Basics of Zoom Lenses (Variable Focal Length Lenses) and Aberration Correction

A zoom lens is an optical device equipped with a variable focal system that allows adjustment of the focal length without changing the shooting distance or working distance. By skillfully arranging multiple lens groups, it enables flexible changes in the field of view while accommodating a wide range of shooting conditions.

The variable focal capability makes it easier to cover various scenarios with a single lens, thereby improving shooting efficiency and productivity. This is especially advantageous in applications such as factory automation (FA) and inspection systems, where it reduces the need for lens changes and allows the field of view to be adjusted quickly.

This article provides a comprehensive explanation of zoom lenses, including their basic structure, aberration correction, types and selection criteria, and maintenance methods for extending their lifespan. By gaining a deeper understanding of the unique features of variable focal lenses, you can create an optimized imaging environment.

Basic Functions of Lenses and the Advantages of Variable Focal Length

In an optical system, a lens is a crucial component that accurately forms an image from light coming from a subject. A variable focal zoom lens offers the advantage of adjusting the field of view freely without changing the shooting distance.

In general, a lens functions by refracting light to form an image on a sensor or film. The mechanism that allows only the focal length to vary while keeping the shooting distance constant is achieved by moving multiple lens groups back and forth. This enables a single lens to cover a wide range of angles, from wide-angle to telephoto.

The advantage of a variable focal length is that it allows fine adjustments of the field of view according to the scene or subject. While prime lenses require lens changes or repositioning, a zoom lens allows focal length adjustments with ease. This is particularly beneficial in settings where reducing shooting time or saving space for equipment is important.

Additionally, in applications such as factory inspection lines or laser processing, the ability to instantly change the field of view according to the size or requirements of the subject without replacing the lens is highly valued. Because of its wide range of applications, a zoom lens offers flexibility that other lenses cannot, from product inspection to precision micro-processing.

Representative Aberrations in Zoom Lenses

While the variable focal mechanism of zoom lenses offers convenience, it also tends to produce multiple types of aberrations. Here, we provide an overview of the most representative ones.

Aberrations refer to phenomena in which an ideal point image does not converge perfectly to a single point, affecting image quality through issues such as color fringing, distortion at the edges, or overall image softness. Since zoom lenses generally have a relatively large number of lens elements, they are more prone to a wide variety of aberrations. In optical design, controlling these aberrations is a critical challenge.

In particular, high-magnification zoom lenses require different aberration corrections at the wide-angle and telephoto ends, so technologies that balance these corrections across the zoom range are essential. For example, in addition to digital correction, combining different types of glass materials is often used to reduce chromatic aberration. Maintaining practical performance necessitates a design that carefully balances the various aberrations while finding an acceptable compromise.

In recent years, advances in manufacturing techniques and the development of computer-based optical simulations have dramatically improved aberration correction technologies. Even so, some designs prioritize certain aberrations depending on the application, making it important to consider your shooting objectives in relation to the lens characteristics when selecting a zoom lens.

Spherical Aberration and Chromatic Aberration

Spherical aberration is a phenomenon in which light rays passing through the center of a lens and those passing through its periphery focus at different positions. Since zoom lenses typically combine many lens elements, the use of aspherical lenses or high-refractive-index glass has become increasingly common to reduce spherical aberration. Such designs help achieve sharp image quality from the center to the edges of the frame.

Chromatic aberration refers to color fringing caused by differences in refractive indices for different wavelengths of light. In particular, when the focal positions of the red, green, and blue (RGB) components do not coincide, the edges of the image may appear colored. Many zoom lenses employ ED (extra-low dispersion) lenses or fluorite elements to minimize chromatic aberration as much as possible.

Reducing these aberrations requires not only optimizing the number and arrangement of lens elements but also carefully selecting the characteristics of each glass material. As a result, high-performance zoom lenses tend to have more complex optical designs, optimized to meet the specific needs of photographers.

Coma, Astigmatism, and Distortion

Coma is an aberration in which point light sources near the edges of the frame appear to streak or trail. It can be particularly problematic in astrophotography, and in zoom lenses, the degree of coma may vary between the wide-angle and telephoto ends. To suppress it, the curvature and relative positions of lens groups must be designed with high precision.

Astigmatism is a phenomenon where the focal points in different directions—such as vertical and horizontal—do not coincide. This causes the image to appear blurred or smeared, which can be critical in precision photography or inspection applications. In zoom lenses, multiple lens groups are carefully moved in coordination to minimize astigmatism as much as possible.

Distortion is an aberration in which the shape of the subject is significantly altered, with barrel distortion typical at the wide-angle end and pincushion distortion at the telephoto end. In zoom lenses, the appearance of distortion varies depending on the shooting scenario, so optical design involves correcting it while balancing the effects across different zoom ranges.

Types of Zoom Lenses

Zoom lenses come in various types depending on their internal structure and intended applications. Choosing the right type for a given use can significantly enhance shooting efficiency and image quality.

The defining feature of zoom lenses is their ability to vary the focal length from wide-angle to telephoto, but there are several mechanisms for achieving this variability. In practical use, lenses are designed to meet different needs, such as enabling quick focusing or prioritizing image quality characteristics.

Depending on the manufacturer and model, the types of glass, coating technologies, and the design of movable elements differ, so it is important to understand these characteristics when selecting the optimal product. For example, some lenses include vibration-reduction mechanisms, while others are compactly designed for easier use on factory lines.

For industrial applications in particular, flexible switching of the field of view and reliable operation are essential. As a result, some models feature sealed internal structures or are built to be robust under harsh conditions, offering a wide range of options to meet various requirements.

Differences Between Varifocal Lenses and Standard Zoom Lenses

A varifocal lens is characterized by the tendency for the focus position to shift when the focal length is changed. Users need to readjust the focus each time, so these lenses are often used for observation or fixed-position shooting. In contrast, standard zoom lenses are designed so that the focus position does not shift significantly when the focal length is adjusted, making them easier to use for video recording or continuous observation.

One advantage of varifocal lenses is their relatively simple structure, which helps keep costs down. However, because they require manual focus adjustment, they are not always suitable for applications where the field of view changes frequently. On the other hand, they work well when a fixed field of view needs to be carefully set or when time can be spent precisely adjusting focus.

Standard zoom lenses are better suited for situations that require quick adjustments in the field, but their optical design is more complex, which tends to increase the cost. It is important to assess the specific needs of the shooting scenario and choose a lens by comparing factors such as cost and ease of use.

Characteristics of High-Magnification Zoom Lenses

High-magnification zoom lenses typically refer to products that cover a wide range with a single lens, such as from a wide-angle of around 18mm to a telephoto exceeding 300mm. They are convenient for a variety of situations, including travel and event photography, as well as industrial applications. However, it is also true that aberration characteristics and resolution can vary between the wide-angle and telephoto ends, making correction more challenging.

To address this, high-magnification zoom lenses often incorporate specialized lens materials and advanced coating technologies to minimize aberrations as much as possible. Many also include mechanisms to compensate for camera shake at the telephoto end, enhancing practical usability.

In exchange for this high convenience, such lenses tend to be larger and heavier. The choice of model depends on whether portability or higher magnification and image quality are prioritized, so careful consideration of the intended use environment is essential to avoid mistakes in selection.

Key Points for Choosing a Zoom Lens

When choosing a zoom lens, it is important to check several key points according to the shooting conditions and the desired image quality.

With the growing variety of variable focal lenses, selecting a model that suits your specific needs requires organizing the factors to focus on. For example, it is essential to clarify in advance the size and distance of the subject, the desired field of view, and the required brightness.

In precision inspection applications such as factory automation (FA), details at the micron level may be required, so it is important to carefully check specifications such as resolution and depth of field. Additionally, environmental factors, such as the need for continuous operation on a high-volume production line or limited equipment space, should also be taken into account.

By considering the actual operating environment, reviewing the lens’s basic performance and correction features, and gathering additional information if necessary, the risk of making a wrong choice can be minimized. Selecting a zoom lens that matches your purpose can help achieve highly cost-effective operation.

Basics of Image Size, Focal Length, and F-Number

Image size refers to the imaging area determined by the size of the sensor or film. When selecting a zoom lens, it is important to consider the corresponding image circle, as the field of view can vary significantly depending on the combination with the camera or imaging sensor being used.

Focal length is the factor that determines the lens’s field of view, so it is essential to choose an appropriate range according to the subject, whether wide-angle, standard, or telephoto. The f-number (F-stop) indicates the lens’s brightness and depth of field: a smaller f-number provides a brighter image with shallower depth of field, while a larger f-number yields greater depth of field. This allows selection based on whether you prioritize background blur or require high depth of field for inspection purposes.

In particular, for inspection equipment used in factory environments, lighting conditions are often limited, making bright lenses necessary. It is crucial to check both the focal length and f-number and consider the combination that best suits your shooting environment and application.

Resolution and Depth of Field

Lens resolution indicates how accurately a lens can reproduce the fine details of a subject. It is often expressed through metrics such as line sharpness, contrast, or the MTF (Modulation Transfer Function), and when selecting a zoom lens, it is important to understand the level of resolution required for your application.

Depth of field refers to the range over which the image appears to be in sharp focus. In inspection or measurement applications, it is often necessary for the entire subject to appear clearly. Increasing the f-number deepens the depth of field, but balancing this with other factors, such as available light and shutter speed, is also important.

Depending on the application, attention must also be paid to the effects of diffraction, which can occur when the aperture is stopped down excessively. To make the most of a zoom lens, it is essential to optimize shooting settings considering both resolution and depth of field.

Checking Distortion and Floating Mechanisms

Distortion is an aberration in which images appear barrel-shaped or pincushion-shaped, which can make it difficult to obtain accurate measurements during inspection or testing. In zoom lenses, the degree of distortion changes across different focal lengths, so it is important to understand the level of correction in each range in advance.

A floating mechanism is a technology designed to maintain lens performance even when the focus distance or zoom position changes. This helps preserve uniform image quality even as the subject distance varies, making it particularly valuable for inspection on production lines.

In situations where high resolution is required or measurement accuracy is critical, the presence of a floating mechanism can significantly affect the final image quality. Choosing a lens that maintains performance during zooming and focusing ensures more stable and reliable results.

Maintenance and Longevity of Zoom Lenses

Zoom lenses, which have many moving parts, can experience performance degradation or failure if not properly maintained. Regular cleaning and inspection are essential.

Because zoom lenses contain precisely arranged groups of lens elements that move relative to each other, they are prone to dust, oil contamination, and wear. By regularly checking the condition of the exterior and moving parts and cleaning them as needed, image quality deterioration can be prevented. In industrial applications, where operating conditions are often harsh, increasing the frequency of maintenance is advisable.

When cleaning, care must be taken not to scratch the sensor or lens surfaces. It is important to use appropriate tools and cleaning solutions and follow proper procedures. In addition to optical surfaces, checking the lubrication of moving parts and the tightness of components helps maintain smooth operation over the long term.

If abnormalities occur or advanced servicing is required, it is recommended to use specialized maintenance services or support. Prompt attention can minimize the extent of any damage and help ensure both longer lens life and cost savings.

Summary

Zoom lenses are convenient optical devices that allow flexible changes in the field of view, but proper understanding and management—such as aberration correction and maintenance—are essential. By selecting a zoom lens that suits your purpose and using it appropriately, you can maintain stable performance over a long period.

Grasping the basic structure of zoom lenses, the common types of aberrations, the various lens types, and selection criteria can significantly expand your shooting capabilities and improve inspection efficiency. To fully take advantage of the benefits unique to variable focal lenses, it is essential to understand lens design principles and aberration correction strategies.

Additionally, performing proper maintenance and continuously optimizing settings and checks according to the operating environment will help preserve lens performance over time. As optical technology continues to advance, mastering the correct use of zoom lenses will provide significant value not only in photography and videography but also in factory automation and inspection applications.