Light Metering

Lead Summary

Light metering is the practice of measuring the intensity of light in order to determine the correct exposure settings — aperture, shutter speed, and ISO — for a photograph. For film photographers, accurate metering is especially important because film tolerates less error than digital sensors, and corrections cannot be made after the fact without affecting the entire roll. Metering methods divide broadly into two families: reflected metering, which reads light bouncing off the subject, and incident metering, which reads light falling onto it. Within each family, a spectrum of hardware implementations exists — from basic selenium cells on early clip-on meters, through CdS cells in prism finders, to sophisticated multi-segment matrix arrays that draw on lens distance data. Understanding these distinctions shapes how photographers approach everything from casual street work to precision studio portraiture.

Core Concepts

Reflected vs. Incident Metering

The foundational split in light metering is between two ways of measuring light.

Reflected metering measures light bouncing off the subject, so the reading varies with the subject's tone and reflectance. Dark subjects reflect less light, causing the meter to recommend more exposure; bright subjects reflect more, pushing the meter toward less exposure. Because every reflected meter is calibrated to render any reading as middle gray, a scene dominated by a dark tone will trend toward underexposure and one dominated by a bright tone toward overexposure — unless the photographer compensates.

Incident metering measures the light falling on the subject, not reflected from it. Because the incident reading depends only on the light source — not on what color or tone the subject happens to be — dark objects and light objects receive exactly the same reading under identical illumination. This is why incident metering produces inherently consistent results: dark subjects render as dark, and light subjects render as light, without requiring photographer interpretation. This consistency is especially valuable with films that have a narrow exposure latitude, such as slide films.

Every reflected meter assumes the scene is middle gray. An incident meter does not know, or care, what the subject looks like.

The Middle-Gray Assumption and the Zone System

The reflected meter's middle-gray assumption is not a flaw but a design choice that requires the photographer to understand it. Spot metering bridges reflected metering and the Zone System by allowing deliberate tonal placement: rather than accepting the meter's default middle-gray rendering, the photographer points the spot meter at a specific area of the scene — a highlight or a shadow — and consciously decides which zone that area should occupy in the final image, adjusting exposure accordingly. This turns a limitation into a precision tool.

Mechanism & Process

Stop-Down Metering

Before full-aperture TTL metering was standardized, and still today when using adapted lenses without aperture coupling, photographers rely on stop-down metering. The lens is physically stopped down to the intended taking aperture, and the camera's meter reads through the lens at actual exposure conditions. The typical workflow is: open fully for composition and focusing, engage the depth-of-field preview lever to stop down, take the meter reading, then expose. Canon FL-mount lenses require this procedure because they transmit no aperture information to the camera body at all — the meter can only see the actual light at the set aperture, not at full aperture.

Full-Aperture TTL Metering

Full-aperture TTL metering allows the photographer to compose and focus with the lens wide open while the meter reads at full aperture, using a coupling mechanism to account for the planned aperture setting. Minolta pioneered this in the SR-T 101 (1966): a coupling lug operated a follower needle in the viewfinder, so the photographer could set aperture directly from the meter reading without stopping the lens down. This was a significant convenience improvement over the stop-down workflow.

Off-the-Film (OTF) Metering

Off-the-film (OTF) metering, first introduced in the Olympus OM-2 in 1975, takes TTL metering one step further: the meter reads light reflected directly from the film surface and the first shutter curtain during the actual exposure, not before it. This real-time measurement allows the camera to adjust shutter speed mid-exposure — a capability particularly valuable in scientific and technical photography such as photomicrography and astronomical photography, where illumination may change during a long exposure. OTF metering also enabled highly accurate automatic flash exposure: flash duration is controlled by cutting power the instant the film has received enough light, measured at the film plane itself.

Matrix Metering

The Nikon FA's introduction of matrix metering in 1983 was the most significant paradigm shift in practical exposure control since TTL metering itself appeared with the Topcon Super D in 1963. The FA's Automatic Multi-Pattern (AMP) system divided the frame into multiple segments and compared readings against a database of scene patterns to infer the most likely subject distribution and recommend an appropriate exposure. Prior to the FA, no camera employed multi-segment evaluative metering. Despite initial resistance from traditionalist photographers, matrix metering was adopted by every major SLR manufacturer within a few years and became the expected default for modern cameras.

The Nikon F5 extended this further with its 3D Color Matrix Metering: a 1005-pixel RGB metering sensor evaluated scene color, brightness, and contrast, while CPU (D-type AF-D) lenses transmitted subject distance data to the camera, refining exposure calculations based on where in the frame the subject was positioned. The practical limitation was that manual-focus lenses, lacking electronic CPU contacts, could not supply distance information, disabling 3D matrix metering and falling back to a simpler pattern.

Historical Development

Selenium and CdS: External to Integrated

Early camera metering relied on clip-on selenium meters attached to the top of the camera body. Selenium cells are photoelectric — they generate a small current proportional to light, requiring no battery — but they are physically bulky and less sensitive than later technologies. The Nikon F launched with clip-on selenium meters. The subsequent Nikon F Photomic moved the meter inside the viewfinder prism itself, switching from selenium to CdS (cadmium sulfide) cells. CdS cells required battery power but were far more compact and sensitive, and this prism-integrated approach became standard across the Nikon lineup.

Minolta became the first manufacturer to build a CdS meter directly into a 35mm SLR body with the Minolta SR-7 in 1962. Four years later the SR-T 101 added full-aperture TTL metering with lens coupling — a combined first that established Minolta as a metering pioneer well before most competitors.

From Match-Needle to LED

Early TTL metering systems displayed exposure through a match-needle system: the photographer adjusted aperture or shutter speed until a needle in the viewfinder reached a center position. The Pentax K1000 carries this philosophy to its most stripped-down form — a single needle moving up or down in response to light, requiring no interpretation beyond centering. Practitioners widely regard it as one of the most intuitive metering displays ever made. The Nikon FM2 updated the concept with an LED -o+ indicator, which the camera's designers noted was easier to read in low or no-light conditions than a mechanical needle.

The Nikon Photomic prism finders incorporated an elegant battery-conservation detail: rather than requiring a separate on/off switch, advancing the film transport lever 30 degrees from the body automatically energized the meter circuit. Returning the lever flush against the body cut power. This integrated metering naturally into the shooting workflow without additional steps.

Mercury Batteries and Their Legacy

Many classic meters were calibrated for 1.35V mercury batteries — a chemistry banned from sale due to environmental concerns. Modern alkaline replacements output 1.5V, which shifts meter readings and introduces systematic error. The practical solutions vary by camera: Wein Cells (zinc-air, 1.35V) drop in as direct replacements; MR-9 adapters allow standard batteries to be used with voltage-compensating circuitry; and some photographers simply accept the offset and recalibrate empirically. The same issue affects Nikkormats, older Canon bodies, and a wide range of other cameras designed in the 1960s and 1970s.

Variants & Subtypes

Center-Weighted Metering

Center-weighted metering distributes sensitivity across the entire frame but biases heavily toward the center, on the assumption that subjects are usually centrally placed. The Minolta SRT series implemented an early variant of this through the CLC (Contrast Light Compensator) system: two CdS cells — one toward the front of the prism and one toward the rear — measured average light while automatically compensating for contrast between the upper (typically brighter sky) and lower (typically darker subject) portions of the frame. This was an early form of multi-cell intelligence, anticipating the segmented approaches that would follow.

Integral (Averaging) Metering

Integral metering averages light across the full frame. Leica R-system cameras use full-field integral metering, which performs well in scenes where light and dark areas are roughly balanced but becomes unreliable in contrasty or backlit situations. For these cameras, selective spot metering is recommended when lighting is uneven — requiring the photographer to take on more deliberate metering technique.

Spot Metering

Spot metering reads a very small angle of view — typically 1 to 5 degrees — from the center of the frame, allowing measurement of specific tonal areas. Used in combination with zone system thinking, spot metering gives precise exposure control: measure a highlight, decide it should fall in Zone VII, and adjust accordingly. It demands more photographer skill but offers the highest degree of precision.

Notable Examples

Minolta CLE

The Minolta CLE illustrates several metering technologies in one compact rangefinder body. Its TTL metering employs a silicon photodiode (SPD) sensor in a dual-mode arrangement: at shutter speeds faster than 1/60s, the camera reads a multi-colored pattern on the shutter curtain itself; at slower speeds, it reads from both the curtain and the film surface. For flash, OTF metering monitors the flash burst at the film plane and automatically cuts duration when correct exposure is achieved, making flash exposure automatic at any aperture. An exposure compensation dial provides ±2 EV adjustment, effectively extending the camera's ISO range and enabling deliberate creative over- or under-exposure.

Hasselblad V-System

The Hasselblad V-system ships without any built-in meter. Exposure must be determined either by a handheld meter or by attaching an optional PME metered prism finder. The prism provides TTL metering but does not link to the camera's controls: the photographer still sets aperture and shutter speed manually on the lens. The metered prism becomes particularly useful for macro work and when using filters, where manual bellows factor calculations would otherwise be required.

Mamiya C Series

The Mamiya C220 and C330 TLR cameras lack TTL metering entirely. When extended bellows are used for close-focus work, significant light loss occurs proportional to extension distance, and this must be calculated and applied manually. Without through-the-lens metering to account for the bellows factor automatically, photographers must apply the exposure compensation formula themselves or use a handheld meter that accounts for this.

Practical Considerations

Using Handheld Meters for Film

For film photography generally, and for slide films specifically, using a handheld external meter in manual exposure mode is the most reliable approach. External incident meters eliminate the tonal-interpretation problem of reflected metering and allow zone system principles to be applied without modification. For Kodak Ektachrome E100 specifically, incident metering with the diffuser pointed toward the shooting position provides tighter exposure consistency than reflective methods — valuable given E100's narrow exposure latitude.

Filter Compensation and TTL vs. Handheld

An important practical difference between TTL and handheld metering emerges when optical filters are placed in front of the lens. A handheld meter does not see through the filter and therefore cannot account for the light absorbed by the filter glass — the photographer must compensate manually (e.g., by shooting at ISO 400 instead of 800 when using an 85B filter over an ISO 800 film). A TTL meter, reading through the lens and therefore through the filter, automatically accounts for the light loss and requires no manual adjustment.

Testing a Meter on a Used Camera

When buying a used metered film camera, meter accuracy should be verified against a known-good reference — a digital camera, a smartphone app, or a calibrated handheld meter — pointed at the same scene. A practical indoor benchmark: 400 ISO, 1/60s, f/2.8 under typical room lighting. Equally important is inspecting the battery compartment for corrosion: green or white residue indicates a battery was left inside and may have leaked, potentially destroying the meter's wiring from the inside. Corrosion damage can sometimes be cleaned, but severe cases require professional repair or render the meter permanently non-functional.