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Introduction

Perhaps few people — myself included — would have guessed that the inspiration for this article came from a single controversial moment at a product launch event.

It was October 15, 2025. OPPO, brimming with confidence, unveiled its latest flagship lineup — the Find X9 Series. With lenses covering multiple focal lengths and featuring Danxia-inspired color reproduction, the company made no attempt to hide its ambitions in mobile imaging. The collaboration with the legendary camera brand Hasselblad further fueled its confidence to aim higher. So, much like its competitors before it, OPPO boldly declared that “you don’t need to bring a camera when traveling,” demonstrating its prearranged success live on stage — and it was precisely this demonstration that sparked debate.

When confronted with this scene, some saw it as another triumph of computational photography — a victory that preserved the convenience of smartphones while pushing technical boundaries. But to others, it was nothing less than a disrespectful mockery of professional-grade cameras worth tens of thousands of yuan. When I first saw the image, I resonated with the latter view. The Canon R7, it seemed, was not performing at the standard one would expect. Why such a conclusion? The key lay in a small, easily overlooked icon at the upper-left (or lower-left) corner of the viewfinder — not the familiar Tv (S), Av (A), or M mode used by photography enthusiasts, but rather the forgotten green symbol: “A+” — the Advanced Automatic Mode. This mode was once the “teacher” of the digital camera era, the very starting point of countless people’s photographic journeys. Yet today, it stands at the center of a heated professional debate. The contrast is as striking as it is thought-provoking.

Compared to today’s smartphones — which combine ultimate convenience with rapid iterative intelligence — the automatic exposure algorithms of cameras may seem outdated. Yet, if we look back to an era when every setting had to be manually adjusted and even natural conditions could constrain a photographer’s creativity, it was precisely this “not-so-smart” starting point that opened the door to the world of photography for countless people. Behind it lies a grand, turbulent saga of technological evolution. So, let us set aside needless debates for a moment and step into this fascinating history together.

Section I: The Development of Light Metering Components

The concept of light metering in photography dates back to 1844 and has evolved for nearly 180 years. Its origin can be traced to the discovery that different photosensitive materials respond differently to light exposure. In the early days, determining proper exposure relied either on specialized devices that required visual inspection or purely on personal experience. It wasn’t until the 1930s that the invention and popularization of photoelectric components provided the technological foundation for objective light measurement.

The selenium photocell — verified as the earliest photoelectric element used for photographic light metering — was suitable for both external handheld light meters and built-in camera systems. Its core principle lay in the photoelectric effect: the stronger the external illumination, the greater the generated electric current. By connecting the selenium cell to an ammeter, the degree of needle deflection could then accurately indicate the intensity of the surrounding light.

The selenium photocell, which required no external power supply and had a spectral response similar to that of the human eye, became an effective means of directly measuring illumination. For this reason, when used in built-in camera metering systems, its surface was often covered with a cluster of small lenses resembling the compound eyes of insects. This design limited incoming light and prevented mirror reflections on the photocell surface, giving early automatic exposure cameras their distinctive appearance, as shown below.

The first-generation Olympus PEN-EE, released in August 1961, featured a ring of selenium photocells around the lens. These cells were mechanically linked to the camera’s internal components, allowing the aperture to be adjusted automatically based on the preset film sensitivity and ambient light conditions. Combined with its zone-focus design (similar to the “SNAP” mode on Ricoh GR cameras) and a fixed shutter speed of 1/60s, users only needed to compose their shot and press the shutter button. Moreover, its affordable price and the ability to shoot twice as many frames thanks to its half-frame format made the PEN-EE one of the most popular early automatic cameras. In other cameras or external light meters that adopted different linkage mechanisms, the selenium photocell similarly allowed users to achieve precise exposure control—much like using an exposure scale in today’s manual mode.

As time went on, the limitations of selenium photocells became apparent. Prolonged exposure to strong light caused degradation and inaccurate readings; their sensitivity to low-light conditions was poor; and they required a certain physical size to function properly. To overcome these issues, smaller and more sensitive cadmium sulfide (CdS) photoresistors were introduced. For example, as shown in the Leica–Minolta CL below, the metering probe positioned in front of the shutter curtain used a CdS cell.

In exposure metering applications, CdS (cadmium sulfide) photoresistors required external power. The system converted changes in their resistance (the stronger the light, the lower the resistance) into voltage or current signals to drive the display. Although CdS cells solved the size limitations of selenium photocells, their sensitivity to low light was still less than ideal. They also suffered from a critical flaw known as the “strong light memory effect”: after measuring bright light, if they immediately measured dim light, the displayed result would read artificially high due to response lag—leading to underexposure of darker subjects.

This inherent defect ultimately limited their range of applications, confining them mostly to mid- and low-end automatic cameras. In high-end models, CdS elements were gradually replaced by superior silicon photodiodes (SPD) and their improved variant, gallium phosphide photodiodes (GPD). The metering systems built on these technologies remain in use in modern digital SLR cameras today.

Like selenium photocells, silicon photodiodes (SPDs) generate a current proportional to the intensity of incoming light. However, because the current produced by SPDs is extremely weak, it must be amplified by an externally powered operational amplifier before being processed by subsequent circuits. SPDs deliver stable and outstanding performance in both bright and dim lighting conditions. They not only overcome the “strong light memory effect” of CdS photoresistors but also offer a smaller physical footprint, making them ideal light-sensing components. The only drawback of SPDs is their excessive sensitivity to red light, which necessitates the use of a blue correction filter in front of the sensor. This flaw was later resolved with the introduction of gallium phosphide photodiodes (GPDs).

From a technical standpoint, digital SLRs—restricted by the optical paths formed by the pentaprism and mirror assembly—continued to rely on discrete autofocus and metering subsystems even in their later years. In contrast, the mirrorless camera architecture pioneered by the Micro Four Thirds (M4/3) system consolidated all these functions within the CMOS image sensor itself. It’s important to note, however, that the accuracy of metering depends not only on the sensor’s intrinsic performance but also on its placement—a topic we will explore in the next section.

Section II: TTL — The Decisive Breakthrough

TTL stands for “Through the Lens.” Its principle lies in measuring the light that passes directly through the camera lens. By unifying the optical path, this method eliminated the inaccuracies of external light meters and perfectly adapted to interchangeable lens systems, thereby becoming the standard for modern cameras. It is worth noting that all in-camera TTL metering systems are reflective—that is, they measure the brightness of light reflected from the subject rather than the light incident upon it.

Although Zeiss Ikon had already introduced a twin-lens reflex camera, the Contaflex 860/24, in 1935 with an integrated TTL metering system and even secured a patent for it, the widespread adoption of the technology did not occur until the 1960s. The key driver behind its popularization was progress in the semiconductor industry, which made it possible to produce compact photosensitive components. One of the earliest mass-produced SLR cameras equipped with built-in TTL metering was Tokyo Kogaku’s Topcon RE Super, launched in 1963.

Another representative among the “earliest” models was the renowned and highly successful Pentax SP, which remains celebrated as a classic entry-level camera for film photography. Its full name, “Spotmatic,” originates from a pioneering concept: at Photokina 1960, Pentax showcased a prototype camera equipped with a TTL system similar to today’s spot metering technology.

During my research, I discovered that the question of “who was first” had once sparked considerable debate. The consensus today is that Pentax took the lead in showcasing a prototype, while Tokyo Kogaku (Topcon) was the first to mass-produce and release a camera equipped with TTL metering. The Pentax SP was officially released in 1964, and its metering system differed entirely from that of the Topcon RE Super.For cost and market considerations, the production version of the Spotmatic abandoned the prototype’s mechanical spot-metering structure. Instead, it adopted a center-weighted average metering system, with two CdS photoresistors placed beside the viewfinder inside the pentaprism. A similar mechanical design later appeared in the Leica-Minolta CL mentioned in the previous section. The concept of positioning metering cells beside the viewfinder also became common in later film and even digital SLR cameras with built-in metering systems, as shown below.

Tokyo Kogaku, however, firmly believed that the metering element should be placed directly behind the lens. To achieve this, the RE Super made a groundbreaking move: it positioned a CdS sensor behind the reflex mirror, and through finely etched patterns on the mirror’s surface, redirected about 7% of the incoming light toward the sensor — all while maintaining sufficient brightness in the viewfinder.This innovative design became an important blueprint for the evolution of subsequent TTL systems. Ironically, despite being technologically ahead of its time — even capable of full-aperture metering, which was remarkable for that era — the RE Super struggled in other functional aspects, leaving its overall competitiveness behind that of Pentax, Nikon, and others. Coupled with Tokyo Kogaku’s worsening financial situation in later years, the Topcon brand ultimately withdrew from the camera market in 1977.

To address the challenge of metering under instantaneous light sources such as flash, the industry began exploring a new concept — measuring the reflected light directly from the film surface. This concept was finally realized by Olympus in 1975 with the launch of the OM-2, featuring a system called “TTL Direct.”

When the shutter was pressed, and the mirror lifted while the front curtain opened to expose the film, the metering sensor — indicated by the green area in the diagram below — received the light reflected off the film surface. To ensure accuracy, the OM-2’s shutter curtain was specially coated to match the film’s reflective properties, enabling precise flash exposure measurement.

Even in the digital era, TTL remains a proven, stable, and highly reliable metering paradigm. For most compact digital cameras and mirrorless systems, while independent metering modules are no longer required, their exposure systems still adhere to the core philosophy of TTL — analyzing the light that passes directly through the lens and reaches the image sensor. This foundational concept of “Through the Lens” metering continues to serve as the technical cornerstone for the evolution and diversification of modern metering modes.

Section III: The Rise of Multi-Zone Metering Technology

As TTL metering systems matured—thanks to the optimization of sensor placement and sensitivity—classic modes such as spot metering and center-weighted average metering became firmly established. Yet, the real world’s ever-changing lighting conditions and photographers’ growing need for creative flexibility demanded even more sophisticated solutions. To meet these needs, a new concept emerged: metering that could intelligently evaluate multiple areas of the frame simultaneously.

In March 1966, Minolta released an improved version of its first SLR with built-in metering, the SR-7, naming it the SR-T101. It introduced a TTL metering system called CLC (Contrast Light Compensation), whose core innovation lay in two vertically aligned CdS light sensors. These sensors could simultaneously measure highlights and shadows within the scene, then output a compensated exposure value. Specifically, the lower sensor was made twice as sensitive as the upper one, giving priority to proper exposure of foreground subjects. This allowed for more accurate exposure in high-contrast scenes. The achievement not only enhanced the shooting experience in complex situations—such as landscapes with bright skies—but also became widely regarded as the direct prototype of modern multi-zone metering technology.

The true transformation of multi-zone metering came with the introduction of microcomputers. These brought algorithmic logic into photography, enabling cameras to rapidly and precisely process complex variables from both the environment and user input.

In 1983, Nikon launched the FA SLR camera, which would go on to win the following year’s European Camera of the Year award. It featured a metering system called AMP (Automatic Multi-Pattern), powered by a built-in microcomputer that analyzed data from two SPD (Silicon Photo Diode) sensors located at the left and right sides of the viewfinder. Each SPD sensor was divided into three detection zones, working in tandem to divide the entire frame into five metering regions. This innovation made the FA the world’s first camera to implement matrix metering. Subsequent models like the Nikon F-801 and F4 refined and expanded upon this system, carrying forward the legacy of the FA’s groundbreaking multi-zone metering technology.

In the diagram below, the fundamental conceptual difference between two metering modes becomes clear: in section A, the Nikon FA divides the frame into five separate zones, evaluating each independently; whereas in section B, center-weighted average metering treats the entire frame as a single whole. The latter is constrained by the technical framework discussed in the previous section—it can only compute an overall brightness average, making it ineffective for complex scenes with uneven lighting distribution.

From that point onward, matrix metering (also known as evaluative metering) became established as a mainstream technological paradigm, coexisting with traditional modes such as spot metering. Driven by the rapid advancement of integrated circuits, its evolutionary trajectory clearly pointed toward finer scene segmentation and more intelligent exposure analysis.

For instance, the Canon EOS 620 released in 1987 integrated previously separate light sensors into a single component divided into six zones for evaluative metering. By 1994, the EOS-1N featured an internal metering sensor capable of analyzing 16 zones. Fast-forward to 2014, the EOS 7D Mark II incorporated a 150,000-pixel RGB metering sensor that divided the frame into 252 zones, with each zone consisting of roughly 600 pixels for exposure evaluation. This trend was also reflected in Minolta’s “Honeycomb Metering” and Nikon’s continuously refined Matrix Metering System, which together compose a technological epic chronicling the evolution of automatic exposure control.

The diagram below shows the real-time display function of the “Honeycomb Metering System” on the Minolta Alpha 7 film camera. In this system, black areas indicate underexposure, white areas represent overexposure, and the numerical values inside each hexagonal cell correspond to the exact number of stops by which that specific region is over- or underexposed. This metering system was also featured in the Konica Minolta Alpha 7 Digital and Alpha 5 Digital, as well as Sony’s first DSLR, the DSLR-A100, continuing the legacy of Minolta’s innovative exposure technology.

Section IV: The Foundation of Calculation — APEX

Looking back at the previous sections, the internal logic of technological evolution becomes clear: photosensitive elements and optical design laid the groundwork for hardware, while microcomputers and integrated circuits endowed cameras with intelligence, allowing the experience of earlier generations to be efficiently reused through automation. Within this process, the selection of a universal and practical exposure reference system became a crucial step.

After a long process of refinement, the APEX system, proposed in 1960 by the American Standards Association (ASA), ultimately prevailed. The letter “A” in APEX stands for “Additive”, hence it is also known as the “Additive System.” Serving as a comprehensive framework for exposure calculation in film photography, APEX was widely adopted throughout the industry.

• Av (Aperture Value) and Tv (Time Value) correspond directly to the “Av” and “Tv” modes found on Canon cameras today.
• Sv (Speed Value) can be derived from the ISO sensitivity setting.
• Bv (Brightness Value) represents scene luminance, where L is brightness and K is a calibration constant.

A detailed formula and calculation example are shown in Figure below¹. For those interested in the mathematical details, further reference materials are highly recommended.

What truly matters is understanding that the APEX system provided both photographers and cameras with a unified, simplified framework for exposure calculation. Its core innovation was the abstraction of exposure into a single EV value—meaning that any aperture–shutter combination producing the same EV would yield an identical exposure. In the film era, this allowed users to adjust just one variable (aperture or shutter speed) while the camera automatically computed the other—a concept that directly gave rise to Aperture Priority and Shutter Priority modes. As digital cameras evolved, ISO sensitivity became an adjustable variable as well, enabling far greater flexibility within the same conceptual framework.

Inside a camera, automatic exposure calculations are typically described using what is known as a “program line.” For example, the diagram below illustrates the program line of the Minolta Alpha 7000, which depicts how the camera calculates exposure when using ISO 100 film in Program Auto (P) mode.

In this diagram, the horizontal axis represents shutter speed, while the vertical axis represents aperture. The diagonal lines across the grid correspond to EV (Exposure Value) levels. For a given EV value, the intersection point between the green and black lines indicates the aperture–shutter combination that achieves that specific exposure. For instance, in this chart, the combinations F2.8 at 1/1000s, F5.6 at 1/250s, and F11 at 1/60s all yield an exposure value of EV = 13.

Different manufacturers design their automatic exposure programs with distinct computational logics. This variation becomes especially apparent in interchangeable-lens systems, where cameras adjust their strategy according to lens focal length and aperture. For instance, the Minolta Alpha 7000 employed separate program lines for wide-angle, standard, and telephoto lenses: at EV 13, the corresponding exposure settings were F8 at 1/125s, F5.6 at 1/250s, and F4 at 1/500s, respectively—each optimized to maintain consistent exposure across varying focal lengths.Canon’s early EOS 650 and EOS 620 models even featured the ability to match exposure programs to specific lenses, a sophisticated capability that anticipated the adaptive metering systems of later generations.

Section V: From Mechanical to Electronic, From External to Internal

To all readers who have followed along this far—please accept my sincere gratitude. As outlined in the preceding sections, every prerequisite for automatic exposure was now in place: ideal hardware, reliable algorithms, and sufficient computing power awaited their unification. This final push would fully open the doors to a new era of camera development—one that would liberate image-making from professional confines and grant ordinary people the power to capture fleeting moments.

Among the milestones of this transition, Canon’s AE-1, released in April 1976, stands as an undeniable classic. It was the first camera to replace traditional mechanical linkages with integrated circuits and a microprocessor, thereby realizing shutter-priority automatic exposure through electronic computation. This innovation reduced the number of mechanical components by over 300 parts², significantly cutting both manufacturing costs and failure rates.

Thanks to its novel electronic operation, compact body, affordable price, and highly successful marketing campaign, the AE-1 captured the spirit of the market perfectly. It ultimately sold 5.7 million units, becoming one of the best-selling SLR cameras in history. Its success not only revolutionized the camera industry but also rescued Canon from the brink of bankruptcy. Five years later, in 1981, Canon released its successor—the AE-1 Program—which introduced a new fully automatic program exposure mode, allowing the camera to autonomously determine both aperture and shutter speed. This advancement marked a decisive step toward the intelligent automation that defines modern photography.

The success of the AE-1 established the irreversible trend toward electronic cameras. In this new technological landscape, the Nikon FA—previously mentioned in Section III in the context of multi-zone metering systems—emerged as a high-end embodiment of this shift, pushing electronic integration even further. It featured an industry-leading 1/4000s electronically controlled shutter, along with every automatic exposure mode available at the time. Combined with its renowned AMP multi-pattern metering system, the FA came to be regarded as a culmination of Nikon’s cutting-edge technologies, a showcase of engineering excellence. Its sales performance ranked just below Nikon’s professional flagship, the legendary F3.

Unfortunately, the brilliance of the FA was short-lived. Just one year after its debut, the camera industry was completely reshaped by the Minolta Alpha 7000, a groundbreaking model that marked a new era. As the world’s first mass-produced autofocus SLR, it compelled the global industry to rethink what a camera could be. The name “Alpha” became synonymous with technological revolution—a legacy that would endure even after the brand changed hands, continuing to define innovation in photography. It is worth noting, however, that the Alpha 7000’s historical significance was primarily founded on its autofocus system. The key breakthrough in automatic exposure technology would instead arrive with its successor: the Alpha 7700i, released in May 1988. Outside Japan, this camera was known as the Dynax 7000i or Maxxum 7000i, representing the next major evolution in the automation of photography.

The Alpha 7700i further enhanced autofocus performance, surpassing even the Canon EOS 650/620 in focusing speed. Its autofocus system employed three CCD sensor arrays, allowing it to achieve precise focus on both horizontally and vertically oriented subjects. The metering system also underwent a significant redesign—five of its six zones were concentrated toward the center, effectively emulating a “spot-linked metering” approach to optimize exposure for the main subject.However, the true milestone of the Alpha 7700i lay not in its hardware improvements, but in the introduction of the “Creative Expansion Card” system—known in Japan as the “Artistic Function Card”.

This innovation went beyond a simple hardware upgrade; it effectively endowed the camera with a “software soul.” By inserting different Creative Cards, users could instruct the camera to enter specific scene modes or unlock unique functions. The system could not only adjust exposure parameters but also actively control lens focus movement, pioneering the concept of software-expandable functionality in cameras—an idea far ahead of its time.

Inspired by Minolta’s Creative Expansion Card system, Canon introduced its own innovation in March 1990 with the release of the EOS 10, which featured a program auto-exposure system centered around the “ART CODE” barcode. Using a dedicated scanner, users could scan barcodes printed in a companion preset book, instantly loading the corresponding shooting parameters into the camera. The camera would then automatically adjust its settings, allowing users to capture images that closely resembled the sample photos shown in the barcode book.

From the perspective of technological history, creative cards and barcodes were transitional solutions unique to a certain era. Yet for every user, the scenarios that evolved from them may have become the gentle starting point of their photographic journey. At that moment, technology faded into the background, serving only one pure purpose — to help you capture a fleeting instant. Whether or not photography remained a part of your life afterward, that moment — clumsily yet earnestly preserved by it — has already become eternal.

Epilogue

“Light — finally within reach.” — Nikon FA advertising slogan

From exposure to focus, one barrier after another has fallen, and the right to record life has been passed down, delivered into the hands of millions of ordinary people. History reminds us not to forget the path we came from; the future urges us to stay aware of where we stand now. Thus we, once mere chasers of light, have become its masters — the creators who shape it.

By understanding the journey that led us here, we recognize the origins of the tools in our hands. By looking toward this era, we see more clearly the direction we are heading. This is a grand epic written by humankind — the story of how we moved from pursuing light to commanding it. And in the vast river of photography, this is but one chapter among many.

At this very moment, perhaps it’s time to let go of all technical debates. Because the best camera will always be the one in your hands, eager to capture life — and it now waits quietly, ready for your next spark of passion.

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• Header image from Pixabay, taken by user “peterscode.” The main text has been refined with AI assistance.
• This article references earlier works from websites such as Xitek and Zhihu columns — heartfelt thanks to those creators for their insights.
• If you find any errors in this text, feel free to point them out in the comments section.

1. In the aperture value formula (Av), N represents the f-number. In the shutter speed value formula (Tv), t represents the shutter speed in seconds, and D is the denominator of that speed.In the sensitivity value formula (Sv), S represents the ISO sensitivity level. In the brightness value formula (Bv), if ambient brightness is measured in candelas, the constant K = 11.4. ↩︎
2. This claim comes from Canon’s official promotional materials. ↩︎