# Aperture, Shutter and Sensitivity

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

## The Stop

A **stop** is a factor of two in the light. Every camera control shares this currency.

| Control | Full-stop series | What else it decides |
|---|---|---|
| **Aperture** | f/1.4 · f/2 · f/2.8 · f/4 · f/5.6 · f/8 · f/11 · f/16 · f/22 | Depth of field |
| **Shutter** | 1/15 · 1/30 · 1/60 · 1/125 · 1/250 · 1/500 · 1/1000 | How movement records |
| **Sensitivity** | ISO 50 · 100 · 200 · 400 · 800 · 1600 · 3200 | Grain or noise |

### The f-number scale

The f-number is focal length divided by entrance pupil diameter. On a 50 mm lens, f/8 gives a 6.25 mm pupil.

Light passes through an **area**. Area scales with the square of diameter. To halve the area, divide the diameter by 1.414. This produces the progression: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Because the f-number is a divisor, a **larger** number means a **smaller** opening.

The shutter and sensitivity series work similarly. Shutter steps roughly halve time: 15, 30, 60, 125, 250, 500. Sensitivity doubles at each step: ISO 50, 100, 200.

### Equivalent exposures

Because all three controls use stops, an adjustment in one balances an adjustment in another.

Suppose the meter reads **f/5.6 at 1/60**. Changing to f/16 loses three stops (5.6 to 8, 11, 16). The shutter must compensate by three stops: 1/30, 1/15, **1/8**.

**Exposure is the thing that is conserved; the picture is the thing that changes.**

### Reciprocity

Exposure equals illuminance multiplied by time. In film, this **reciprocity law** holds across standard ranges but fails during very long or short exposures.

> **Common pitfall:** ISO does not add light. Aperture and shutter govern light reaching the sensor; sensitivity amplifies the signal, adding noise.

## Practice questions

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

### 1. A photographer opens the aperture by two stops and shortens the shutter by two stops. The negative density comes out unchanged. Why is the photograph nevertheless a different photograph?

A. Density is only nominally identical as rounding errors in f-numbers accumulate
B. A wider aperture alters lens perspective and spatial relationships of subjects
C. Exposure is conserved, but depth of field and motion blur change by two stops
D. Faster shutters lower recorded dynamic range and introduce reciprocity defects

**Answer:** C. Exposure is conserved, but depth of field and motion blur change by two stops

**Why:** Because each control carries a second job. Two stops wider throws the background further out of focus; two stops shorter freezes movement that was previously blurred. Exposure is what the trade preserves, and everything that makes the picture look the way it looks is what it changes.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/a-photographer-opens-the-aperture-by-two-stops-and-shortens-the

### 2. Complete the account of the exposure controls.

**Answer:** A stop is a factor of **two** in the light. The f-number is the focal length divided by the diameter of the entrance **pupil**, and the series climbs by the square root of two because light passes through an **area**. The failure of the reciprocity law at very long exposures is a property of the individual **emulsion** and is published for each one.

**Why:** The third blank is the one that makes the series memorable rather than arbitrary. Halving an area means dividing a diameter by the square root of two, so the f-numbers are just 1 multiplied by 1.414 again and again, rounded for printing on a barrel.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/complete-the-account-of-the-exposure-controls

### 3. A one-stop change means a factor of two in the light, whichever of the three controls is used to make it.

**Answer:** True

**Why:** **True**, and it is the reason the triangle works at all. A stop of aperture, a stop of shutter and a stop of sensitivity are the same size, so any one can pay for any other exactly. Without a shared unit there would be no such thing as an equivalent exposure.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/a-one-stop-change-means-a-factor-of-two-in-the-light-whichever-of

### 4. "Exposure is the thing that is conserved; the picture is the thing that changes." What does this trade-off mean when trading shutter duration for aperture size?

A. Image sharpness stays uniform whilst overall frame illumination is altered
B. Total sensor illuminance remains constant whilst signal amplification varies
C. Image brightness stays fixed whilst motion blur and depth of field alter
D. Total optical density is preserved whilst entrance pupil size remains fixed

**Answer:** C. Image brightness stays fixed whilst motion blur and depth of field alter

**Why:** Photometric exposure is balanced across stops, but secondary optical qualities differ: shutter alters motion recording and aperture alters depth of field. Sensitivity governs noise rather than aperture trade-offs, and pupil size directly changes with f-number.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/exposure-is-the-thing-that-is-conserved-the-picture-is-the-thing

### 5. How many whole stops separate f/2 from f/11?

**Answer:** 5

**Why:** **Five.** Each step multiplies the f-number by about 1.414 and halves the light: 2, 2.8, 4, 5.6, 8, 11. Five stops is a factor of 32 in the light, which is why a lens that opens to f/2 is worth so much more than its half-stop-larger price tag suggests.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/how-many-whole-stops-separate-f-2-from-f-11

### 6. Arrange these shutter speeds from the longest exposure to the shortest.

**Answer:**

1. 1/15
2. 1/30
3. 1/60
4. 1/125
5. 1/250
6. 1/500

**Why:** Each step is one stop, so the whole run from 1/15 to 1/500 is five stops, a factor of 32 in the time. The printed numbers are nominal: 1/15 is really about 1/16 and 1/125 about 1/128, and the powers of two are what the mechanism is actually doing.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/arrange-these-shutter-speeds-from-the-longest-exposure-to-the

### 7. By what factor does the light reaching the film fall when the aperture is closed from f/4 to f/8?

**Answer:** 4 (within ±0.5)

**Why:** By a factor of **four**. From f/4 to f/8 is two stops, and each stop halves the light, so a quarter arrives. The diameter has halved and the area, which is what actually admits light, has fallen to a quarter, the square relation that the whole f-number series is built on.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/by-what-factor-does-the-light-reaching-the-film-fall-when-the

### 8. Which are true of the reciprocity law and its failure?

A. Digital sensors show the same reciprocity failure as film
B. Exposure is illuminance multiplied by time, so halving one and doubling the other should be equivalent
C. The size of the departure differs from emulsion to emulsion and is published per film
D. Film departs from the law at very long and very short exposures

**Answer:** B. Exposure is illuminance multiplied by time, so halving one and doubling the other should be equivalent; C. The size of the departure differs from emulsion to emulsion and is published per film; D. Film departs from the law at very long and very short exposures

**Why:** Sensors do not lose reciprocity, they accumulate thermal noise over long exposures instead, which looks bad for an unrelated reason. The third statement is the practical one: there is no universal correction, only the manufacturer's figures for the emulsion in the camera.

Page: https://tryals.app/practice/photography-i/aperture-shutter-and-sensitivity/which-are-true-of-the-reciprocity-law-and-its-failure