“A good photograph is knowing where to stand. A great photograph is knowing how to shape the light.” – Annie Leibovitz
In the cover photograph, the subject is a young Dutch girl. She is in focus. The windmills in the background are out of focus. This results in a dreamy bokeh diffusion of the background resulting from a wide open aperture. Using the aperture in this manner brings an intended isolation to the subject. It is a creative choice. If a small aperture was selected, the windmills could be in sharp focus too. It makes the image too busy. The viewer does not know where to look. By using the wide open aperture, the viewer is led to see the girl first. This also shapes the light and shifts the context to the girl’s eyes.
Understanding the Aperture in Photography
In photography, aperture refers to the opening within a camera lens that controls how much light enters and reaches the sensor. It is measured using the f-number or f-stop, which is technically the ratio of the lens’s focal length (F) to the diameter of the entrance pupil (D), represented as f = F/D. For instance, a 50 mm lens with a 25 mm aperture opening has an f-number of f/2.0. This relationship is inverse, meaning that a lower f-number (e.g., f/1.8) corresponds to a larger aperture and allows more light into the camera, while a higher f-number (e.g., f/16) means a smaller aperture and less light.
The aperture is one of the three pillars of exposure, along with shutter speed and ISO, and plays a critical role not only in light regulation but also in visual aesthetics. One of the most significant creative effects influenced by aperture is depth of field, the range of distance in a scene that appears acceptably sharp. A wide aperture (low f-number) produces a shallow depth of field, effectively isolating a subject from the background and foreground by rendering them out of focus. This is especially useful in portrait photography where subject separation enhances visual impact. Conversely, a narrow aperture (high f-number) increases depth of field, making it ideal for landscape photography where both foreground and background need to be sharp.
The f-to-d Ratio: A Technical Breakdown of Aperture in Photography
In optics and photography, the f-number, often written as f/N, is a dimensionless number that quantifies the aperture setting of a lens. It is calculated using the focal length (F) of the lens divided by the diameter (D) of the entrance pupil (the effective aperture):
f/N = F / D
Where:
- F = focal length of the lens (in millimetres)
- D = diameter of the entrance pupil (in millimetres)
This equation shows that the f-number is a ratio, not an absolute measurement. It describes how “open” the lens is relative to its focal length. The smaller the f-number, the larger the aperture opening; the larger the f-number, the smaller the aperture.
Example Calculation
Consider a 100 mm focal length lens:
- If the aperture diameter is 25 mm, then:
f-number = 100 mm / 25 mm = f/4 - If the aperture diameter is reduced to 12.5 mm, then:
f-number = 100 mm / 12.5 mm = f/8
Why the f-to-d Ratio Matters
- Exposure Control
The f-number directly affects exposure, the amount of light that reaches the sensor. A larger aperture (smaller f-number) lets in more light and results in a brighter image, which is crucial in low-light conditions. - Optical Consistency Across Lenses
The f-to-d ratio provides a standardized way to compare exposure across lenses of different focal lengths. For example, f/2.8 on a 35 mm lens and f/2.8 on a 200 mm lens both admit the same light intensity per unit area on the sensor, even though the physical aperture diameters differ. - Depth of Field and Aesthetic Control
Because the f-number is tied to aperture size, it also governs depth of field. A wide aperture (e.g., f/1.4) creates a shallow depth of field, isolating subjects beautifully with background blur (bokeh). A narrow aperture (e.g., f/16) increases depth of field, keeping more of the scene in focus. - Lens Design Constraints
Lenses with lower f-numbers (e.g., f/1.2 or f/1.4) are more complex and costly to manufacture because they require larger glass elements to maintain a wide entrance pupil relative to focal length. These are often referred to as “fast lenses” because they allow faster shutter speeds at lower light.
Visual Analogy
Think of focal length as the length of a pipe and the aperture as the diameter of the pipe’s opening. The wider the pipe’s diameter relative to its length, the more water (light) can flow through. The f-to-d ratio helps determine how much water flows relative to the pipe’s length.
Conclusion
Understanding the f-to-d ratio gives photographers a fundamental grasp of how lenses gather light and shape images—both technically and artistically. Whether balancing exposure, controlling focus range, or choosing the right lens for a specific application, the f-number is a critical concept rooted in this simple yet powerful ratio.
From a technical standpoint, aperture also affects lens performance and diffraction. While lenses tend to be sharpest a few stops down from their maximum aperture (often around f/5.6 to f/8 for many lenses), going too narrow (e.g., f/22) can introduce diffraction, reducing overall image sharpness. Additionally, aperture size directly affects exposure values, where each full stop change either halves or doubles the light entering the lens.
Understanding the aperture and its f-to-d ratio is fundamental for controlling both technical exposure and creative expression in photography. Mastery of aperture settings empowers photographers to manipulate light, sharpness, and mood within their images.
About the Author:
Michael Martin is the Vice President of Technology with Metercor Inc., a Smart Meter, IoT, and Smart City systems integrator based in Canada. He has more than 40 years of experience in systems design for applications that use broadband networks, optical fibre, wireless, and digital communications technologies. He is a business and technology consultant. He was a senior executive consultant for 15 years with IBM, where he worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He is a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).
Martin served on the Board of Directors for TeraGo Inc (TGO: TSX) and on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). He has served as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology. He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) [now Ontario Tech University] and on the Board of Advisers of five different Colleges in Ontario – Centennial College, Humber College, George Brown College, Durham College, Ryerson Polytechnic University [now Toronto Metropolitan University]. For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section.
He holds three master’s degrees, in business (MBA), communication (MA), and education (MEd). As well, he has three undergraduate diplomas and seven certifications in business, computer programming, internetworking, project management, media, photography, and communication technology. He has completed over 50 next generation MOOC (Massive Open Online Courses) continuous education in a wide variety of topics, including: Economics, Python Programming, Internet of Things, Cloud, Artificial Intelligence and Cognitive systems, Blockchain, Agile, Big Data, Design Thinking, Security, Indigenous Canada awareness, and more.