Lens Focal Length Demystified (Visual Examples)
Discovering the perfect shot in photography often seems like a puzzle.
One crucial piece in understanding that puzzle is the concept of lens focal length.
In this article, we aim to demystify the topic of lens focal length by providing some visual examples to help clarify this seemingly complex concept.
Focal length, measured in millimeters, defines the distance between the optical center of the lens and the camera sensor or film plane when focused at infinity.
It greatly impacts the magnification and field of view of a given lens.
Different focal lengths are suitable for varied photography styles and subjects, from wide-angle landscapes to close-up portraits.
Exploring lens focal length and its effects on your photos will heighten your photography skills, enabling you to capture truly stunning images.
To help you grasp this concept even better, we will dive into some visual examples that illustrate how various focal lengths affect your images.
Understanding Focal Length
Basic Concepts of Focal Length
Focal length is a fundamental characteristic of lenses in photography.
It is typically represented in millimeters (mm) and describes the optical distance from the point where light rays converge to form a sharp image of an object to the camera’s digital sensor or 35mm film at the focal plane.
In simpler terms, it determines the magnification and field of view of a lens.
There are two main types of lenses, based on their focal length:
- Prime lenses: These lenses have a fixed focal length, e.g., 50mm or 90mm. They offer excellent image quality and are generally preferred for their faster maximum apertures, enabling better low-light performance.
- Zoom lenses: These lenses have variable focal lengths, e.g., 18-55mm. They provide the flexibility to cover a range of focal lengths within a single lens, allowing you to quickly switch between wide-angle, standard, and telephoto perspectives.
Focal Length in Relation to Lenses
Focal length impacts several aspects of photography, including:
- Magnification: Longer focal lengths yield greater magnification or a more “zoomed-in” appearance. For instance, a 200mm lens will bring the subject closer than a 35mm lens.
- Angle of view: A lens with a longer focal length will have a narrower angle of view, capturing less of the scene, while a shorter focal length will give you a wider angle of view, encompassing more of the surroundings.
- Perspective and distortion: Shorter focal lengths can potentially create more noticeable distortion, such as stretching objects near the edges of the frame. Longer lenses tend to compress the distance between objects, providing a more natural perspective.
| Focal Length | Magnification | Angle of View | Distortion |
|---|---|---|---|
| Short (Wide-Angle) | Low | Wide | High |
| Long (Telephoto) | High | Narrow | Low |
Types of Lenses
There are several types of camera lenses, each with its own unique characteristics and functions.
These three main lens types are wide-angle lenses, standard lenses, and telephoto lenses.
Wide-Angle Lenses
Wide-angle lenses have a focal length between 14mm and 35mm, providing a wider field of vision than standard lenses.
They offer a broader scope from side to side, with a more panoramic perspective, similar to widescreen cinematography.
Some common features of wide-angle lenses include:
- Large angle of view, ideal for landscapes and expansive scenes
- Concave lens shape, causing light rays to spread out
- Shorter focal point, resulting in a greater depth of field
Wide-angle lenses can be further categorized as:
- Standard wide-angle: Typically 28mm to 35mm
- Super wide-angle: Between 14mm and 28mm, offering an even larger angle of view
Standard Lenses
Standard lenses have a focal length similar to that of the human eye, typically ranging from 35mm to 70mm.
This lens type closely resembles the natural perspective we experience, making them versatile and suitable for various photography genres.
Characteristics of standard lenses include:
- Moderate angle of view, conveying a natural perspective
- Convex lens shape, allowing light rays to meet at a single focal point
- Balanced depth of field, capable of creating slight background blur
Telephoto Lenses
Telephoto lenses possess a longer focal length than standard lenses, typically anywhere from 70mm to 300mm or even more.
This lens type is designed for capturing subjects at a distance, making telephoto lenses perfect for wildlife, sports, and portrait photography.
Some key features of telephoto lenses include:
- Narrow angle of view, allowing you to capture distant subjects
- Convex lens shape, similar to standard lenses
- Longer focal point, resulting in a shallower depth of field for background blur
Telephoto lenses can be subdivided into:
- Short telephoto: Ranging from 70mm to 200mm
- Long telephoto: Ranging from 200mm to 300mm, or even more for extreme zoom capabilities
Each lens type caters to specific photographic needs and scenarios, allowing photographers to achieve a variety of effects and perspectives.
Factors Affecting Focal Length and Image Quality
Objective and Ocular Lenses
Objective lenses are the primary lenses in a camera or optical system, which work to gather light and project an image.
Ocular lenses, also known as eyepieces, are used mainly in telescopes and microscopes.
They magnify the image produced by the objective lenses.
The interplay between objective and ocular lenses contributes to the focal length and image quality.
- Objective lenses have a significant impact on focal length because their physical properties, such as diameter and curvature, determine the angle of view.
- Ocular lenses can have an effect on focal length when used in conjunction with objective lenses, for example in a telescope; they change how much magnification you achieve.
Lens Power and Millimeters
Lens power, typically measured in diopters, describes the ability of a lens to converge or diverge light, influencing overall image quality.
The focal length, expressed in millimeters, is an essential factor of lenses that directly affects image quality and determines how wide or tight the angle of view will be.
| Focal Length | Type of Lens |
|---|---|
| 14-35mm | Ultra-Wide Angle Lens |
| 35-70mm | Wide Angle to Standard/Normal Lens |
| 70-200mm | Telephoto Lens (including short and medium telephoto) |
| 200mm+ | Super Telephoto Lens |
- Prime lenses have a fixed focal length, which means they have only one focal length to work with. They are known for providing better image quality because of their simple, optimized design.
- Variable focal length or zoom lenses offer a range of focal lengths, which provide increased flexibility in framing and composition, but may suffer from slight image quality loss due to more complex designs.
A few considerations:
- Shake: Camera shake or vibrations can impact image quality at longer focal lengths, making image stabilization feature in lenses or cameras essential to maintain sharpness.
- Depth of field: The depth of field changes with the focal length; wider angles have a larger depth of field, while telephoto lenses have a shallower depth of field, making them great for isolating subjects from backgrounds.
Creative Applications of Focal Length
Shallow Depth of Field and Portraiture
Shallow depth of field is an effect in photography where only a small portion of the image is in focus while the rest is blurred.
This effect is often used in portraiture to emphasize the subject and separate them from the background. To achieve this:
- Use a lens with a longer focal length (e.g., a telephoto lens) as it can produce more prominent depth-of-field effects.
- Shoot with a wide aperture setting (lower f-number) to let more light into the camera and create a shallow depth of field.
For example, a 70-200mm telephoto lens used at f/2.8 aperture can produce stunning portraits with a beautifully blurred background.
Wide-Angle and Telephoto Effects
Different lenses produce different effects and give photographers creative options for composition. We’ll explore wide-angle and telephoto lenses below:
- Wide-Angle Lenses: These lenses have shorter focal lengths (e.g., 15mm on full-frame cameras, 10mm on APS-C, or 7.5mm on MFT cameras). Wide-angle lenses are useful for capturing expansive landscapes or taking group photos in tight spaces. The larger field of view presents a unique perspective that adds depth and intensity to your images. However, keep in mind that wide-angle lenses can cause distortion, especially near the edges of the frame. This effect can be used creatively or corrected in post-processing if needed.
- Telephoto Lenses: These lenses have longer focal lengths (e.g., 70-200mm) and allow photographers to capture subjects from a distance. In addition to the previously mentioned shallow depth of field, telephoto lenses can compress the perceived distance between objects in the frame, creating a flattering effect in portraiture. They are also ideal for sports, wildlife, and event photography where it’s important to get close to the action without physically being there. On the downside, telephoto lenses can be larger and heavier, which may impact the portability and handling of your camera setup.
Optical Terminology and Concepts
Convex Lenses and Index of Refraction
Convex lenses are a type of lens that bulges outward, away from the center.
These lenses are thicker in the middle and thinner at the edges, allowing them to focus incoming light rays onto a single point.
They are commonly used in cameras, magnifying glasses, and eyeglasses for farsightedness.
The index of refraction is a measure of how much a lens can bend light.
It’s a dimensionless quantity that represents the speed of light in the lens material compared to the speed of light in a vacuum.
A higher index of refraction means that the lens will have a greater ability to bend and focus light, resulting in a shorter focal length.
Parallel Rays and Millimeter Scale
When parallel rays of light pass through a convex lens, they converge at a single point called the focal point.
The distance from the center of the lens to the focal point is the focal length, usually expressed in millimeters (mm).
Focal length determines the lens’s magnification and field of view, with shorter focal lengths providing wider fields of view and longer focal lengths offering greater magnification.
Lens focal length can fall into two general categories:
- Prime lenses: These lenses have a fixed focal length and typically offer better image quality and faster apertures compared to zoom lenses. Examples include 35mm, 50mm, and 85mm lenses.
- Zoom lenses: These lenses have a variable focal length, allowing you to change the magnification and field of view without swapping lenses. Examples include 24-70mm and 70-200mm lenses.
| Focal Length | Prime or Zoom | Field of View |
|---|---|---|
| 35mm | Prime | Wide |
| 50mm | Prime | Standard |
| 85mm | Prime | Telephoto |
| 24-70mm | Zoom | Wide-Standard |
| 70-200mm | Zoom | Telephoto |
Understanding these basic optical concepts and terminology helps you better assess the capabilities and characteristics of various lenses for different photographic needs.
