the knife edge test

Question 1

what is the role of the knife edge test ?

Answer 1

simple method of testing the quality of lens

Question 2

what is ideal ( paraxial ) image formation or ideal optics ?

Answer 2

. rays travel close to the axis and it leads to ideal or stigmatic image formation
. this means that all the rays coming from one object point end up at just one image point
. objects and image are geometrically similar

Question 3

how does paraxial optics relate to the knife edge test ?

Answer 3

. if all rays coming from one object point end up at just one image point -
. when we move a knife edge across the beam of light at the focus because all of the light from that one object point is concentrated at that one focus
as knife edge cuts across the focus would immediately cut of all the light
. we can test how good the quality of focus is by moving knife edge across it
. and whether it cuts the light of instantly or produces different pattern will tell us about the quality of focus or quality of lens

Question 4

what kind of system is the eye ?

Answer 4

. the eye is an ideal positive system which forms real image on retina that drive our photoreceptors and therefore give us visual perception

Question 5

what happens in real image formation ?

Answer 5

. real image formation shows errors at focus which are called aberrations
. not all the rays ended up at the same point

Question 6

how often do systems exhibit ideal image formation ?

Answer 6

. very few systems exhibit ideal image formation

. we can make pupil size smaller , we make the rays travel close to optical axis and that is the ideal proximation

Question 7

what is the largest problem affecting image quality ?

Answer 7

. the largest problem affecting image quality is an error of focus ( defocus )

Question 8

what do we call error of focus at the eye ? ( defocus )

Answer 8

ametropia

Question 9

what is the effect of high-order aberrations compared to error of focus ( defocus ) ?

Answer 9

. the effect of higher-order aberrations is smaller compared to defocus
.

Question 10

what is the knife edge test ?

Answer 10

. it’s physically moving a knife edge into a focusing beam of light to test its quality

Question 11

what is apparatus involved in the knife edge test ?

Answer 11

. consists of point source of light - which comes from a laser
. positive lens as the eye is positive
. aperture ( like pupil of eye ) to produce near paraxial imaging
. knife edge ( razor blade ) - can move knife edge up and down and in and out
. screen - at the end of optical system to see what happens to the shadow as knife edge moves across the beam

Question 12

what happens when we place knife edge at focus ?

Answer 12

. when we raise the knife edge and lower it again
. the bright light disappears quickly and the light we are left with is due to reflection ,
. if all the light is focused at this one point , as soon as the knife edge crosses that point the light will disappears

Question 13

what happens when we place knife edge inside focus ?

Answer 13

. we get a with movement - shadow is moving in the same direction as knife edge

Question 14

what happens when we place knife edge outside focus ?

Answer 14

. as we move knife edge up , the shadow comes down
. as we move knife edge down the shadow moves up
. we get an against movement