Introduction — Three Essential Terms for Understanding Photography
Anyone starting out with an interchangeable-lens camera will encounter a large number of technical terms. With so much unfamiliar terminology, it can be difficult to know where to begin.
This article focuses on just three fundamental concepts:
- Aperture
- Shutter speed
- ISO sensitivity
Photography begins with capturing light and recording it as an image.
These three concepts provide the foundation for understanding how a camera receives light and produces a photograph.
Each has a distinct role. Understanding those roles makes the rest of photographic terminology much easier to approach.
Let’s examine them one at a time.
1. What Is Aperture?
Aperture is the mechanism that controls how much light passes through a lens.
Inside a camera lens is a mechanism called the diaphragm, which adjusts the size of the opening through which light travels.
When this opening becomes larger, more light passes through per unit of time. When it becomes smaller, less light passes through.
- Opening the aperture increases the amount of light passing through the lens per unit of time.
- Closing down the aperture reduces the amount of light passing through the lens per unit of time.
Aperture is therefore one of the fundamental controls affecting the amount of light recorded in a photograph.
Understanding the f-Number
The aperture setting is expressed as an f-number, commonly written as f/2.8, f/4, f/5.6, and so on.
One aspect of f-numbers often confuses beginners.
- A smaller f-number means a larger aperture opening and more light.
- A larger f-number means a smaller aperture opening and less light.
For example, with the same lens, f/2.8 allows more light through than f/4.
The numerical relationship may initially seem counterintuitive.
An f-number is defined by the relationship between the lens’s focal length and the apparent diameter of its entrance pupil.
As the aperture opening becomes larger relative to the focal length, the f-number becomes smaller.
For now, remembering that a smaller f-number corresponds to a larger aperture opening is enough.
The mathematical reason for this relationship, including why light intensity varies with the inverse square of the f-number, is explained in a separate article.
2. What Is Shutter Speed?
Shutter speed describes the length of time during which film or an image sensor collects light for an exposure.
It is also commonly referred to as exposure time.
The amount of light recorded depends partly on how long that light is allowed to reach the recording medium.
- A faster shutter speed means a shorter exposure time.
- A slower shutter speed means a longer exposure time.
Typical settings include:
- 1/4000 second
- 1/1000 second
- 1/250 second
- 1/60 second
An exposure of 1/4000 second is much shorter than an exposure of 1/250 second.
Under otherwise identical conditions, a shorter exposure time results in less recorded light.
This reveals an important distinction between aperture and shutter speed.
Aperture controls the amount of light passing through the lens per unit of time, while shutter speed controls how long that light is collected.
Together, they determine the amount of light delivered to the recording medium.
3. What Is ISO Sensitivity?
ISO sensitivity indicates the relationship between the amount of light needed for an exposure and the resulting photographic response or image brightness.
In practical terms:
- A higher ISO allows a photograph of a given brightness to be produced with less exposure.
- A lower ISO generally requires more exposure to produce a photograph of the same brightness.
For example, ISO 400 is more sensitive in photographic terms than ISO 100.
The underlying mechanism differs between film and digital cameras.
ISO in Film Photography
In film photography, ISO describes a characteristic of the light-sensitive material itself.
A higher-speed film generally requires less exposure to produce a usable photographic result.
The sensitivity is determined by the film stock.
Changing film sensitivity normally means loading a different type of film.
ISO in Digital Photography
In a digital camera, the image sensor converts incoming light into electrical signals.
Those signals undergo amplification and image processing to produce the recorded photograph.
The ISO setting relates the exposure received by the sensor to the camera’s signal levels and image output.
Modern digital cameras allow ISO to be adjusted from one photograph to the next.
Although film and digital cameras achieve sensitivity in different ways, they share the same practical relationship:
A higher ISO setting allows a specified image brightness to be achieved with less exposure.
4. Summary — Three Terms, Three Different Roles
The three concepts discussed in this article describe different aspects of the photographic process.
| Term | Fundamental role |
|---|---|
| Aperture | Controls the amount of light passing through the lens per unit of time |
| Shutter speed | Controls the time during which light is collected |
| ISO sensitivity | Relates the amount of exposure to the photographic response or resulting image brightness |
Together, these concepts provide the foundation for understanding how light becomes a photograph.
The next step is to understand how they relate to one another.
In particular, aperture and shutter speed jointly determine the amount of light received during an exposure.
That relationship can be expressed mathematically, making the underlying structure of photography much clearer.
Related Articles — Take Your Understanding Further
1. How Aperture, Shutter Speed, and ISO Work Together
Why does exposure depend on the inverse square of the f-number? How can different aperture and shutter-speed combinations produce the same exposure?
The following article explains these relationships step by step.
▶ Understanding Exposure ★ How Aperture, Shutter Speed, and ISO Work Together
2. Understanding Camera and Lens Specifications
Sensor size, megapixels, focal length, and maximum aperture determine important characteristics of cameras and lenses.
Learn what these specifications mean and how to interpret them.
▶ Camera Basics for Beginners ★ Sensor Size, Megapixels, Focal Length, and Maximum Aperture
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