How do Aberrations Manifest Differently At Different Focal Lengths?

Aberration refers to the deviation between the actual image formed by an optical system and the ideal image; it is an unavoidable phenomenon in optical system design.

Lenses with different focal lengths exhibit significant differences in the types and manifestations of aberrations due to variations in their angle of view, lens curvature requirements, and physical structure.

The following are the specific manifestations of several common types of aberrations as they vary with focal length:

1.Distortion

The relationship between distortion and changes in focal length is direct and obvious. At short focal lengths, lenses are prone to barrel distortion, with the degree of distortion increasing as the focal length decreases; at long focal lengths, lenses may exhibit pincushion distortion, though the magnitude is generally minor; and at focal lengths of 85mm or greater, distortion becomes virtually negligible.

2.Spherical aberration

Spherical aberration is inversely proportional to the cube of the focal length; in other words, the shorter the focal length, the more pronounced the aberration. Therefore, short focal length lenses with small focal lengths are more affected by spherical aberration, which mainly manifests as a significant difference between the focal points of the center and the edges, forming obvious blurred circular spots.

This effect is even more severe under large aperture conditions. While spherical aberration decreases relatively as the focal length increases, issues may still persist when using large apertures.

Aberrations vary with focal length

3.Coma aberration

Coma is a function of the field of view, which is inversely proportional to the focal length: the shorter the focal length, the wider the field of view.

Therefore, the large field of view of short focal length lenses results in a large incident angle of off-axis rays, leading to significant coma, which manifests as a comet-like tail in the image of a point light source; while long focal length lenses have a small field of view and relatively small coma.

4.Astigmatism

Astigmatism is a function of the field of view and typically occurs during wide-angle shooting. In wide angle lenses with short focal lengths, light rays passing far from the optical axis traverse asymmetric surfaces; this can result in differing focal lengths for the meridional and sagittal planes, causing the image to appear blurred in one direction while remaining relatively sharp in the other.

In contrast, telephoto lenses are less affected by astigmatism because the light rays within the field of view travel closer to the optical axis.

5.Field curvature

Field curvature refers to the tendency of the image plane to curve naturally, a phenomenon particularly pronounced in short focal length lenses. Wide angle lenses have a large angle of incidence of light at the edge of the field of view, and the optimal focal plane is obviously curved, which can cause the center and the edge of the image to not be in sharp focus at the same time.

Manifestations of various types of aberrations

6.Chromatic aberration

Chromatic aberration primarily manifests as light of different wavelengths focusing at different points, a phenomenon with a complex relationship to focal length. Differences in the refractive index for various wavelengths cause the focal points of different colors to separate (longitudinal chromatic aberration) or result in varying magnification levels (lateral chromatic aberration).

Short focal length lenses have a shorter optical path and exhibit less chromatic aberration, but due to their wide angle of view, they are prone to displaying purple or green fringes at the edges.

Telephoto lenses have a long optical path, resulting in more noticeable chromatic aberration. This manifests as purple or green outlines around the edges of objects in focus, and this chromatic aberration is evenly distributed across the entire image. The purple fringing is particularly severe when shooting high-contrast objects.

Furthermore, the effect of aperture changes on aberration control varies at different focal lengths.

For example, stopping down the aperture of a wide-angle lens can improve astigmatism and chromatic aberration at the edges, but the improvement in distortion is negligible; stopping down the aperture of a telephoto lens can improve axial chromatic aberration and spherical aberration, but usually requires stopping down 1-2 stops to achieve optimal resolution and eliminate spherical aberration.