Visualising lens aberrations — one at a time, Siemens Star

I’ve written a program to simulate lens aberrations, singly and in combination. For some of the limitations of the program, see the preceding post on this blog. In this post, I’ll use a grid of sinusoidal Siemens stars as the target. Later on, I’ll show results on real-world images. This post presents the main aberrations one at a time.

Lateral chromatic aberration, or LaCA, occurs when a lens focuses different wavelengths of light at slightly different positions in the image plane, but all in the same focal plane. This leads to color fringes near the edges of high-contrast features, especially near the corners of the image. Unlike longitudinal chromatic aberration, LaCA does not blur the image but causes misregistration of the red, green, and blue components. It is often strongest at the edges of the field and minimal or absent in the center. Correction typically involves aligning the color channels digitally or through lens design choices.

Longitudinal chromatic aberration, or LoCA, arises when different wavelengths of light come to focus at different distances along the optical axis. This results in color-dependent blur: for example, red light may be in focus while blue light is slightly front- or back-focused. LoCA is most noticeable in out-of-focus regions and can cause magenta or green fringes around blurred edges. Unlike LaCA, which shifts colors laterally across the image, LoCA changes the sharpness of each color independently. It is most prominent at wide apertures and difficult to correct in postprocessing, making lens design the primary method of control.

Spherical aberration (SA) occurs when light rays passing through the outer parts of a lens are focused at different points along the optical axis than rays passing through the center. This leads to a softening of the image, particularly around bright highlights, and can reduce overall contrast. SA is most apparent when the lens is used wide open and tends to decrease as the aperture is stopped down. It affects all colors equally and is typically most noticeable in the center of the image. Some lenses deliberately include controlled amounts of SA to produce a specific rendering style, especially in portrait photography.

Astigmatism in a lens causes points of light to blur into lines rather than circles, with the blur orientation depending on the direction of the rays. It arises when the lens focuses horizontal and vertical details at slightly different distances, leading to a mismatch between sagittal and tangential focal planes. This results in image areas, especially away from the center, appearing stretched or smeared in one direction. The effect typically worsens toward the edges of the frame and can vary with focus distance and aperture.

Coma is an aberration that affects off-axis points, causing them to appear as asymmetrical, comet-like smears rather than sharp points. It occurs because rays from different parts of the lens aperture are focused at slightly different positions in the image plane. The result is a flaring or tailing effect that radiates away from the image center, often most noticeable in high-contrast point sources like stars or distant lights. Coma increases with distance from the optical axis and is more pronounced at wider apertures.

Field curvature is an aberration where the sharp focus plane is not flat but curved, typically forming a shallow bowl shape. This means that when the center of the image is in focus, the edges and corners fall slightly out of focus, and vice versa. It arises because a simple lens projects a curved image surface rather than a flat one. In practical photography, field curvature can cause subjects at the edges of the frame to appear soft even if they lie in the same geometric plane as the center. Some modern lens designs partially correct for it, though not always completely.

Tilt, in the context of aberration simulation, refers to a linear variation in focus across the image plane caused by the lens’s focal surface being angled relative to the sensor. Instead of being perpendicular to the optical axis, the focal plane is slanted, so one side of the image comes into focus sooner than the other. This can occur due to lens misalignment, manufacturing tolerances, or intentional use in tilt-shift photography to manipulate depth of field and focus placement. In simulations, tilt is often modeled as a planar defocus gradient across the frame.