Exposure Basics
Photography Fundamentals 🏆
Course 1 · Chapter 1 · Exposure Basics: Aperture, Shutter Speed and ISO
What Exposure Is
Every photograph is a measured amount of light. Wikipedia's Exposure article defines exposure as the amount of light per unit area reaching a frame of film or the surface of an electronic image sensor, and says it depends on the exposure time, the lens f-number and the luminance of the scene. This chapter is about those three controls and how to trade one against another without changing the brightness of the picture.
| Control | What it changes | Side effect |
|---|---|---|
| Aperture | Size of the lens opening | Opening it up increases exposure but decreases depth of field |
| Shutter speed | How long the sensor is exposed | A slower shutter increases exposure but also increases motion blur |
| ISO | Sensitivity setting (film speed on film) | A higher ISO needs less exposure for a readable image; noise is covered later in the course (check) |
Stops: Doubling and Halving
Photographers measure changes in light in stops. One stop more is twice as much light; one stop less is half as much. The article describes a "+1" exposure compensation setting as one stop more, meaning twice as much exposure.
Aperture and the f-number
The f-number N is the focal length divided by the diameter of the entrance pupil: a 100 mm lens with a 50 mm pupil is f/2. A larger f-number means a smaller opening and less light. The usual full-stop series is:
Each step multiplies the number by roughly the square root of 2 (about 1.414). That shrinks the opening diameter by about 0.7071 and, because area goes with the square of diameter, halves the area and so the light. The exact values are powers of the square root of 2; the printed numbers are rounded (the exact value near 5.66 is written 5.6, and near 22.6 is written 22).
Shutter Speed
The standard series from the Shutter speed article is 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2 and 1 second. Each step is a 2:1 ratio, so it changes the light by one stop. Slower speeds record more movement as blur. A hand-holding rule of thumb in the article: the slowest speed you can usually manage without noticeable shake is the one numerically closest to the lens focal length, so about 1/60 s for a 50 mm lens and no slower than 1/200 s for a 200 mm lens. Demanding enlargements may need faster speeds.
Trading One for Another: Exposure Value
Exposure Value (EV) puts aperture and shutter time on one scale: EV = log2(N squared / t), where N is the f-number and t is the time in seconds. EV 0 is 1 second at f/1.0. Different pairs with the same EV give the same exposure at a given ISO. The tables in the Exposure value article assume ISO 100; for another ISO S, EV(S) = EV(100) + log2(S/100), so ISO 400 adds 2.
The script below computes EV, and I ran it to get the output shown.
Typical EV values at ISO 100
| Scene | EV |
|---|---|
| Light sand or snow in full sunlight | 16 |
| Typical scene, full or slightly hazy sunlight | 15 |
| Typical scene, cloudy bright (no shadows) | 13 |
| Typical scene, heavy overcast | 12 |
| Bright street scenes | 8 |
| Home interiors | 5 to 7 |
The Sunny 16 Rule
The Exposure article gives a quick no-meter starting point: on a sunny day at f/16, use a shutter speed of one over the film speed (or the closest equivalent), so ISO 100 film gets 1/100 s. Checking against the script: f/16 at 1/100 s is EV 14.64, which sits between the table's 15 for sunlight and 13 for cloudy bright, so the rule is a sensible starting point rather than an exact match (my own observation; check).
Not Yet Verified, and Sources
- Sources. Wikipedia articles Exposure (photography), Exposure value, F-number and Shutter speed. The F-number page text was read to about 100,000 of 106,003 characters; the last 6,000 were not read.
- Not in those pages. The phrase "exposure triangle"; the effect of ISO on noise; image stabilisation (all marked check).
- Not tested. I did not test the focal-length rule with real lenses.
Exercises
Starting from f/2.8, list the next four full stops to a smaller opening. For each step say what happens to the light, and how many stops separate f/2.8 from f/11.
A correct exposure is f/8 at 1/125 s. You want a wider opening of f/4 to blur the background. Which shutter speed from the standard series keeps the exposure the same? Then use the EV script to confirm both pairs give nearly the same value.
The article gives night sports as EV 9 at ISO 100. What is its EV at ISO 400 using EV(S) = EV(100) + log2(S/100)? And what is the longest shutter speed (nearest in the standard series) you would pick for a handheld 100 mm lens, using the focal-length rule of thumb?
Quick Reference
- Exposure: light per unit area reaching the sensor
- One stop: double or half the light
- f-numbers: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22
- Shutter: 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1 s
- EV: log2(N squared / t); EV 0 = 1 s at f/1.0
- Sunny 16: f/16 and 1/ISO seconds