical path significantly reduces the physical length of the lens relative to its focal length. A 500 millimeter catadioptric lens, for example, may be only a fraction of the length of a comparable refractive telephoto lens. The use of mirrors also eliminates chromatic aberration, as reflection is wavelength independent. However, the presence of a central obstruction in the optical path introduces unique diffraction effects that influence image rendering.
Image Characteristics and Performance
The most recognizable signature of a catadioptric lens is its distinctive out of focus rendering. Because of the central obstruction caused by the secondary mirror, point light sources in the background are rendered as ring shaped highlights, commonly referred to as donut bokeh. This effect can be aesthetically interesting in certain contexts but is often considered distracting in general photography.
Sharpness in catadioptric lenses can be surprisingly high in the center of the frame, particularly when the optical alignment is precise. However, contrast is typically lower than in modern refractive lenses due to internal reflections and scattering within the system. Additionally, these lenses usually have a fixed aperture, commonly around f 8 or f 11, which limits exposure flexibility and depth of field control.
Focusing is manual in most designs, and the relatively slow effective aperture reduces the amount of light reaching the viewfinder or autofocus sensors. This makes precise focusing more challenging, especially in low light conditions or when tracking moving subjects.
Advantages and Engineering Strengths
From an engineering perspective, catadioptric lenses offer several compelling advantages. Their compact size and reduced weight make them highly portable compared to traditional super-telephoto lenses. This is particularly beneficial in applications such as wildlife observation, surveillance, and amateur astronomy, where mobility is important.
The absence of chromatic aberration is another notable strength. Since mirrors reflect all wavelengths equally, there is no dispersion of light into color fringes, which is a common issue in refractive optics. This allows catadioptric systems to maintain color fidelity without the need for complex multi element correction groups.
Manufacturing can also be more cost effective for certain focal lengths, as large glass elements are replaced by mirrors, which can be lighter and require less material. This historically enabled the production of relatively affordable long focal length lenses for consumer markets.
Limitations and Practical Drawbacks
Despite their advantages, catadioptric lenses suffer from several inherent limitations that have constrained their widespread adoption. The fixed aperture is one of the most significant drawbacks. Without an adjustable diaphragm, photographers cannot control exposure or depth of field in the same way as with conventional lenses. This also limits performance in varying lighting conditions.
The central obstruction reduces overall contrast and introduces diffraction artifacts that degrade image quality. While sharpness may be acceptable, the perceived clarity and microcontrast often fall short of modern refractive designs. The donut shaped bokeh further restricts their use in situations where smooth background rendering is desired.
Autofocus compatibility is another major issue. Most catadioptric lenses are manual focus only, and their slow effective aperture challenges modern autofocus systems. This makes them less suitable for fast paced photography such as sports or wildlife action.
Market Decline and Modern Relevance
The decline of catadioptric lenses can be attributed to advancements in conventional lens design and manufacturing. Modern telephoto lenses benefit from advanced low dispersion glass, aspherical elements, and sophisticated coatings that significantly reduce aberrations while maintaining high contrast and sharpness. Although these lenses are larger and more expensive, they deliver superior image quality and greater operational flexibility.
Digital image processing has also reduced the need for compact optical compromises. Cropping high resolution images and using computational photography techniques can achieve similar results without the optical trade offs associated with mirror lenses. Furthermore, improvements in sensor sensitivity and stabilization systems have mitigated some of the challenges associated with large refractive lenses.
Today, catadioptric lenses persist in niche applications and among enthusiasts who appreciate their unique rendering characteristics. In engineering terms, they remain an elegant solution to the problem of long focal length compression, but one that has been largely superseded by more versatile and higher performing optical systems.
Summary
Catadioptric lenses represent a fascinating intersection of optical physics and practical engineering. Their ability to deliver long focal lengths in compact forms is achieved through a clever use of mirrors and refractive correction. However, their limitations in aperture control, contrast, and focusing have led to their decline in mainstream photography. While they continue to hold value in specialized contexts, their role in the modern imaging landscape is largely historical, a testament to an innovative approach that has been overtaken by advances in optical and digital technologies.
About the Author:
Michael Martin is the Vice President of Technology with Metercor Inc., a Smart Meter, IoT, and Smart City systems integrator based in Canada. He has more than 40 years of experience in systems design for applications that use broadband networks, optical fibre, wireless, and digital communications technologies. He is a business and technology consultant. He was a senior executive consultant for 15 years with IBM, where he worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He is a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).
Martin served on the Board of Directors for TeraGo Inc (TGO: TSX) and on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). He has served as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology. He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) [now Ontario Tech University] and on the Board of Advisers of five different Colleges in Ontario – Centennial College, Humber College, George Brown College, Durham College, Ryerson Polytechnic University [now Toronto Metropolitan University]. For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section.
He holds three master’s degrees, in business (MBA), communication (MA), and education (MEd). As well, he has three undergraduate diplomas and seven certifications in business, computer programming, internetworking, project management, media, photography, and communication technology. He has completed over 60 next generation MOOC (Massive Open Online Courses) continuous education in a wide variety of topics, including: Economics, Python Programming, Internet of Things, Cloud, Artificial Intelligence and Cognitive systems, Blockchain, Agile, Big Data, Design Thinking, Security, Indigenous Canada awareness, and more.