Imagine discovering that one of the world’s most celebrated camera lenses is… radioactive. It sounds like something from science fiction, yet it’s completely true. Some of Leica’s most famous lenses—including early versions of the legendary 50mm Summicron—contain thorium glass that emits measurable levels of radiation.

Before you panic, these lenses are perfectly safe to use. In fact, photographers and collectors actively seek them out because they represent one of the most fascinating periods in Leica’s history—a time when optical engineers would use almost anything to create the sharpest lens on Earth.

So why did Leica intentionally make radioactive lenses?

The answer lies in the race to produce the finest optics ever created.

Leica’s Quest for the Perfect 50mm Lens

Following World War II, Leica faced an enormous challenge.

Its respected 50mm f/2 Summitar, designed by the legendary Max Berek in the late 1930s, was beginning to show its age. At the same time, Leica engineers were secretly developing what would become one of the most influential cameras ever built: the Leica M3.

A revolutionary camera deserved a revolutionary lens.

Leica tasked two brilliant optical designers, Gustav Kleinberg and Otto Zimmermann, with creating a successor that would outperform every standard lens available anywhere in the world.

The result would become the Leica 50mm f/2 Summicron, arguably one of the greatest standard lenses ever produced.

The Problem: Physics Was Working Against Leica

Designing lenses is always a balancing act.

Engineers want glass that bends light dramatically while also keeping different colours perfectly aligned. These characteristics are known as:

• High refractive index – bends light efficiently.
• Low dispersion – prevents colour fringing (chromatic aberration).
• High Abbe number – indicates excellent control over dispersion.

Unfortunately, during the late 1940s, there was only one practical way to achieve these properties.

Add rare earth elements to the glass.

Enter Thorium: The Radioactive Ingredient

Before modern exotic optical materials existed, lens manufacturers discovered that adding thorium oxide dramatically improved optical performance.

Thorium allowed designers to create lenses that were:

• Sharper
• Better corrected
• Smaller
• Faster
• Higher contrast

There was just one catch.

Thorium is naturally radioactive.

While the amount of radiation is relatively low, it’s enough that many early Leica lenses will register on a Geiger counter today.

This wasn’t unique to Leica.

Other manufacturers—including Canon, Kodak, Asahi Pentax, Zeiss and Fujinon—also used thorium glass throughout the 1950s, 60s and 70s.

At the time, the optical benefits far outweighed any concerns.

The Rare “Star Summicron”

One of the most desirable Leica lenses among collectors isn’t actually labelled “Summicron.”

Before Leica had access to its preferred lanthanum glass, the first production run used thorium glass supplied by Chance Brothers & Co. in England.

These transitional lenses are identified by a small engraved asterisk after the word Summitar.

Today they’re known as the Star Summitar.

Only a limited number were produced, making them one of Leica’s rarest standard lenses.

Soon after, Leica began producing collapsible Summicrons using the same thorium glass before transitioning to its own lanthanum formulation.

Why Do Some Vintage Leica Lenses Turn Yellow?

If you’ve ever looked through an early radioactive Leica lens, you may notice a warm yellow or amber tint.

This isn’t fungus.

It isn’t ageing coatings.

It’s caused by the thorium itself.

Over decades, radiation changes the structure of the glass, causing it to slowly yellow.

Fortunately, collectors discovered a simple solution.

Exposing the lens to ultraviolet light—or simply placing it in direct sunlight for several days—can reverse much of the yellowing by bleaching the glass back towards its original clarity.

Many photographers periodically “sunbathe” their vintage lenses to restore colour accuracy.

Lanthanum Glass: Leica’s Long-Term Solution

Thorium was never Leica’s preferred choice.

Their goal was to use a specially developed lanthanum crown glass, later known as LaK9, created in Leitz’s own glass laboratories before eventually being licensed to Schott.

Lanthanum offered many of the same optical advantages while being significantly less radioactive.

Once production of LaK9 became possible in the early 1950s, Leica transitioned away from thorium.

This change coincided with continual improvements to the optical formula under legendary designer Walter Mandler, resulting in progressively sharper and more refined versions of the 7-element Summicron.

Interestingly, most improvements came from refining the optical design rather than simply changing the glass itself.

Walter Mandler and the Evolution of the Summicron

Few names are as respected in Leica history as Walter Mandler.

Working first in Germany before later leading Leitz Canada, Mandler refined many of Leica’s greatest lenses.

His work included:

• Improved 7-element Summicron designs
• The famous 6-element Summicron
• Numerous Leica M lenses
• Military Elcan optics
• Leica R lenses

Many of these designs continued using various forms of lanthanum glass to maximise image quality.

Even today, Mandler-designed lenses remain highly sought after for their distinctive rendering and character.

The 50mm f/2.8 Elmar: Rare Earth Glass in a Classic Design

Rare earth glass wasn’t limited to premium Summicrons.

Leica also wanted to modernise its legendary 50mm Elmar.

Engineers discovered that increasing its speed from f/3.5 to f/2.8 while maintaining image quality would only be possible using lanthanum glass.

The resulting 50mm f/2.8 Elmar, introduced in 1957, used LaK9 glass in its front and rear elements.

While not quite as sharp wide open as later designs, it became renowned for:

• Beautiful colour rendition
• Outstanding sharpness when stopped down
• Smooth, natural bokeh
• Remarkably compact size

Today it remains one of Leica’s underrated classic lenses.

Canada’s Secret Weapon: The Elcan Military Lenses

Perhaps the most mysterious chapter in Leica’s optical history belongs to Leitz Canada.

During the Cold War, Leica’s Canadian factory produced specialist military lenses under the Elcan name (E. Leitz Canada).

These weren’t ordinary Leica lenses.

They were designed for military surveillance, reconnaissance and extreme environmental conditions.

One of the rarest combinations was the Leica KE-7A, a military version of the Leica M4 built for the U.S. military.

Only around 550 cameras were ever produced.

Its matching 50mm f/2 Elcan featured:

• Four air-spaced elements
• Ten-bladed diaphragm
• Lanthanum optical glass
• Infrared absorbing elements
• Weather-resistant construction
• Exceptional image quality

Collectors consider it among the finest Leica lenses ever produced.

The Legendary 66mm f/2 Elcan

If there is a mythical Leica lens, it’s probably the 66mm f/2 Elcan.

Produced in tiny numbers for military applications, it reportedly offered extraordinary resolution and was designed for demanding reconnaissance work.

Other elusive Elcan lenses included:

• 90mm f/1.0
• 75mm f/2 Elcan-R
• 180mm f/3.4 Elcan-R
• 450mm f/5.6 Elcan-R

Most photographers will never even see one in person.

Are Radioactive Leica Lenses Dangerous?

This is the question almost everyone asks.

The short answer is:

No, not during normal use.

Although thorium lenses emit measurable radiation, the exposure photographers receive while shooting is extremely low.

Health concerns are generally limited to situations where the glass is ground, broken or inhaled as dust—conditions that simply don’t occur during normal photography.

Many photographers have safely used thorium lenses for decades.

Collectors routinely own multiple radioactive lenses without concern.

Why Leica Doesn’t Make Lenses Like This Anymore

Modern Leica lenses are astonishingly good.

So why not recreate these famous designs?

Several reasons make that impossible.

First, environmental regulations largely ended commercial production of heavy metal optical glasses containing lead and thorium.

Second, digital camera sensors have very different optical requirements from film.

Modern Leica lenses are specifically designed so light strikes digital sensors at more perpendicular angles, reducing colour shifts and edge performance issues.

Finally, reproducing specialist rare-earth military optics today would be extraordinarily expensive.

The demand simply wouldn’t justify the investment.

Why Collectors Love Rare Earth Leica Lenses

Despite being over seventy years old, these lenses remain highly desirable.

Collectors appreciate them because they represent:

• The birth of the Leica Summicron
• An important chapter in optical engineering
• Incredible craftsmanship
• Unique rendering impossible to perfectly replicate
• Genuine historical significance

For many Leica enthusiasts, owning one isn’t simply about photography.

It’s about holding a piece of engineering history.

The idea of a radioactive camera lens sounds bizarre today, but in the 1950s it represented cutting-edge optical science.

Thorium and lanthanum glass helped Leica push beyond the limitations of conventional lens design, producing optics that became benchmarks for generations of photographers. While modern materials have replaced these rare-earth formulations, the legendary Summicron, Elmar and Elcan lenses remain enduring reminders of an era when Leica pursued absolute image quality above everything else—even if that meant creating lenses that still make a Geiger counter click over seventy years later.

For Leica collectors, that’s not a flaw—it’s part of the story.