Measuring Light

In Lesson 6 we established that exposure is a precise quantity — the right amount of light, for the right duration, on film of a given sensitivity. But knowing the equation is useless unless you can measure the first variable: how much light is actually present in the scene you want to photograph. Your eyes cannot do this reliably, because they adapt constantly. Walking from a dim hallway into bright sunlight, your pupils constrict and your visual cortex adjusts its gain, so both scenes look adequately bright within moments. Film has no such adaptive circuitry. It responds to the absolute quantity of photons that strike it, and it does not forgive errors. A light meter bridges this gap — it measures the objective brightness of a scene and translates it into the aperture and shutter speed settings you need.

Two Ways to Measure

There are two fundamentally different approaches to light measurement, and understanding the distinction between them is one of the most important things you can learn about photographic exposure.

Reflected Light Metering

Reflected metering measures the light bouncing off the subject toward the camera. This is what a camera's built-in meter does (and what a handheld meter does when you point it at the scene from the camera position). The meter reads the luminance of everything in its field of view — the bright sky, the dark shadows, the subject's face, the grass — and averages that information into an exposure recommendation.

The strength of reflected metering is convenience. You measure from the camera position, looking at what you are going to photograph. You do not need to walk up to the subject or hold a meter in the scene. Every camera with a built-in meter uses reflected metering, as do smartphone light meter apps like metergeist.

Incident Light Metering

Incident metering measures the light falling on the subject, rather than the light reflected from it. An incident meter has a white translucent dome (called a lumisphere) that you hold at the subject's position, pointing it back toward the camera. The dome collects light from a hemisphere — roughly matching the light that falls on a three-dimensional subject — and the meter reads the illuminance.

The critical advantage of incident metering is that the reading is independent of the subject's reflectance. Whether you are photographing a white wedding dress or a black tuxedo, the incident meter gives the same reading, because the same amount of light is falling on both. The white dress will be recorded as white, the black tuxedo as black, and the exposure will be correct for both. This is a powerful simplification.

The 18% Gray Standard

Every reflected light meter is calibrated around a single assumption: that the average scene reflects approximately 18% of the light falling on it. This figure, sometimes called middle gray or 18% gray, represents the reflectance of a “typical” scene containing a mix of lights, darks, and middle tones. When a reflected meter gives you an exposure recommendation, it is telling you the settings that will render the metered area as middle gray on the film.

This calibration works remarkably well for most scenes. A landscape with blue sky, green trees, brown earth, and a few bright clouds averages out close to 18% reflectance. A city street with buildings, pavement, cars, and sky does too. The meter's assumption holds.

But it fails spectacularly for scenes that are predominantly very light or very dark. Point a reflected meter at a field of fresh snow, and it will recommend an exposure that renders the snow as 18% gray — a dull, muddy gray instead of brilliant white. The meter does not know the scene is supposed to be bright; it only knows how much light is coming in, and it assumes that amount represents a middle-toned subject. The fix is to add exposure — typically 1.5 to 2 stops over the meter's recommendation — to push the snow back up to its correct brightness.

The reverse happens with a very dark subject. Photograph a black cat on a dark sofa, and the reflected meter will overexpose, trying to bring the scene up to middle gray. The cat will come out as a washed-out charcoal rather than true black. Here, you subtract exposure — 1 to 2 stops under the meter's recommendation.

Metering Patterns

Camera manufacturers have developed increasingly sophisticated ways to handle the reflected metering problem. These metering patterns determine which parts of the frame the meter considers, and how it weights them.

Averaging (Full-Frame)

The simplest pattern: the meter reads the entire frame and averages the luminance. This is what most handheld meters and early built-in meters do. It works well for evenly lit scenes but struggles when the frame contains large areas of very different brightness — a landscape with a bright sky, for example, will cause the meter to recommend less exposure than the foreground needs.

Center-Weighted

Developed in the 1960s and popularized by the Nikon F Photomic FTn in 1968, center-weighted metering reads the entire frame but gives extra emphasis to the central portion — typically 60% to 75% of the reading comes from the center circle, with the remaining 25% to 40% from the edges. The assumption is that the main subject is usually in the center. This is a significant improvement over simple averaging for portraits and many other common compositions.

Spot Metering

Spot metering reads only a very small area of the frame — typically 1% to 5% of the total image area. This gives the photographer precise control: you can meter the shadow, the highlight, the subject's face, or any specific element, and know exactly how much light is coming from that area. Spot metering is the foundation of the Zone System (Lesson 18) and is invaluable for challenging lighting situations. The Pentax Spotmeter V, introduced in 1960, was the gold standard for decades.

Matrix / Evaluative

Modern SLRs and digital cameras use multi-segment metering that divides the frame into zones (Nikon's matrix metering, introduced in the FA in 1983, used five segments; modern cameras use thousands). The camera compares the brightness pattern across all segments against a database of known scenes and tries to recognize the situation — backlit portrait, landscape with bright sky, snow scene — and adjust the exposure accordingly. This sophisticated approach works remarkably well in many situations, but it is essentially a black box: the photographer cannot easily predict or override its logic.

The EV System and Its History

As we discussed in Lesson 6, the Exposure Value system was patented by Friedrich Deckel in 1954 and formalized in the APEX (Additive System of Photographic Exposure) standard. But the practical adoption of EV extended well beyond camera design.

EV charts became a standard reference tool. Published in camera manuals, meter instruction sheets, and photography textbooks, these charts listed typical EV values for common scenes at ISO 100:

• EV −4: Deep star fields, aurora
• EV 1: Candle-lit scenes, distant city skylines at night
• EV 5: Interior of a home with artificial lighting
• EV 8: Brightly lit interiors, shop windows at night
• EV 10: Shade on a clear day, heavily overcast
• EV 12: Overcast day, open shade in sun
• EV 13: Slightly overcast, no shadows
• EV 14: Hazy sun, soft shadows
• EV 15: Full sun, distinct shadows
• EV 16: Bright sun on light sand or snow

Many handheld light meters display their readings in EV. The Sekonic L-308X, a popular modern incident/reflected meter, gives readings directly in EV, which the photographer then translates to an aperture-shutter pair. Some meters show all three: EV, and the corresponding aperture and shutter speed for the set film speed.

Practical Metering for TLR Photographers

Most twin-lens reflex cameras have no built-in light meter. The Rolleiflex 2.8F has a selenium cell meter coupled to the controls, and a few later models like the Yashica-Mat 124G include CdS meters, but the majority of TLRs — Rolleicords, Autocords, Minolta TLRs, most Mamiya C-series bodies — are meterless. This is not a disadvantage; it is simply a different workflow.

Before every shot (or before a series of shots in consistent light), you need to take a reading. Your options include:

Handheld light meter. A dedicated meter like the Sekonic L-308 or Gossen Digisix gives reliable incident and reflected readings. Many photographers prefer incident metering for the consistency it provides.

Smartphone light meter app. Modern phone cameras have calibrated sensors, and apps like metergeist use them to provide reflected light readings. You hold the phone at the camera position, point it at the scene, and read the suggested aperture and shutter speed. For most outdoor and well-lit situations, a smartphone meter is remarkably accurate.

Sunny 16 and experience. In consistent daylight, the Sunny 16 rule and its variations (Cloudy 8, Heavy Overcast 5.6, Shade 4) can get you within a stop of correct exposure without any meter at all. Many experienced photographers use this as a sanity check even when they have a meter — if the meter suggests something wildly different from what you would expect for the conditions, investigate before committing a frame.

A Metering Workflow

Here is a practical workflow for TLR photography with an external meter:

1. Set your film speed on the meter. If shooting Tri-X, set ISO 400. If shooting Portra 160, set ISO 160.
2. Take a reading in the light where your subject is. For incident metering, hold the meter at the subject's position with the dome pointing toward the camera. For reflected metering, point the meter at the scene from the camera position.
3. Note the EV or the suggested aperture/shutter pair.
4. Choose your combination. If you want shallow depth of field, open the aperture and use a faster shutter. If you want maximum depth, stop down and slow the shutter. Ensure the shutter speed is fast enough for handheld shooting if you are not using a tripod.
5. Set the camera. On a TLR, the aperture and shutter speed rings are on the taking lens barrel. Set both to your chosen values.
6. Recheck if conditions change. If the light shifts — clouds cover the sun, you move from open shade into direct light — take a new reading. Do not assume the last reading still applies.

This workflow becomes second nature with practice. Experienced TLR photographers can meter, compose, focus, and shoot in under ten seconds. The external metering step adds perhaps three seconds to the process. In exchange, you develop a deep intuitive understanding of light that photographers relying entirely on automatic exposure often miss.

In the next lesson, we will turn from the physics of capturing light to the chemistry of recording it. We will examine how silver halide crystals in the film emulsion respond to photons, form a latent image, and give photography its name — literally, “writing with light.”