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Lens Characteristics: Flare, Ghosting and Aberration
Essentially, lenses contain a variety of items that affect image quality. For example, when a lens is directed at a strong light source, a whitish flare is produced. When unneeded light reflects off the lens surface and mirror frame, ghosting occurs. Additionally, the lens' spherical front surface causes some degree of image distortion and contains aberration that cause problems such as color bleeding. In response to these problems, manufacturers of high-quality lenses do their utmost to reduce image degradation by using coatings, aspherical lenses and special types of glass.

Flare
When a lens is directed at a strong light source like the sun, unneeded rays of light reflected from the lens surface and the camera mirror's frame can have a blanketing effect on the lens. When this happens, all or part of the image turns whitish and sharpness is lost.
 
Cleanly captured image, without flare.
 
Sunlight enters the lens, causing flare.
 
Ghosts
Commonly regarded as a type of flare, the phenomenon known as ghosting occurs when light repeatedly reflects off the surface of the lens and is seen in the image. Reflections occurring in front of and behind the lens' aperture give the ghost the same shape as the aperture
 
Cleanly captured image, without ghosting.
 
As sunlight reflects inside the lens, ghosting occurs.
 
Aberrations
When light refracts inside the lens, image quality is degraded. This is called a lens aberration, or flaw. A wide variety can occur, such as curvilinear, spherical, coma, astigmatic, and chromatic aberration.
Example of barrel distortion   Barrel distortion
(Curvilinear aberration)
  Pincushion distortion
(Curvilinear aberration)
 
 
When objects consisting of straight lines are photographed, the effect of curvilinear aberration is strongly felt. This photo is an example of curvilinear aberration.
 
When the periphery of the image bulges outwards, it is called barrel distortion. This often occurs at the widest angle of a zoom lens.
 
This is the opposite of barrel distortion. It also appears at the edges of an image, where lines appear pulled in towards the center. Generally, this is most often seen at the telephoto end of a zoom lens.
 
Other Types of Aberration
There are also a number of other types of aberration. Let's have a look at the sample images. Each image's white circles show the kind of deformation caused by that type of distortion.
      Ideal image  
 
These images are taken from the corner of a piece of photography, which is an area that is prone to aberration.
 
     
  Spherical aberration   Coma aberration   Astigmatism   Chromatic aberration  
 
 
 
 
 
 
This is an inevitable type of aberration when using spherical lenses. Differences occur between the light in the center of the lens and the light at the edge of the lens, creating a halo-like blur.
 
This type of distortion occurs at the edge of lenses designed to correct for spherical aberration. The name "coma" is used because the blur caused by this aberration looks like a comet's tail.
 
This type of aberration is caused by the lens used to correct spherical and coma aberrations. It occurs when the lens shifts from the optical axis, causing points on the edge of the lens to become either elliptical or linear.
 
Based on the same principle as a prism breaking light into a rainbow spectrum, these aberrations cause color shifts. They are seen especially in telephoto lenses and increase as the power of the lens increases.
 

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