What is the difference between normal lenses and defocusing lenses

Introducing Normal Lenses and Defocus Lenses

In the fields of optics and image capture, the lens is a core component responsible for guiding and shaping light to form an image on a sensor or retina. While all lenses are based on the same laws of physics, their design goals and final effects can vary widely. This article will focus on two fundamentally different types of lenses: the Normal Lens and the Defocus Lens.

• Defining the Core Concepts:
  • Normal Lens: These lenses are designed to achieve maximum image sharpness and minimum aberration. Their goal is to project the scene's light onto the target plane accurately and clearly, providing the least distorted image, which closely resembles natural human vision. In photography, they are standard equipment; in vision correction, they are the foundation for clear vision.
  • Defocus Lens: This is a category of specialized optical elements intentionally designed to introduce a controlled amount of blur or "Defocus." This defocus is not an optical flaw but a feature of the design. Its purpose is not to pursue ultimate clarity but to achieve specific artistic effects (in photography) or produce particular physiological effects (in vision correction).
• Purpose of Comparison:

  The aim of this article is to deeply analyze the key differences in the optical design, light processing, image formation, final application, and unique effects of the Normal Lens and the Defocus Lens. Understanding these differences is crucial for optics professionals, photographers, and consumers interested in visual health, especially Myopia Control technologies like DIMS lenses.

The Standard Bearer: In-Depth Look at the Normal Lens

The Normal Lens represents a classic paradigm in optical design, where the core philosophy is faithful reproduction. They are the workhorses of photography and standard vision correction, with a design philosophy of "what you see is what you get," aiming for the most natural and least intrusive visual experience.

Definition and Characteristics of a Normal Lens

In photography, the term "Normal Lens" specifically refers to lenses with a particular focal length that provides a viewing angle and perspective most closely matching the natural view of the human eye in a relaxed state.

• Core Function: To project the scene's light onto the sensor with minimal radial distortion and extremely high clarity.

Focal Length and Field of View (FoV)

The "normal" focal length is not an absolutely fixed value; it is closely related to the size of the image sensor or film used.

• Definition of Focal Length: Focal length (f) is the distance at which light rays converge onto the focal plane (sensor/film) when the lens is focused at infinity.
• Determination of Normal Focal Length: In photography, a lens whose focal length is approximately equal to the diagonal length of the imaging medium is generally considered a normal lens.
  • For example, for traditional 35mm full-frame (diagonal approximately 43mm), lenses between 40mm and 58mm are considered normal, with 50mm being the most representative.
• Field of View (FoV): Normal focal length lenses typically offer a Field of View between approximately 40° and 50°. This angle closely matches the visual range of the human eye when concentrating on an object, making the image perspective and spatial compression feel very natural and balanced. Compared to wide-angle or telephoto lenses, a normal lens neither exaggerates spatial depth nor overly compresses space, hence its reputation as the "standard" view.

Optical Design Principles of a Normal Lens

The optical design of a Normal Lens is an art of precision and correction, where all efforts are focused on ensuring light converges perfectly at the focal point.

• Pursuit of Image Sharpness: Designers use multiple lens elements to eliminate or minimize various optical aberrations.
• Key Aberrations Corrected Include:
  • Spherical Aberration: Ensuring light rays entering through different parts of the aperture converge at the same point.
  • Chromatic Aberration: Ensuring different wavelengths of light (colors) focus at the same point, preventing colored fringing.
  • Coma and Astigmatism: Ensuring that even at the edges of the image, a point source still images as a point, rather than a comet shape or a line.
• Glass Materials: Special glass with high refractive index and low dispersion (such as extra-low dispersion glass) is often used to assist in improving image quality.

Common Applications

• General Photography: Due to their natural perspective and high clarity, Normal Lenses are ideal for street, documentary, portraiture, and everyday photography. They train photographers to compose with an "eye-level" view.
• Vision Correction: Standard prescription lenses are optically Normal Lenses. Their design goal is to accurately focus light onto the retina to correct refractive errors (such as myopia, hyperopia, astigmatism), thereby providing clear vision.

The Specialized Tool: Exploring the Defocus Lens

The Defocus Lens takes a completely different path from the Normal Lens which strives for ultimate clarity. Their goal is not to eliminate all aberrations or blur, but to utilize defocus itself as a functional or artistic tool.

Definition and Characteristics of a Defocus Lens

A Defocus Lens is a category of specialized optical lenses characterized by intentionally producing a non-uniform or specific type of light dispersion on the image plane. This defocus is not caused by focusing error or manufacturing defects; it is an optical effect determined by the lens structure itself.

• Key Function: To deliberately manipulate the light beam's convergence point or beam cross-section to achieve softening, enhanced background blur, or peripheral field defocus on the image or retina.

How Defocus Lenses Work: Creating Intentional Blur

Defocus lenses change the traditional focusing mode of light rays through complex optical structures. They usually do not rely on moving lens elements for focus, but rather on fixed structures to influence light distribution.

• Manipulating the Point Spread Function (PSF): In optics, the PSF describes how a lens images an ideal point source. The Normal Lens seeks to compress the PSF to the smallest possible point; the Defocus Lens intentionally expands and reshapes the PSF into a specific shape, gradient, or blur spot.
• Introducing or Utilizing Aberrations: Unlike the Normal Lens which corrects spherical aberration, some photography Defocus Lenses may deliberately introduce or retain uncorrected spherical aberration. This causes light entering through different apertures to focus at different points, producing a soft, dreamy blur effect.
• Light Splitting and Superposition: For Defocus Lenses used for Myopia Control, they achieve light splitting by setting different refractive powers in different areas of the lens. This allows one part of the light to focus clearly on the retina (providing clear vision), while another part focuses in front of the peripheral retina (creating therapeutic defocus).

Types of Defocus Lenses in Detail

The varieties of Defocus Lenses are numerous, mainly classified by the function they are intended to achieve and their application area.

A. Photography-Focused

• Apodization Filters:
  • Principle: These lenses often integrate a filter element with a radial density gradient near the aperture. The element's light transmission gradually decreases from the center to the edge.
  • Effect: It smoothly reduces the light intensity at the aperture edge, making the edges of out-of-focus light spots (Bokeh) softer and more uniformly transitioned, eliminating the "donut" or hard-edge light spots common in traditional lenses, significantly improving the quality of the blur.
• Defocus Smoothing Lenses (Soft Focus Lenses):
  • Principle: Typically designed to retain a suitable amount of spherical aberration, or manufactured with special processes (like grinding) to make the lens surface deliberately imperfect.
  • Effect: Produces a soft, overall slightly blurred image. This "soft focus" effect is especially suitable for artistic portraiture, softening skin details and textures to create an ethereal atmosphere.

B. Vision Correction/Myopia Control

• Defocus Incorporated Multiple Segments (DIMS Lenses):
  • Principle: This is one of the key technologies applied in Myopia Control. The central area of the lens provides standard corrective power for clear vision. Surrounding the central area are multiple tiny, functional optical zones (usually dots or rings) with peripheral defocus power.
  • Effect: When the wearer looks through the lens, the central field of view remains clear (handled by the central zone), while the peripheral field simultaneously receives a myopic defocus signal (handled by the peripheral zones). This peripheral defocus has been clinically shown to slow down the eye's axial length elongation, thereby achieving the goal of slowing down myopia progression.

Core Parameter Comparison: Defocus Lens vs. Normal Lens

Key Differences Between Normal and Defocus Lenses

Although both the Normal Lens and the Defocus Lens belong to the family of optical lenses, there are fundamental differences in their design philosophy, how they process light, and their final impact on the image. These differences are manifested in three main aspects: optical design, image formation, and final application purpose.

1. Optical Design

Optical design is the basis for determining a lens's characteristics. The design of a Normal Lens is subtractive (eliminating aberrations), while the design of a Defocus Lens is deliberately additive (introducing or distributing defocus).

• Normal Lens: Traditional Lens Design for Sharp Focus
  • Goal: Lens groups are precisely calculated and manufactured to ensure that all incident light rays, regardless of which part of the lens they pass through, converge exactly at the same focal point.
  • Manufacturing Precision: Extremely high requirements are placed on the curvature of the lens surface, the uniformity of the glass material, and the distance between lens groups to ensure the light wavefront is as flat as possible.
  • Mathematical Model: The design is based on Gaussian optics and rigorous aberration theory, aiming to achieve diffraction-limited performance—meaning clarity is limited only by the wave nature of light, not by defects in the lens itself.
• Defocus Lens: Modified Design to Induce Controlled Blur
  • Goal: The design intentionally deviates from the traditional correction path to achieve a predetermined blur distribution.
    • Photography Applications: May use asymmetric curvature or special filter layers to ensure a point light source has a smooth, soft halo around the focus point, without hard edges.
    • Vision Correction Applications: Uses complex micro-optical arrays or multifocal/multi-zone designs, such as DIMS lenses, to simultaneously split the image of an object into a sharply focused portion and a peripherally defocused portion, achieving a dual optical function.
  • Structural Complexity: While Normal Lenses are complex, the complexity of Defocus Lenses (especially those for myopia control) lies in the design of their functional zones and microstructures, rather than purely the elimination of aberrations.

2. Image Formation

The differences in optical design between these two lenses directly determine how they form images on the imaging plane (be it a sensor or the retina).

• Normal Lens: Creates a Sharp, Focused Image on the Sensor/Retina
  • Focal Characteristics: When focused accurately, a point on the object corresponds to a minimum blur spot (or a theoretical point) on the imaging plane. This ensures that the high spatial frequencies (i.e., details and textures) of the image can be clearly resolved and recorded.
  • Effect on the Retina: For vision correction, the Normal Lens (standard single-vision lens) ensures light falls precisely on the foveal retina, providing clear central vision. However, in the peripheral field of view, if the eye has refractive issues, the image may still fall behind the retina (peripheral hyperopic defocus).
• Defocus Lens: Produces a Blurred or Softened Image
  • Focal Characteristics: The Defocus Lens intentionally causes light to not converge perfectly at a single point. In photography, a point light source is imaged as a soft blur spot with gradually transitioning edges, creating smooth Bokeh.
  • Effect on the Retina:
    • DIMS Technology: This type of lens forms a unique image on the retina: the central field of view is clear (0D defocus), but the peripheral field is deliberately made to have myopic defocus (typically a refractive power of -2.0D to -3.5D). Physiological studies indicate that this myopic defocus (image falling in front of the retina) acts as a signal to inhibit axial eye growth, thus slowing down myopia progression.

3. Applications

The usage of these two lenses reflects their respective optical characteristics.

• Normal Lens: General Photography, Standard Vision Correction for Clear Vision
  • Versatility: Suitable for most tasks requiring accurate recording and clear imaging. Whether shooting identification photos, precise scientific images, or daily documentation, the Normal Lens is the preferred choice.
  • Optical Goal: In any application, the goal is to provide the most clear and accurate visual information.
• Defocus Lens: Photographic Special Effects, Myopia Treatment
  • Photographic Applications: Specifically used in scenes requiring artistic softening or special background rendering, such as soft-focus portraits, or artistic creations that emphasize creamy smooth Bokeh.
  • Myopia Treatment (DIMS Lenses): Defocus Lenses have achieved groundbreaking progress in vision correction. They are not merely correcting refractive errors, but more importantly, utilizing the defocus signal to treat or control the physiological development of myopia, serving as a lens with therapeutic efficacy.

Applications of Defocus Lenses: Beyond Clarity

The special optical properties of Defocus Lenses give them irreplaceable value in specific fields. Their application is no longer solely focused on clarity but serves artistic expression and physiological intervention.

1. Photography

In the art of photography, clarity is not always the highest goal. The Defocus Lens provides photographers with a powerful tool for controlling depth of field and the overall image atmosphere.

• Creating Bokeh Effects:
  • The Normal Lens relies on a wide aperture to achieve blur outside the depth of field. But the Defocus Lens can further optimize the quality of the blur.
  • Principle: As discussed, through mechanisms like Apodization Filters, the Defocus Lens can smooth the blur spots (Bokeh) outside the focus point. The edges of the light spots are no longer the harsh circles formed by traditional lenses but show a soft, gradual transition, creating a smooth, painterly or creamy feel.
  • Purpose: This optimized bokeh eliminates distracting background clutter, makes the subject stand out more, and enhances the overall aesthetic value of the image.
• Softening Portraits:
  • History and Purpose: The soft-focus effect has a long history in portrait photography. The Defocus Lens introduces a trace of spherical aberration or other controlled defocus, causing high-frequency details (like skin texture, pores) to be softened, while low-frequency structures (like facial contours, eyes) retain sufficient clarity.
  • Effect: This optical softening effect is more natural than post-processing software treatment, lending the image a dreamy, luminous quality, which is crucial for specific artistic styles or aesthetic requirements.

2. Myopia Control - DIMS Lenses

In the field of vision correction, the Defocus Lens is not just about correcting vision but serves as a therapeutic tool for managing and slowing down the progression of myopia.

• How Defocus Lenses Slow Myopia Progression:
  • Physiological Basis: The main cause of myopia is excessive increase in axial eye length. Conventional wisdom suggests that the blurred signal received by the eye in the peripheral field (often peripheral hyperopic defocus, where the image falls behind the retina) stimulates the eye to continue elongating.
  • Role of Defocus Lens (DIMS): DIMS lenses, through their special design, actively introduce myopic defocus (image falling in front of the retina) in the peripheral field of view. Studies show that this myopic defocus signal can inhibit eye growth, thereby effectively slowing the rate of myopia progression.
  • Optical Structure: The core structure of the DIMS lens is a central clear vision zone surrounded by hundreds of defocus functional micro-units. These micro-units are responsible for providing the peripheral defocus signal.
• Clinical Evidence and Effectiveness:
  • Clinical trial data show a significant reduction in both the rate of myopia progression (diopter change) and axial eye elongation in children wearing Defocus Lenses (such as DIMS lenses) compared to a control group wearing standard Normal Lenses (traditional single-vision lenses).
  • Parameter Indicators:
    • Axial Elongation: The rate of axial eye elongation is typically reduced by about 30\% to 60\% when using a Defocus Lens compared to a Normal Lens. Axial elongation is the key physiological indicator of myopia progression.
    • Refractive Change: The lenses effectively slow down the annual increase in refractive power (myopia degree).

3. Other potential applications

• Specialized Applications: In certain customized medical or scientific instruments (such as specific microscopes or diagnostic devices), the Defocus Lens can be used to create an optical slicing effect or perform depth mapping on specific layers of a sample without physically moving the focus.

Benefits and Drawbacks: A Balanced Comparison

When choosing optical components, whether for photographic equipment or vision correction, it is crucial to understand their inherent advantages and limitations. The Normal Lens and the Defocus Lens each excel and fall short in different areas.

1. Normal Lenses

The Normal Lens is a cornerstone of the industry due to its excellent optical fidelity and versatility.

• Benefits:
  • Excellent Sharpness and Clarity: The design goal is aberration elimination, ensuring they provide the highest resolution and detail-capturing capability at the focus point. This is a core advantage in scenarios requiring precise information recording, scientific measurement, or ultimate image quality.
  • High Fidelity and Low Distortion: The perspective provided by the Normal Lens is most similar to the human eye, and geometric distortion in the image is minimal. This makes them ideal for architectural, news, and general documentary photography, ensuring accurate lines and proportions.
  • Versatility: Due to their natural perspective and high clarity, Normal Lenses can adapt to most photography genres, from landscapes to portraits, making them the "all-purpose" lens in the toolkit of both beginners and professionals.
  • Fundamental Role in Vision Correction: Standard single-vision lenses (optically Normal Lenses) are the primary choice for correcting myopia, hyperopia, and astigmatism, providing the wearer with the most basic and clearest corrected vision.
• Drawbacks:
  • Can be Too Sharp: In some cases (especially high-resolution portrait photography), the extreme sharpness of the Normal Lens may reveal too many skin details and imperfections, making the image appear less soft or flattering.
  • Limited Artistic Blur: Although background blur can be achieved with a wide aperture, the quality of the Bokeh is generally ordinary compared to specialized Defocus Lenses, lacking a smooth and aesthetically pleasing artistic quality.
  • No Ability to Intervene in Myopia Progression: In vision correction, while providing clear vision, traditional Normal Lenses cannot actively intervene in the growth of the eye's axial length, thus being ineffective for Myopia Control.

2. Defocus Lenses

The Defocus Lens is a specialized tool created to achieve specific effects or functions, and its value lies in its non-traditional optical output.

• Benefits:
  • Strong Artistic Effects: Specially designed defocus mechanisms (such as apodization) can produce extremely soft and pleasing Bokeh, giving the background blur a unique artistic beauty. Soft-focus lenses can easily create a dreamy, romantic, or vintage photographic atmosphere.
  • Myopia Control Capability: This is the most revolutionary advantage of the Defocus Lens in vision correction. Like DIMS lenses, they utilize therapeutic peripheral myopic defocus signals to significantly slow the rate of myopia progression in children and adolescents.
  • Innovation and Specialization: These lenses solve specific problems (artistic softening, therapeutic intervention needs) and represent innovative design in the field of optical engineering.
• Drawbacks:
  • Not Suitable for General-Purpose Photography: To achieve softening or multifocal purposes, the Defocus Lens must sacrifice overall maximum resolution and edge clarity. Therefore, they are unsuitable for scenes requiring precise detail recording (e.g., landscape, scientific photography).
  • Specialization and Application Limitations: These lenses are highly specialized tools. Photographic Defocus Lenses are only used for specific artistic creation; Myopia Control Defocus Lenses are only suitable for wearers with a need for myopia management. Neither can serve as the primary everyday lens like a Normal Lens.
  • Complexity and Cost: The special optical design and manufacturing processes (such as micro-lens arrays) often make the production of Defocus Lenses more complex, which may lead to higher manufacturing costs.

FAQ: Normal Lenses vs. Defocus Lenses

This section aims to answer the most common questions about the Normal Lens and the Defocus Lens and their applications, helping readers better understand the choices and functions of these two types of lenses.

Q: Can a Normal Lens create bokeh?

A: Yes, a Normal Lens can certainly create bokeh (the aesthetic quality of the blur in the out-of-focus areas), primarily by utilizing a wide aperture (low f-number, e.g., f/1.8 or f/1.4).

• Distinction: While the Normal Lens can produce background blur through shallow depth of field (wide aperture), the Defocus Lens (such as lenses with apodization technology) optimizes and beautifies the bokeh quality through special optical design. They make the edges of the light spots softer and the transition smoother, whereas the out-of-focus light spots from a Normal Lens at wide aperture might still have relatively harsh edges or an onion-ring texture.

Q: Are all soft-focus photography lenses considered Defocus Lenses?

A: Essentially, yes, from an optical function standpoint. A soft-focus lens is designed to intentionally introduce controlled optical imperfections (most commonly, calculated amounts of uncorrected spherical aberration). This controlled imperfection causes light rays to converge over a wider area rather than a single point, resulting in the characteristic softening or defocus effect. Therefore, by definition of introducing controlled defocus as a feature, they fall under the broad category of Defocus Lenses used for artistic purposes.

Q: What is the main difference in the use of a Defocus Lens for photography versus for Myopia Control?

A: The difference lies in the working area and purpose of the defocus signal:

• Photographic Defocus Lens (e.g., Soft Focus):
  • Working Area: Typically affects the light outside the entire focal plane, aiming to blur the background or soften the subject (artistic effect).
  • Purpose of Defocus: Artistic expression and visual aesthetics.
• Myopia Control Defocus Lens (e.g., DIMS):
  • Working Area: The central area remains clearly focused (0D defocus) to ensure normal vision; the peripheral area actively introduces myopic defocus.
  • Purpose of Defocus: Therapeutic treatment of myopia. Peripheral defocus acts as a physiological signal to inhibit axial eye growth.

Q: Does using a Defocus Lens for Myopia Control affect central vision?

A: No, modern therapeutic Defocus Lenses like DIMS are specifically designed to minimize impact on central vision.

• Design Assurance: These lenses have a dedicated central clear optical zone that provides the exact corrective power needed by the wearer. This ensures that light falls accurately on the foveal retina, guaranteeing the core image sharpness and clarity required for daily activities.
• Functional Separation: The defocus function is primarily concentrated in the peripheral areas or microstructures of the lens. Wearers achieve clear central vision in daily life while the peripheral visual system receives the therapeutic defocus signal that inhibits axial growth. Thus, this achieves effective separation and simultaneous operation of visual function (clarity) and therapeutic function (defocus).

Q: Should I choose a Normal Lens or a Defocus Lens for my child's vision correction?

A:

• If the child only requires clear corrected vision and the myopia degree is stable with no risk of progression: A traditional Normal Lens (standard single-vision lens) is the appropriate choice.
• If the child has myopia and it is progressing (myopia is increasing): It is highly recommended to consult an eye care professional and choose a Defocus Lens (such as a DIMS lens). This is because the Normal Lens cannot control myopia progression, while the Defocus Lens can provide clear vision while effectively slowing down axial growth, making it one of the main non-pharmacological methods for myopia intervention today.