Understanding Spherical Aberration

In pho­tog­ra­phy, Spher­i­cal Aber­ra­tion is a defect in cam­era lens­es that caus­es light from dif­fer­ent points on the object being pho­tographed to come to a focus at dif­fer­ent points. Spher­i­cal Aber­ra­tion caus­es soft­ness of the image and some F‑stop num­bers to be incor­rect. Spher­i­cal Aber­ra­tion can be cor­rect­ed with lens ele­ments made of high­er qual­i­ty glass.

There are two kinds of lens aber­ra­tions: chro­mat­ic (the inabil­i­ty to focus dif­fer­ent wave­lengths of col­or at the same spot) and mono­chro­mat­ic (when lens­es are unable to focus a sin­gle col­or of light). One of the most preva­lent mono­chro­mat­ic sub­types is spher­i­cal aberration.

What is spherical aberration?

Spher­i­cal aber­ra­tion is a lens aber­ra­tion caused by the focus­ing of all incom­ing light rays after pass­ing through a spher­i­cal sur­face. Light rays that pass near the lens’s hor­i­zon­tal axis are refract­ed less than rays clos­er to the edge or “periph­ery” of the lens, and they sub­se­quent­ly appear at dif­fer­ent loca­tions across the opti­cal axis​.In oth­er words, after pass­ing through the lens, incom­ing light’s par­al­lel light rays do not con­verge at the same spot. Because of this, spher­i­cal aber­ra­tion can degrade pic­ture qual­i­ty and clar­i­ty, mak­ing it dif­fi­cult to cap­ture clear pho­tos. An exam­ple of spher­i­cal aber­ra­tion is shown below:

How does spherical aberration occur?

Spher­i­cal aber­ra­tion occurs when light rays pass through spher­i­cal lens and focus at var­i­ous loca­tions on a cam­er­a’s sen­sor. It’s a form of mono­chro­mat­ic aber­ra­tion, which is defined as an opti­cal flaw caused by a lens con­cen­trat­ing on just one col­or of illumination.

Man­u­fac­tur­ers fre­quent­ly use spher­i­cal sur­faces for lens­es and curved mir­rors because it is sim­pler and less expen­sive to man­u­fac­ture spher­i­cal sur­faces than aspher­i­cal or gra­di­ent-index ones.

Here’s how spher­i­cal aber­ra­tion occurs in tech­ni­cal terms:

1. Spher­i­cal sur­faces that pass light near the hor­i­zon­tal axis (parax­i­al rays) refract less than those that pass clos­er to the edge (periph­er­al rays). As a con­se­quence, par­al­lel light rays nev­er con­verge as they trav­el across the opti­cal axis.
2. The most sig­nif­i­cant dif­fer­ence is that when a wave­front is spher­i­cal aber­rat­ed, periph­er­al rays focus clos­er to the lens than parax­i­al rays do. The dis­tance between where the two types of rays end up focus­ing is a method to assess the sever­i­ty of spher­i­cal aber­ra­tion in a system.

Keep­ing your cam­era lens clean can help avoid aber­ra­tion. You might also notice col­or fring­ing and sharp­ness issues if your lens has aber­ra­tion issues.

Closing

Spher­i­cal aber­ra­tion is a wave­length-inde­pen­dent type of aber­ra­tion. That means it occurs con­stant­ly, regard­less of the col­or or wave­length of light pass­ing through it. Spher­i­cal aber­ra­tion affects only flat wave­fronts and the con­di­tion con­tin­ues to hold true no mat­ter how far from the lens you’re look­ing. In short, spher­i­cal aber­ra­tion affects incom­ing light rays. As a pho­tog­ra­ph­er it is impor­tant to be aware of this phe­nom­e­non and how it can affect the final image.