Ultimate Guide：Common Optical Defects in Camera Lens Systems

The performance of a lens has a profound effect on the outcome of photography, and the quality of the lens directly influences the final image. Over the years, advancements in optics have significantly reduced many common defects in lens systems, but they have not been completely eliminated. 

On-axis Lens Errors

On-axis errors are defects that affect light rays passing through the center of the lens, often resulting from the lens' inability to focus different wavelengths or the geometry of the lens itself.

Chromatic Aberration

Chromatic aberration occurs when a lens is unable to focus all wavelengths of light onto the same focal plane. This happens because lenses have different refractive indices for different colors of light. As a result, images may display color fringes, particularly around high-contrast edges, due to the failure of the lens to converge all colors at the same point. Modern lenses use specialized coatings and extra-low dispersion glass to mitigate chromatic aberration, but it remains an issue, especially in cheaper or older lenses.

Spherical Aberration

Spherical aberration arises when light rays passing through the edges of a spherical lens do not converge at the same point as those passing through the center. This is due to the shape of the lens, which causes different parts of the lens to focus light unevenly. Spherical aberration can result in a blurry image, particularly towards the edges, and is more pronounced in lenses with wide apertures. Using aspherical elements in lens design helps counteract this problem, improving sharpness across the frame.

Off-axis Errors

Off-axis errors are more complex because they affect light rays passing through the edges of the lens and can cause a variety of distortions that are more noticeable towards the corners of the image.

Coma

Coma is an aberration that causes point sources of light, such as stars or streetlights, to appear distorted into comet-like shapes, especially toward the edges of the image. This effect is most prominent when shooting wide open with lenses that are not well-corrected for this aberration. High-end lenses often include elements designed to reduce coma, making them a better choice for night sky photography or other applications involving pinpoint light sources.

Astigmatism

Astigmatism in a lens system occurs when light rays from different planes do not focus at the same point, causing image points to appear stretched into lines. This defect results in images that are sharp in one direction but blurry in another, often visible in off-center areas of the frame. Astigmatism can affect the quality of both landscape and portrait images, although modern lens designs attempt to minimize this issue.

Field Curvature

Field curvature occurs when the lens projects the image in a curved manner, rather than onto a flat plane, causing the center of the image to be in focus while the edges are blurry, or vice versa. This effect can be problematic in both wide-angle and telephoto lenses, where keeping all areas of the frame in sharp focus is essential. Corrective elements or multi-lens systems can help reduce field curvature, providing a flatter field of focus.

Barrel Distortion

Barrel distortion causes straight lines to bow outward, especially toward the edges of the image. This defect is most commonly observed in wide-angle lenses, where the distortion is most exaggerated. Barrel distortion is particularly noticeable when photographing subjects with straight lines, such as buildings or horizons. While some level of barrel distortion is common in wide-angle lenses, software corrections can be used to compensate for this effect in post-processing.

Pincushion Distortion

Pincushion distortion is the opposite of barrel distortion, where straight lines bend inward toward the center of the image. This type of distortion is more common in telephoto lenses and can give the impression that the image is being "pinched" at the center. Like barrel distortion, pincushion distortion can be corrected using lens profiles in post-processing software.

Optical Defects Examples

Certain types of lenses are more susceptible to specific optical defects due to their design and intended use. Here are examples illustrating which lenses may be prone to the mentioned defects:

Chromatic Aberration

Example: Standard Zoom Lenses
Standard zoom lenses, especially those with a wide zoom range, often exhibit chromatic aberration. This is due to the varying refractive indices of the different glass elements used across the zoom range. Higher-end models may incorporate low-dispersion glass to mitigate this issue, but budget options may struggle with color fringing, particularly at the edges.

Spherical Aberration

Example: Portrait Lenses
Certain portrait lenses with large apertures (e.g., f/1.2 or f/1.4) can suffer from spherical aberration, especially if they are designed with simple spherical surfaces. While these lenses produce beautiful bokeh, they may lack sharpness at wider apertures. Manufacturers often address this by using aspherical elements to improve focus across the frame.

Coma

Example: Wide-Angle Lenses
Wide-angle lenses, particularly those with fast apertures, are prone to coma, which can distort point light sources, such as stars in astrophotography. For instance, a fast wide-angle lens might show significant coma in the corners of images taken at wide apertures. Advanced designs and aspherical elements help reduce this effect in higher-end models.

Astigmatism

Example: Vintage Lenses
Many older or vintage lenses can exhibit astigmatism, where vertical and horizontal lines are focused at different points. This defect was more common in designs that did not account for complex light paths. Modern lens designs, especially those aimed at high-resolution sensors, aim to correct this issue with more sophisticated glass arrangements.

Barrel and Pincushion Distortion

Example: Fisheye Lenses
Fisheye lenses are known for extreme barrel distortion, which causes straight lines to bow outward. While this is a characteristic of fisheye lenses that photographers often embrace for creative purposes, it exemplifies how specific designs can amplify geometric distortion. Conversely, telephoto zoom lenses can exhibit pincushion distortion, particularly at longer focal lengths, where straight lines appear compressed inward.

Summary

Camera lenses have been developed for hundreds of years. With the advancement of human optics, many optical defects have been improved. Buying advanced lenses can give you the best picture quality. However, you can also help improve optical defects such as flare, ghosting, etc. by purchasing a matte box. Check out this blog: K&F CONCEPT Matte Box: Elevate Your Filmmaking Experience to learn about K&F CONCEPT Matte Box.