zoom lenses (original) (raw)

Definition: lenses with variable focal length and the ability to keep focus while zooming

Alternative terms: zoom objectives, parfocal lenses

Category: article belongs to category general optics general optics

Related: focal lengthlensesoptical aberrations

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DOI: 10.61835/kfe Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

Key questions:

What is a Zoom Lens?

tele objective

Figure 1: A tele zoom objective for an SLM camera.

A zoom lens is an optical lens with a variable focal length that keeps the focus (image sharpness) during focal length changes, without requiring additional adjustment. This ability is known as parfocal behavior.

A zoom lens is almost always a multiple-element lens, meaning it comprises several lenses, some of which must move to adjust the zoom. This makes the term “zoom lens” somewhat misleading. Most zoom lenses also function as zoom objectives — they serve as objectives, gathering light from a subject and forming the initial image.

A broader category is that of varifocal lenses, which have variable focal lengths but typically do not maintain focus. As a result, the image’s longitudinal position shifts during zooming, and sharpness is lost unless focus is manually or automatically corrected. Despite this distinction, the term zoom lens is sometimes used loosely to refer to simpler varifocal designs as well.

A zoom range is often expressed as a ratio between the maximum and minimum focal lengths. For example, a 1:4 zoom lens has a maximum focal length 4 times greater than the minimum. While some lenses exceed a 1:10 zoom ratio (called superzooms), they often sacrifice image quality, compactness, and manufacturing simplicity for flexibility.

In photography, zoom objectives are categorized by focal length ranges:

Switching between these specialized lenses can deliver significantly better image quality than a single superzoom lens, though it may be less convenient and overall more costly.

Applications of Zoom Lenses

Zoom lenses are primarily used in imaging applications where varying the focal length is necessary, which directly affects the field of view and magnification. Zooming in means increasing the focal length, which narrows the field of view (for a given image sensor size) and enlarges the captured image.

Typical imaging applications include photo cameras, video cameras, binoculars, and telescopes. A notable non-imaging application is in variable beam expander systems, where the beam diameter (often of a laser beam) can be varied.

Maintaining focus while zooming — known as parfocal performance — is especially important in video cameras, where manual refocusing during continuous recording is impractical. In contrast, for still-image photo cameras, parfocal behavior is less critical, especially if the camera includes a fast and reliable autofocus system.

Technical Realization of Zoom Lenses

Multiple Lens Systems

Most zoom lenses consist of multiple lenses, some of which move during zooming while others remain fixed. In simpler designs, a group of elements may move together. More complex systems involve moving multiple lens elements at different rates and possibly even along nonlinear paths.

One common design is an adjustable afocal zoom group (typically two movable lenses and one fixed lens), followed by a separate fixed focusing lens. More advanced systems may involve additional optical groups and exhibit increased mechanical complexity.

Zoom mechanisms are often operated manually, usually via a rotating ring or lever. Motorized zoom lenses also exist, using actuators controlled electronically — for example, by computers in remote surveillance systems.

Tunable Lenses

Although it is theoretically possible to build a zoom system using a single tunable lens (with a variable focal length), such designs are rare due to significant limitations in image quality, light gathering capability, and zoom range.

Digital Zoom

Some basic smartphone cameras rely solely on digital zoom, which works by cropping to the central portion of the captured image. This reduces the effective pixel count, leading to noticeable quality loss. Even when interpolation techniques are used to restore the original pixel dimensions and image sharpening algorithms are applied, the result remains inferior to true optical zoom.

In some cases, a limited optical zoom is combined with digital zoom to extend the total zoom range, though image quality still degrades during the digitally magnified portion.

Another common approach in smartphones is to use multiple cameras with different fixed focal lengths or narrow zoom ranges. While switching between cameras with different magnifications does not constitute true zooming, it can deliver similar results.

Design Trade-offs of Zoom Lenses

Designing a true zoom lens with parfocal behavior presents significant challenges, particularly when high image quality is required. Various types of optical aberrations (such as chromatic aberrations, astigmatism, coma, field curvature, etc.) are difficult to fully eliminate even in fixed focal length systems. These issues become even more complex in variable focal length designs, especially when a wide zoom range is required.

To address this, one requires not only advanced optical design methods and computer software, but also needs to carefully balance requirements for optimum compromises. Performance aspects that are less perceptible in the intended use case may be sacrificed to improve more critical qualities. For instance, some amount of barrel and pincushion distortion may be tolerated if minimizing loss of image sharpness is a higher priority — particularly for video applications, where distortion is less disturbing than image blur.

In addition to optical quality, other design goals include minimizing size, weight, mechanical complexity and shock sensitivity, depending on the application. Despite such efforts, zoom objectives are inherently more complex, larger, and heavier than fixed focal length lenses — and still usually offer lower image quality.

For high-performance applications, such as professional TV broadcast cameras, zoom systems may contain several dozens of lens elements in multiple moving groups. These systems are large, intricate and expensive, but necessary for maintaining high image quality across a wide zoom range.

Suppliers

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Edmund Optics, supplier of zoom lenses

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zoom lenses

Explore Edmund Optics’ broad selection of high-performance zoom lenses designed for flexible magnification in demanding imaging applications. From motorized to manual models, these lenses support a wide range of machine vision, inspection, and research tasks. They are engineered to accommodate varying working distances and field-of-view requirements, enabling seamless integration into dynamic optical systems.

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