Visualizing lens aberrations, one at a time, revisited

I have now constructed a more accurate lens simulator than the one I talked about last month. The new one takes phase effects into account, and uses 31 wavelengths between 400 and 700 nanometers on 10 nanometer spacing instead of three wavelengths. This is the first post with images from the simulator.

When analyzing the effects of lens aberrations, it can be surprisingly difficult to see what’s really going on just by looking at natural photographs. Real-world images are full of detail and variation, which can mask or muddle the impact of individual aberrations. To better understand how specific aberrations affect image formation, it’s helpful to isolate them and examine their influence under controlled conditions.

In this post, I’ll compare the results of applying a set of common optical aberrations one at a time with two simple synthetic targets: point spread function (PSF) grids and sinusoidal Siemens stars. These tools offer a much clearer view into how aberrations distort the image plane. PSF grids show how a lens blurs or distorts a scene of idealized point sources across the field, while Siemens stars reveal changes in resolution and contrast as a function of angle and spatial frequency.

This post won’t cover natural images, although that’s my eventual goal. For now, we’ll focus on visual clarity and building intuition with the cleanest possible examples before tackling the complexity of real-world scenes.

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.

Defocus occurs, as you would expect, when the image is out of focus.

Trefoil is an aberration that shows three lobes, and typically gets stronger away from the lens axis.

Tetrafoil is similar to trefoil, except that there are four lobes.