Lens aberration; Aberration; Aberrations (of a lens)
All lenses suffer from aberrations. There is no such thing as a perfect lens. However, the high degree of engineering involved in a lens reduces the lens aberration problems to an acceptable level.
Lens makers have developed a very high level of technical skill with lens grinding and polishing. One manufacturer makes amazing claims. They say if elements from their photographic lenses were the size of a football stadium the surface would vary no more than the thickness of a business card.
Types of lens aberration
- Spherical aberration: The surface is not an ideal shape and/or inaccurate. Parallel beams do not focus at the same place.
- Coma: The image manifestation is a ring shaped image circle creating a v-shaped light flare.
- Chromatic aberration: The separation of colours by the lens as a result of using materials of a poor optical quality. It creates fringes of colour around the edges of objects seen in the image.
- Field curvature : The image plane takes the form of an arc and differs to the sensor plane which is flat creating a loss of focus at the lens edges.
- Barrel, pincushion and moustache distortion : The symmetry of the lens is imperfect slightly distorting the image (each type has a specific distortion).
- Astigmatism: Where images are distorted in way that makes them appear twisted from the shape of the original object when other lenses would not repeat the distortion.
- Pinhole diffraction; Diffraction; : Can cause distortions in lenses, mostly at a very tiny scale. Photo-lenses are affected by diffraction. The effect is a softening of the image. These are normally found when pinhole apertures are used of when there is special types of material used in the lens making. This type of lens aberration is of little importance for the photographer.
Reducing lens aberration
In general an individual or a simple optical lens can be corrected. Improvements include a higher level of technical manufacture. Improved materials (higher quality glass) make a big difference. The lens coatings, and extra corrective elements in the lens all help improve lens aberration. One or more of these solutions can be applied to an individual optical lens or an in individual element in a photo-lens.
Combining lenses to correct aberrations is one of the reasons for creating compound lenses (multiple lenses placed together in lens groups). Essentially, an aberration on one lens element is off-set by another lens element to counteract it. See: Definition: Optical lens; Lens (optical).
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