Aberration is the deviation of light rays passing through the lens causing image defects. In a perfect world, a perfect lens would project every point of the subject to the corresponding point on a plane. Every lens has some form of defect that can be blamed on properties of light. Ultimately these aberrations must be corrected to make the lens useful.
Photographic lenses produce many kinds of aberrations including – Spherical, Lens Distortions, Coma, Astigmatism, and Chromatic. All forms of aberrations require understanding its nature in order to produce high quality glass.
Spherical aberrations occurs when light ray entering a spherical lens do not meet at the same point on the image plane. The center of the lens is called the optical axis. It is a line passing through the perfect center perpendicular to the lens. This is where the light enters the lens at a 90 degree angle. Light rays start to bend as they move further away from the axis. The focus point of all rays changes according their entry point. Rays passing through the center of the lens focus further than rays passing away from the optical axis.
This area of different focus points (Fa and Fb) along the optical axis is known as circle of confusion. The image formed within this area is most likely to be free on spherical aberration. In practice using smaller aperture opening may also help to reduce this defect.
This form of aberration occurs when horizontal and vertical areas of the subject are in focus at different distances on a focal plane. Light rays entering the lens at different points of subject show variations in focal length. The green rays in the image represent y-axis and yellow x-axis. Notice the shift in focus areas of x and y is the difference of focal length. This shift in focus is known as astigmatism. The result is formation of an elliptical image.
Ralf Roletschek [GFDL 1.2]
This type of distortion causes images to appear curved predominantly around the edges. There are three type of lens distortion known as pincushion, barrel, and pinch. The cause is magnification changing across the image height. The effect is pincushion and referred to as positive when magnification decreases from the center. The outward or decrease in magnification is known as barrel effect.
This type of distortion can be corrected with image editing software like Adobe Photoshop.
Coma is mostly apparent in night shots. Light sources such as bulbs seems to leave comet like blurs on the image. This type of aberration occurs when ray of light traveling are oblique to the optical axis and cannot meet at the focal plane. The scattering of rays light produces comet like trails with a bright center. Coma can be positive if the off axis rays focus away from the optical axis and negative when they focus close to the optical axis.
Chromatic aberration or chromatic distortion is the inability of the lens to focus all colors to the same point on focal plane. It is the same principal as a prism splitting the white light into a rainbow through a process called dispersion. This is more common is lenses with very long focal lengths such as telephoto lenses. Common signs of chromatic aberration are color blur in image.
Lens aberration will always be present in all lenses. Modern lens manufacturing techniques have greatly minimized these defects. Digital cameras employ built in software that work in conjunction with the optics to reduce aberrations. Also the pixel density of the modern sensors have increased in huge amounts.
Early digital camera sensor were built with a special optic called low pass filter. Its job is to eliminate certain aberrations that created a moiré effect. Low pass filter also takes away certain detail from the image giving it a softer look. Due to adavancements in technology, most companies are now starting to manufacture cameras without the low pass filter. I hope this article has given you some knowledge about lens aberratiosn. Please leave any questions or comments below.