Aberration of Lens(Part 2)

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4. Field curvature

When shooting objects on a plane perpendicular to the optical axis, the image formed by the lens is not in an image plane, but on a curved surface symmetrical to the optical axis. The defect of this imaging is field curvature. Field curvature is an aberration that is independent of aperture. Reducing the aperture does not improve the blur caused by field curvature.

When taking pictures with a lens with field curvature, when the focus is adjusted to the clear image at the center of the screen, the image around the screen is blurred; when the focus is adjusted to the clear image around the screen, the image at the center of the screen begins to blur again, unable to obtain a clear image with the center and surroundings on a straight image plane. Therefore, in some special cameras, the film is intentionally placed in an arc position to reduce the influence of field curvature. Since the field curvature of the wide-angle lens is larger than that of the general lens, a slightly arc-shaped stand arrangement is used when taking group photos (a wide-angle lens is often used), in order to improve the image quality of the peripheral field of view.

5. Distortion

Refers to the deformation of the image formed by the object. Distortion does not affect the clarity of the image, but only affects the similarity between the image and the object. Due to the distortion, a straight line on the object side becomes a curve on the image side, causing distortion of the image. Distortion can be divided into two types of pillow distortion and barrel distortion. The fundamental cause of distortion is that the lateral magnification of the central area of the lens image field is inconsistent with the lateral magnification of the edge area. As shown in the figure below, if the edge magnification is greater than the central magnification, pincushion distortion will occur, otherwise, barrel distortion will occur.

Distortion has nothing to do with the F-number of the lens' aperture, only the field of view of the lens. Therefore, the distortion of a wide-angle lens is generally larger than that of a standard lens or a telephoto lens. No matter what kind of lens, which kind of distortion, reducing the aperture will not improve the distortion.

It is important to note that the distortion aberration of the lens is not the same as the perspective distortion. The distortion of the lens is caused by the imaging of the lens, and various methods (such as aspheric mirrors) can be adopted to reduce the distortion when designing the lens. Perspective distortion is determined by factors such as viewpoint, angle of view, and lens pointing (pitch). This is the law of perspective. No matter what kind of lens, if the viewpoint is the same, the angle of view is the same, and the lens pointing is the same, the resulting perspective distortion is the same.

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