Optical lens - types and main differences

Optical lens - types and main differences

August 26, 2020

Optical Lens - types and main differences 

A lens is a transparent material (glass or plastic) with at least one curved surface. The word “lens” derives from the Latin word for "lentil" bean which has the shape of a Bi-convex lens.

The lens shape causes light rays to bend in a specific way as they pass through the lens itself, whether converging to a specific point or diverging as if from a specific point.

  • A convex (converging) lens - converges parallel rays of light at a single point on the optical axis, on the opposite side of the lens.  
  • A concave (diverging) lens - diverge parallel rays. All rays that enter the lens, parallel to its optical axis diverge.

Index of refraction and Snell’s law - The difference between the index of refraction of the lens and the surrounding air causes the actual bending of the light rays. The bending phenomenon is described by Snell’s law for refraction. The law describes the relationship between the angle of incidence and refraction when passing a boundary between two different materials. It relates to the difference in the angle between the incident and refracted light rays to the indices of refraction for two materials.

  • Converging lens, refraction rule - Parallel rays traveling to the principal axis of a converging lens (biconvex, plano-convex) refracts through the lens and travel through the focal point on the opposite side of the lens.
  • Diverging lens, refraction rule - Parallel rays traveling to the principal axis of a diverging lens (biconcave, plano-concave) refracts through the lens and travel in line with the focal point 

Four main spherical lens type 

Biconvex lens is the best option when the object and the image are at an equal (or near-equal) distance from the lens which creates a magnification ratio of 1:1. The symmetry reduces spherical aberration as well as canceled chromatic aberration. The same radius of curvature on both spherical surfaces reduces the spherical aberration

  • Converging or positive lens
  • Curvature on both sides of the lens
  • Thicker at its center than at its edges
  • The focal length is defined as positive
  • Two focal points 
  • Forming a real or a virtual image

 

Plano Convex lens is the best option to collect and collimate light - focus parallel rays of light to a single point. When the object and the image are located at a different distance from the lens the asymmetry of the Plano Convex lens reduces the spherical aberration. This lens is best used when the object is placed at infinity and the image is a focused point. It is recommended to place the curved surface facing the largest object distance to reduce spherical aberration.

  • Converging or positive lens
  • Curvature on one side of the lens
  • Thicker at its center than at its edges
  • The focal length is defined as positive
  • One focal point 
  • Forming a real or a virtual image

 

Biconcave lens is the best option when the object and the image are at an infinite conjugate ratio closer to 1:1 with a converging input beam. The output beams will diverge from the virtual image (located on the object side). The focal length is the distance between the virtual image and the lens.

  • Diverging lens
  • Curvature on both sides of the lens
  • Thinner at its center than at its edges
  • The focal length is defined as negative
  • Two focal points
  • Forming a virtual image -  smaller than the object itself

 

Plano Concave lens is used to expand light or to increase focal lengths or to balance aberrations from other lenses within a system. Spherical aberration and coma are reduced when used at the infinite conjugate ratio with a collimated light incident upon the concave side. Plano concave lens is the best option when the object and the image are at an infinite conjugate ratio greater than 5:1 and less than 1:5. It is recommended to place the curved surface facing the largest object distance to reduce spherical aberration.

  • Diverging lens
  • Curvature on one side of the lens
  • Thinner at its center than at its edges
  • The focal length is defined as negative
  • One focal point 
  • Forming a virtual image -  smaller than the object itself
  • Increase focal lengths 
  • Balance aberrations from other lenses within a system

 

Let’s do a comparison between the different lenses 

Biconvex Vs Biconcave

Biconvex lenses are focusing light where Biconcave lenses disperse the incident energy. 

 

Biconvex

Biconcave

  • Converging 
  • Curvature on both sides of the lens
  • Thicker at its center than at its edges
  • Positive focal length 
  • Two focal points 
  • Forming a real or a virtual image
  • Used as magnifying or condensing lenses
  • Used as objectives or magnifiers
  • Used in imaging systems such as telescopes, monocular, microscopes, binoculars, cameras, projectors
  • Human eyes 
  • Used in image relays
  • Diverging
  • Curvature on both sides of the lens
  • Thinner at its center than at its edges
  • Negative focal length 
  • Two focal points
  • Forming a virtual image 
  • Used to magnify objects - telescope, peepholes, light projection, and binoculars 
  • Used in eyeglasses - nearsightedness

 

Biconvex Vs Plano Convex 

Both biconvex and plano-convex are positive, converging lenses 

 

Biconvex

Plano Convex

  • Two focal points
  • Finite conjugate - the object being imagined is much closer to the lens
  • Common use - Microscopy 
  • One focal point 
  • Infinite conjugate - the object being imaged is far away from the lens
  • Common use -  focusing light from a star
  • Better to deal with spherical aberrations
  • More economic

 

Biconcave Vs Plano Concave

Both biconcave and plano-concave are negative, diverging lenses 

Biconcave 

Plano Concave

  • Two focal points 
  • Used in laser beam expanders, optical character readers, viewers, and projection system
  • One focal point 
  • Used to balance aberrations from other lenses within a system