Any lens-based imaging system, whether it’s a telescope, microscope, projector, or simply a mobile phone lens, will have light converge imperfectly along the theoretical geometric optics path. This is because a real lens will always have some limitations in terms of its shape, thickness, and refractive index, which will cause it to have some errors in imaging, known as aberrations. Furthermore, aberrations include spherical aberration and coma aberration. This blog post will guide you through a detailed understanding of the differences between these two phenomena.
You can simply think of aberration as a plastic film covering the lens, which obstructs the light source, preventing light from traveling along the designed path due to certain special factors.
What is a Spherical Aberration
Spherical aberration is defined as the deviation of focus due to different optical paths when a beam passes through a spherical lens. The light on one side of the lens, called the off-axis light, focuses at a different point from the light on the other side, called the near-axis light, thus creating a focus that is in a string distribution instead of a single point.
For example, one person described it as shining a penlight onto a hole, where if the beam focuses differently in the middle versus the outside of the light, what hits the wall is a fuzzy spot rather than a bright one.
Spherical aberration may produce a blurry imaging spot and a blurred imaging spot with a lowered clarity of the middle portion within an imaging area. Spherical aberrations occur mostly in spherical lenses or monolithic lenses without correction, and the larger the aperture, the stronger the spherical aberration effect.
To alleviate spherical aberrations, one can choose to lower the numerical aperture value of the optical system or adopt aspherical lenses. The curvature of aspherical lenses depends on the distance from the optical axis, thus functioning as a spherical aberration correction tool by concentrating the light and forming a clearer image due to a reduced aberration effect.
What is a Comet Aberration?
Coma aberration is normally a situation where the point source is located outside the optical axis. There is a failure of the lens to focus the light correctly when it is off-axis, and the imaging spot will appear with a comet-like bright spot trailing behind it.
For example, if coma aberration is present in a camera lens when photographing the night sky, each star will no longer appear as a sharp point but instead as a point with a small, comet-like tail.
The effect of coma aberrations can be seen mostly in lenses with a wide diameter and a broad field of view, especially when photographing the starry night sky, wherein the tail effect of the starry points will impact the aesthetic effect of a photograph. Coma aberration is a problem that is related to both aesthetic and resolution aspects concerning the accuracy of optical measurement and imaging, especially because it is most dominant against the off-axis light, and it is not centered on the optical axis, unlike spherical aberrations.
Why consider both spherical aberration and coma at the same time?
As a matter of fact, spherical aberrations and coma often coexist when used in practical applications. Spherical aberrations will impact the resolution of the central region, but coma will impact the imaging resolution of the marginal region. Even if only one is corrected, the imaging effect may present a clear centre with a blurred edge, a blurry centre with a clear edge, and a tailing effect simultaneously.
However, in modern optical lenses, to control both aberrations, it is common to use multi-element lenses, aspherical lenses, and symmetrical lenses. This can be used not only for enhancing the clarity of the imaging area, but also for ensuring the symmetry of the edge light imaging so that the entire imaging area meets the precision requirements.
Conclusion
Spherical aberrations create a blurry and fuzzy light source located in the center of the optical axis, impacting the total sharpened effect. Coma aberration causes a fuzzy light source located off-axis to appear similar to a “comet tail,” impacting the total edge imaging effect.
The latest advancements in optics must address both spherical aberrations and comets in order for an overall balanced, sharpened effect with a potential edge imaging effect. Understanding both spherical aberrations and comets is very valuable for understanding superior optical devices and serves as the background education required for learning about other related topics, including optical imaging, photography, and astronomical observations.
