L11-stops and pupils

Question 1

what is aperture stop ( AS) ?

Answer 1

. limits the amount of people that can enter an optical system
. on - axis marginal rays determine the A.S.

Question 2

what is the entrance pupil ?

Answer 2

. E is the image of the A.S. as seen from the object through all the optical elements in front of the A.S.

. E is also an imaginary structure that tells us the size of the beam that can be sent into the optical system

Question 3

what is the exit pupil ?

Answer 3

is the image of the A.S. as seen from the exit of the optical system ( formed by all the optical elements behind the A.S.)

Question 4

what is the field of view ?

Answer 4

The Field of View of an optical system is the extent of the object plane that can be imaged by the system. The FoV can be given either as an angle or as a unit of length.

Question 5

what is field stop ?

Answer 5

The Field Stop is the structure in an optical system that limits the field of view of the system

Question 6

what is the field stop of camera ?

Answer 6

in the case of the camera it is simply the size of the digital sensor collecting the image (or the size of the film if you have a vintage camera!)

Question 7

what is our optical system composed of ?

Answer 7

. camera with lens
. object ( people )
. real image ( picture )

Question 8

what do object and image rays do when they approach lens ?

Answer 8

. object rays diverge

. image rays converge into image screen

Question 9

what happens if lens is the only optical aperture ?

Answer 9

As the lens is the only optical aperture, then it must also be the aperture stop of the optical system and therefore determines the bundle of rays that can go through.

Question 10

what can we only observe in a real system ?

Answer 10

in the real system we can only observe the part of the image that falls on the image screen. The finite size of the screen therefore limits how much of the field of view we can observe

Question 11

what does a larger screen mean ?

Answer 11

the larger the screen the more of the object plane we can see. This means the screen is the field stop of the optical system

Question 12

what happens if object is at finite distance ?

Answer 12

. If the object is large but the field stop limits the field of view, then we will not be able to see the entire object,

Question 13

what happens to points closer to optical system ?

Answer 13

The points on the object closer to the optical axis are imaged on the screen. But the object point further away from the optical axis is imaged outside the image screen and therefore is not observed.

Question 14

what does it mean if image screen is larger ?

Answer 14

larger image screen captures more of the object

Question 15

what is the effect of a stop placed between the lens and image plane on objects with a small angle ?

Answer 15

for the object point with the smallest angular subtense all the rays that go through the lens (which is the aperture stop) also go through the stop. Therefore for object points close to the optical axis, the stop has no effect whatsoever.

Question 16

what happens as angle size gets larger when a stop is placed between the lens and image plane ?

Answer 16

As the angular subtense of the object increases we notice that not all rays that go through the lens manage to go through the stop

Question 17

what is the effect as the angular subtense of object increases ?

Answer 17

the chief ray (which goes through the centre of the aperture stop, i.e. the lens) just about manages to pass through the stop, which means that exactly half the beam goes through the stop and the remaining half is blocked . The result is that we still get an image at the image plane, but only half the light is reaching this image point so the image is not as bright as image points closer to the optical axis. We call this the point of Half Illumination

Question 18

what is full illumination ?

Answer 18

all the light that goes through aperture stop also goes through to reach the image

Question 19

what happens as the angular subtense of object increases further ?

How does the location of field stop affect illumination?

Answer 19

As the angular subtense of the object increases further , we note that none of the rays that go through the aperture stop (i.e. lens) manage to get past the stop, and therefore none of this light will reach the image plane. Hence we get no image at all. This is the point of Zero Illumination. The stop placed between the lens and the image plane is therefore the structure that is limiting the field of view of the optical system, hence it is the field stop for this system.

Question 20

what is the region of full illumination and what happens as object angle or field angle increases ?

Answer 20

the region of full illumination represents the part of the object field for which the field stop does not block any part of the beam reaching the image. As the object angle (or field angle) increases, the field stop will start to block a part of the beam, reducing the image brightness, until at a sufficiently large field angle, all the beam is blocked by the field stop resulting in no image at that point
The type of images depends on where the fields stop is located.

Question 21

what happens when field of view is not located at image plane ?

Answer 21

Therefore when the field stop is not located at the image plane, the field of view falls off gradually.

Question 22

what happens when field of stop is placed at the image plane ?

Answer 22

all rays are either blocked or none of rays are blocked , therefore we get an abrupt end to the field of view , the image is bright until the edge when it suddenly stops

Question 23

what does the point of half illumination indicate ?

Answer 23

is used to indicate the size of the field of view

Question 24

what are aperture stops based on ?

Answer 24

based on the axial marginal rays

Question 25

what must every optical system have ?

Answer 25

aperture stop and field view

Question 26

how is field stop determined ?

Answer 26

based off-axis rays

Question 27

what should happens in ideal optical system ?

Answer 27

all light that exits an exit pupil of one system enters entrance pupil of other system

Question 28

what happens when using a direct ophthalmoscope when both eyes are emmetropic ?

Answer 28

Since we are assuming emmetropic eyes for both patient and observer, the light rays will emerge from the patient’s eye as parallel rays, and therefore as they enter the observer’s eye they will converge towards the observer’s retina. Therefore an image of the patient’s retina is formed on the observer’s retina – i.e. suggesting that the observer can see the patient’s retina.

Question 29

why can we not see someone’s retina when looking at their eye ?

Answer 29

this is because there isn’t enough light coming out from a person’s eye which is why pupil appears black
in direct ophthalmoscope by illuminating a patients retina using a light source

Question 30

what happens when a stop is placed in between patient’s and the observer’s eye ?

Answer 30

stop is the sight hole at the end of the direct ophthalmoscope
as sight hole ie stop has a smaller diameter than either patient’s or observer’s pupils, then it is clear that this stop acts as an aperture stop for the whole optical system (comprising both eyes).

Question 31

what do the on-axis points on ophthalmoscope represent ?

Question 32

what do the off-axis points on ophthalmoscope represent ?

Answer 32

off-axis object points, we note that part of the beam emerging from the patient’s eye will get blocked by the sight hole, until at some angle, all the light is blocked. The sight hole is therefore acting as a field stop for the direct ophthalmoscope.

• Some light does not make it through
• However the further away the off-axis object is means no light makes it through and will to see this point on the retina
• field of view is limited because of the gap between 2 pupils some of the light exiting the patients pupil is lost and does not make its through the site hole.

Question 33

why must examiner move closer to eye ?

Answer 33

the examiner is required to move close to the patient for ideal viewing with the direct ophthalmoscope. (maximise field of view) As the sight hole moves closer to the patient’s eye, light that would have been blocked by the distant sight hole can now go through, thus increasing the field of view of the instrument.

Question 34

how to get the most light in direct ophthalmoscope ?

Answer 34

the better the match between the Exit Pupil of the first system and the Entrance Pupil of the second, the more efficiently light is collected. In the direct ophthalmoscope this match is improved by bringing the two pupils as close as possible. However there is a limit on how close these pupils can be. Ideally we would like them to be in the same plane so that we get the most efficient coupling between the two.

Question 35

what difference between direct and indirect ophthalmoscope ?

Answer 35

in indirect exit and entrance pupil can be in the same place this is because in the indirect an additional lens is used between px eye and observer’s eye