Home > NEWS > What Is Spherical Aberration? Make The Right Lens Choice for Your Imaging System
Even with sophisticated lens systems, it can be difficult to get an entire image in focus. With any spherical surface, light will bend differently depending where on the surface the light is passing through. Light is refracting at different angles, where the steeper the curve on the lens, the larger the angle of refraction. In the case of a camera lens that uses spherical glass, the surface near the edge of the glass will always refract light at a steeper angle. This will cause some incident light, primarily the light closer to the edge of the curved surface, to focus before the desired focal plane. This phenomenon is called spherical aberration.
Spherical aberration is the blurriness at the edge of an image. Using a spherical lens on a camera causes light near the edge of the lens (farther from the optical axis) to converge closer to the lens. The light rays near the optical axis are referred to as paraxial rays and the rays near the periphery of the lens are the marginal rays. It should be noted that almost all optics found in cameras use spherical lenses.
Image clarity is the biggest reason why spherical aberration must be considered when designing a vision system. When not corrected, the resulting image can have slight or even strong blur along the edge. In some cases, this can be to the point that the image may "cut off" a vital part of the target or environment. Without edge-to-edge sharpness the process of image analysis can become much more difficult.
In applications such as precision agriculture that use a very wide field of view from high altitude, it is critical to ensure that the same edge-to-edge sharpness is always present. This is especially true when post processing is done to help with the analysis, such as NDVI for monitoring the health of crops, or by using software that takes advantage of higher bit depths to actually produce the analytics itself.
In aerial imaging, software stitching is used to combine images into larger mosaics. This can be done with multiple cameras to multiply the number of pixels that are capturing the image data. However, if spherical aberration is involved this could prove difficult due to the lack of edge sharpens. When stitching images together, a level of overlap is required to ensure that the images align properly. Without enough of the edge of each image in focus, the ability for software to align and stitch the images will be affected. Therefore, more of each image may need to be used for overlapping to compensate for a lack of clarity, effectively reducing the number of pixels being used to image unique information.
In early optical systems that used single lens elements, the solution to spherical aberration was to add a small aperture. By having a narrow path for light to pass through, the out of focus light coming from the edges of a lens will be blocked and only allow light to pass near the center of the lens. This results in a sharper image, but also reduces the brightness. Therefore, using a lens system that corrects for spherical aberration helps prevent poor image quality while retaining the desired amount of light. However, even with modern lens designs that try to adjust for spherical aberration, using a smaller aperture is still a common solution for getting the full frame in focus when there is adequate lighting.
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