# Exposure and Metering

Photography I · Photographic Language and the Taking of the Image · https://tryals.app/learn/photography-i/exposure-and-metering

## Metering Fundamentals

A **reflected-light** meter measures light bouncing off the subject. It assumes every scene is a **mid grey**. Therefore, it turns white snow dark and black coal pale.

Conventionally, that target is an **18 per cent** reflectance card. Different camera makers calibrate slightly below this value, causing minor discrepancies.

An **incident-light** meter points from the subject toward the camera. It measures light falling onto the scene. Because subject reflectance is ignored, tones render accurately.

| Method | Measures | Fooled by |
|---|---|---|
| Reflected, averaging | Whole frame | Scenes far from average reflectance |
| Reflected, centre-weighted | Mostly the centre | Backlight or bright sky |
| Reflected, spot | One small area | User error in target choice |
| Incident | Light falling on subject | Meter lit differently from subject |

> **Common pitfall:** assuming a spot meter is automatically more accurate. It only tells you how to make a tiny patch mid grey.

## Exposure Rules and Flash

Exposure Value (EV) combines settings into a single figure at ISO 100:

* **Bright sun:** EV 15
* **Heavy overcast:** EV 12
* **Lit street at night:** EV 8
* **Living room:** EV 6

**Sunny 16** provides EV 15 directly. In bright sun, set f/16 and match shutter speed to ISO:
* ISO 100: f/16 at 1/125
* ISO 200: f/16 at 1/250

### Flash Calculations

A flash's **guide number** equals distance multiplied by the correct f-number.

* A guide number of 32 at ISO 100 gives f/8 at 4 metres ($32 / 4$).

This relies on the **inverse-square law**. Light spreads over a sphere, so intensity falls with the square of distance. Doubling distance cuts the light to one quarter, losing two stops.

## Practice questions

8 of this lesson's 14 practice questions, with answers. The full set is in the app.

### 1. Why does a reflected-light meter under-expose a snow field? Select every statement that is part of the reason.

A. The meter cannot distinguish a bright subject from a brightly lit one
B. The meter reports the exposure that renders whatever it sees as a mid grey
C. Snow reflects far more than mid grey, so the meter calls for less exposure than the snow needs
D. Snow reflects light unevenly and confuses the meter cell

**Answer:** A. The meter cannot distinguish a bright subject from a brightly lit one; B. The meter reports the exposure that renders whatever it sees as a mid grey; C. Snow reflects far more than mid grey, so the meter calls for less exposure than the snow needs

**Why:** The fourth is a red herring: the cell is not confused, it is working perfectly. It has been asked a question it cannot answer, is this a bright thing or a well-lit thing?, and answers the only way it can, by assuming mid grey. Correcting it means opening up about two stops.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/why-does-a-reflected-light-meter-under-expose-a-snow-field-select

### 2. A reflected-light meter is unaffected by how reflective the subject is.

**Answer:** False

**Why:** **False**: that describes the *incident* meter, which reads the light falling on the subject through a diffusing dome. A reflected reading confounds illumination with reflectance, which is exactly why it renders snow grey and coal grey and can tell you nothing about which it was looking at.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/a-reflected-light-meter-is-unaffected-by-how-reflective-the-subject

### 3. Complete the vocabulary of exposure measurement.

**Answer:** A standard grey card reflects **eighteen** per cent of the light falling on it. The metering method that is indifferent to the subject's reflectance is **incident** metering. A flash's **guide** number is the product of the distance and the correct f-number at a stated film speed. Illuminance falls as one over the distance **squared**, which is the inverse-square law.

**Why:** The four terms cover both halves of the lesson: what the meter assumes, the method that avoids the assumption, the shorthand for flash output, and the geometry that makes distance matter so much more than intuition suggests.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/complete-the-vocabulary-of-exposure-measurement

### 4. Arrange these metering methods from the widest area measured to the narrowest.

**Answer:**

1. Averaging over the whole frame
2. Centre-weighted
3. Partial, covering the central few per cent
4. Spot, about one degree

**Why:** Narrowing the measured area does not make the reading more correct; it makes it more specific. A spot reading answers a precise question about one patch, and the photographer still has to decide what that patch should look like in the print, which is the subject of the zone system in the second unit.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/arrange-these-metering-methods-from-the-widest-area-measured-to-the

### 5. A flash is moved from 2 metres to 4 metres from its subject. How many stops of light are lost?

**Answer:** 2 (within ±0.4)

**Why:** **Two stops.** Doubling the distance quarters the illuminance, because the same light is spread over four times the area. This is why moving a light a little in a small studio changes the exposure so much, and why a flash on a camera is so unforgiving of subjects at different distances.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/a-flash-is-moved-from-2-metres-to-4-metres-from-its-subject-how-many

### 6. Sunny 16 at ISO 100 gives f/16 at 1/125. Loading ISO 200 film instead, what shutter denominator does the rule give at the same aperture?

**Answer:** 250

**Why:** **250.** Doubling the sensitivity means half the light is needed, so the shutter halves the time: 1/125 becomes 1/250. The rule is usually stated as one over the film speed, which gives 1/200, the nearest marked speed is 1/250, and a third of a stop is well inside negative film's tolerance.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/sunny-16-at-iso-100-gives-f-16-at-1-125-loading-iso-200-film

### 7. An incident-light meter measures illumination falling onto a subject rather than light bouncing off it. What practical vulnerability arises from this design principle during a shoot?

A. It fails if the meter dome receives light different from the subject
B. It miscalculates exposure whenever the subject has high reflectance
C. It underexposes subjects that occupy only a small part of the frame
D. It requires manual compensation for dark tones within the main scene

**Answer:** A. It fails if the meter dome receives light different from the subject

**Why:** Incident metering separates subject luminance from illuminance, making surface reflectance irrelevant. The vulnerability is entirely spatial: if cloud or a flag shadows the meter position while the subject remains in direct sun, the exposure calculation breaks down.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/an-incident-light-meter-measures-illumination-falling-onto-a-subject

### 8. Sort each scene by what a straight reflected-light reading does to it.

**Answer:**

- Comes out too dark: Full-frame snow field, Bride in white, pale wall
- Comes out too light: Black cat on black sofa, Close-up of a coal heap
- Comes out about right: Flat-lit street scene, Grey card in subject light

**Why:** The first column needs opening up and the second needs closing down, which is the reverse of most people's instinct. The rule that survives is: give more exposure to light subjects and less to dark ones, exactly opposite to what the meter says, because the meter is arguing you back towards grey.

Page: https://tryals.app/practice/photography-i/exposure-and-metering/sort-each-scene-by-what-a-straight-reflected-light-reading-does-to-it