Lens Aberration

lens aberration is an optical defect of lens due to which light rays not focused at a point result in distort image.

1. Introduction to Lens Aberration By –Rajat Bansal B.OPTOM UTTAR PRADESH UNIVERSITY OF MEDICAL SCIENCES

2. LENS ABERRATION Aberration  in optics refers to a defect in a  lens  such that light is not focused to a point, but is spread out over some region of space, and hence an image formed by a  lens  with  aberration  is blurred or distorted. the nature of the distortion depending on the type of  aberration. An image-forming optical system with aberration will produce an image which is not sharp

3. Types of Aberration

4. C hromatic aberration 1- Chromatic aberration is color related 2- It is an effect resulting from  dispersion in which there is a failure of a lens to focus all colors to the same convergence point. 3-  It occurs because lenses have different   refractive indices for different  wavelengths of light. 4- It cause an image to have a color fringes along boundaries that separate dark and bright parts of the image 5- Each color in the optical spectrum cannot be focused at a single common point.

5. Longitudinal chromatic aberration- A point light (eg- White light) forms a series of point image along optical axis. Shorter wavelength (blue) deviate more. Longer wavelength (red) deviate less.

6. Transversal chromatic aberration- Image of different size form at the focal length of lens. Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane he magnification  and distortion of the lens also varies with wavelength

7. Correction of Chromatic Aberration Achromatic Lens- Definition- A Lens would be considered totally without chromatic aberration is called chromatic aberration. Principle- All of the wavelength across the visible spectrum focused at one point when pass through achromatic lens. Construction of lens- An achromatic lens is constructed from two different material i.e. a Plus lens of one material is used in combination with a minus lens of another material with a different refractive index to counteract the effect of chromatic aberration.

8. Monochromatic Aberration or Seidel Aberration Aberration of lens that occur due to single color of light that entering the lens system. These are also called 3 rd order aberration. There are total five types of seidel aberration. Paraxial equation- F = L’ – L or L’ = L + F This equation is derived from Snell’s law with assumption that for small angle sin theta = theta

9. Spherical Aberration- Caused by increase prismatic effect towards periphery of lens. Object point is on the optical axis of the system Peripheral rays deviate more than central ray due to which peripheral ray focus earlier than central rays. Circle of least confusion is ¼ the distance between the marginal and paraxial focus. Two types- Positive and Negative spherical aberration.

10. Positive" spherical aberration means peripheral rays are bent too much. " Negative" spherical aberration means peripheral rays are not bent enough

11. Correction in optical system- 1- By using Stops to occlude peripheral rays. 2- By using Aplantic or Aspheric lenses. 3- By using lens doublet ( Weak lens + a weaker opposite weaker power lens of different R.I)

12. Correction in eyes- By iris which act as stop. Anterior surface of cornea is flatter at periphery. difference in R.I. from nucleus to cortex of crystalline lens.

13. Oblique Astigmatism- Occurs when rays from off-axis point pass through the spectacle lenses. cause the light to focus as two focal line images known as Tangential and saggital images. The distance between two focus is known as strum's interval which give the degree of astigmatism. When expressed in diopters , this difference is known as oblique astigmatism error. Effect is less with meniscus lenses. Effect is worse with high power lenses.

14. Correction in optical system- Oblique astigmatism is reduced by finding the optimum base curve for a given lens power. Correction in Eyes- By Aplanatic surface of cornea. Retina is curved not flat so circle of least confusion fall on retina By prescribing appropriate Pentascopic tilt to lens.

15. Coma Occurs when object point is off the axis of the lens. Difference in the magnification for rays passing through different zones of lens. Result in the image appear comet or ice cream cone shaped. The point of the cone points towards the optic axis. Ocular coma is unimportant in eyes (Pupil).

16. Object plane Image plane Lens Correction- Avoid by limiting the rays to axial areas

17. Curvature of Field or Power error Occurs when light entering the peripheral areas of the lens not focus on the far point sphere. Instead it focus on the Petzval surface. It arises when wearer looking through the periphery of lens. This aberration cause the spherical component of the lens to have the effect of being off-power in the periphery when worn the lenses. Also called Petzval curvature of field or Image shell Error.

18. Lens Image shell error (Petzval surface) Correction- 1- Cannot be totally corrected under practical condition. 2- In eyes can be corrected by Accommodation and Curvature of Retina.

19. Distortion Occurs because of difference in magnification at different areas of periphery of lens . For + lenses magnification towards the periphery. For – lenses magnification towards the periphery. It is of three types- 1- Barrel distortion 2- Pincushion distortion 3- Mustache distortion

20. Barrel distortion- I mage  magnification decreases with distance from the optical axis. Straight lines are curved inwards in a shape of a barrel. It happens because the field of view of the lens is much wider than the size of the image sensor and hence it needs to be “squeezed” to fit. As a result, straight lines are visibly curved inwards, especially towards the extreme edges of the frame . Present on most wide angle prime lenses and many zoom lenses with relatively short focal lengths. Correction- Post-processing software such as Lightroom and Photoshop, can easily fix barrel distortion issues.  

21. Pincushion distortion-  image magnification increases with the distance from the optical axis   straight lines are curved outwards from the center . The field of view is smaller than the size of the image sensor and it thus needs to be “stretched” to fit. As a result, straight lines appear to be pulled upwards in the corners in the shape of pincushion. This type of distortion is commonly seen on telephoto lenses. Correction- Post-processing software such as Lightroom and Photoshop , can easily fix barrel distortion issues.

22. Mustache distortion- A mixture of both type of distortion i.e. barrel and pincushion distortion. Also called complex distortion It is less common but not rare. It starts out as barrel distortion close to the image center and gradually turns into pincushion distortion towards the image periphery H orizontal lines in the top half of the frame look like a  handlebar mustache .