Sunday Read: What are Lens Aberrations?

Sunday Reads

Laura Currie
18th December 2022

When researching lenses, you’ll often come across terms such as 'chromatic aberration' and 'coma', along with descriptions of how good the lens is at thwarting these pesky distortions. But what are lens aberrations? These are effects caused by wavelengths within the rays of light that pass through a lens but don’t land in the same place on the sensor. Whilst no lens is perfect (it’s just not physically possible to create the ‘perfect’ lens) certain lenses do perform better than others. We thought it would be helpful to examine the key types of aberrations, how they occur, and which types of lenses are more prone to these effects.

In this Sunday Read, we will be discussing:

• Chromatic aberration

• Spherical aberration

• Coma

• Field curvature

Chromatic aberration

Light travels in different coloured wavelengths and they all behave differently. When travelling through glass, they are refracted/bent in alternative ways – red light bends the least, green is in the middle and the bendiest colour is blue. Because of this, they don’t all line up at exactly the same point on the sensor. This causes coloured fringing and is especially visible when photographing something dark on a bright background, such as trees against a bright sky.

By Andreas 06 - Own work, Wikipedia

By Wilder Kaiser at English Wikipedia, CC BY-SA 3.0

One technique that manufacturers use to combat this effect is the employment of achromatic elements in the design of the lens. These elements have been designed to bring the different wavelengths together, and are usually positioned in pairs, in what’s known as an achromatic doublet. Within the pair, one lens is concave while the other is convex; one is high dispersion while the other is low dispersion, so they effectively cancel out each other’s aberrations.

Spherical aberration

As light travels through a curved lens, it is refracted more strongly at the edges of the glass than it is in the centre because the rays are hitting the surface at an angle. This means that on the other side, the light converges in different places, causing the image to be out of focus.

Made by Mglg, uploaded to English Wikipedia

 

Here we see the path of light that causes spherical aberration, with the rays not converging in the same place.

Large aperture lenses can be prone to spherical aberration, but by stopping the aperture down, the effect can be reduced as the aperture blades block the peripheral rays of light. Modern lenses have come a long way correcting this aberration; these effects are often a lot more evident in older lens designs.

Coma

Coma is an effect caused by light from a point entering the lens at an angle, which prevents it from remaining a point, instead it sort of fans out, rendering it as a streak – like a comet. As with many types of aberration, it becomes more pronounced the closer you are to the edge of the frame, and similarly to spherical aberration, a smaller aperture will help reduce this effect.

Credit

By Llamnuds at English Wikipedia

Field curvature

Field curvature is a little trickier to explain, but here goes…

This is where the edges of the frame are out of focus because many curved lenses can’t project uniformly onto the flat surface of a sensor. Light rays entering at a perpendicular angle converge properly on the centre of the sensor, but light which enters at an angle refracts differently so falls short and away from the centre of the sensor.

Hopefully this diagram demonstrates this, the blue light coming in at an angle converges too far away from the sensor to be sharp:

By Ben Frantz Dale - own work, CC BY-SA 3.0

You might initially wonder why camera manufacturers don’t just make curved sensors, but, because the curvature of the sensor would have to be a perfect match for that particular lens, it would mean no other lenses would be compatible with that camera.

Enter, the Planar lens. The Zeiss Planar lens was designed by Paul Rudolph at Carl Zeiss in 1896 and it was capable of projecting a perfectly flat image onto what would have been film back then. The lens was comprised of a set of elements that were symmetrical in design and positioning from front to back, which you can see in this cross-section diagram:

By Tamasflex - Own work, CC BY-SA 3.0

The lens was known to be pin sharp throughout the frame, however it suffered badly from lens flare. In the 1950s, anti-reflective element coatings came into existence, offering far superior quality to the first model.

You can get seriously deep when learning about these effects, I certainly ended up down a bit of a rabbit hole, it was a wonderful learning experience for me. Hopefully this goes some way to giving you a deeper understanding of these aberrations and what causes them, without needing a degree in physics!