Lecture 5 - Aberrations Flashcards
What is aberrations
Deviation from perfection - no optical system is perfect
What is aberration part of
Optical imaging system e.g. lens, eyes
What happens in ideal image formation
All rays meet at 1 point = SHARP + CLEAR IMAGE
Rays converge to 2nd focal point
Parallel rays hitting different part of lens - refracts by different amounts
What happens in non - ideal image formation
BLURRED IMAGE
Rays converge to several points on optical axis - not all at one point - only 2 most extreme rays ( top and bottom ) converge at 2nd focal point, instead of all rays - REALITY - NOT PERFECT
What does the level of blur in an image depend on
How many aberrations in optical system
Is ideal lens aberrated or non - aberrated
Unaberrated
What happens in ideal lens
ALL parallel rays of light to point image - perfect
Wavefront 90 degrees to each other
Spherical wavefront
What are monochromatic aberrations
Aberrations that arise due to geometry ( shape of lens/mirror )
Do monochromatic aberrations depend on wavelength of light
No
What are the 5 types of monochromatic aberrations and what are they called
Spherical aberration Coma Astigmatism Field Curvature Distortion These are called Seidal aberration
What are the effects of seidal aberrations
- Expanded size for a point image - blurred
- Curved image plane
- Extended images no longer geometrically similar to object
Spherical aberration
- Extreme rays - misbehaving - blurred image point ( all image rays dont converge at one point )
- Real/ non - ideal
- Includes aperture stop - gets rid of extreme rays by adding aperture ( hole ) = clear image
- Paraxial region - area of extreme rays - outside rays misbehaving
- Want to limit outside rays outside of dotted line to limit spherical aberrations
Coma
- Comet like appearance
- Greater angle - more coma distortion on image plane
- Coma increases for OFF AXIS RAYS - rays coming into optical axis at an angle = closer together in image plane and closer to optical axis
Astigmatism
- Cylindrical lens - different powers according to different meridians
- 2 planes - horizontal and vertical - choose point in between these 2 planes
Field curvature
- Curved field of focus
- Distortion looking curved
- Field curvature increases for OFF AXIS RAYS
- Advantage: Retina - curved at back - helps off axis rays focus in front of retina = clear image - minimise this type of aberration
- CURVED IMAGE PLANE = FOCUS, CLEAR IMAGE
