L11-stops and pupils Flashcards
(35 cards)
what is aperture stop ( AS) ?
. limits the amount of people that can enter an optical system
. on - axis marginal rays determine the A.S.
what is the entrance pupil ?
. 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
what is the exit pupil ?
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.)
what is the field of view ?
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.
what is field stop ?
The Field Stop is the structure in an optical system that limits the field of view of the system
what is the field stop of camera ?
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!)
what is our optical system composed of ?
. camera with lens
. object ( people )
. real image ( picture )
what do object and image rays do when they approach lens ?
. object rays diverge
. image rays converge into image screen
what happens if lens is the only optical aperture ?
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.
what can we only observe in a real system ?
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
what does a larger screen mean ?
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
what happens if object is at finite distance ?
. 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,
what happens to points closer to optical system ?
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.
what does it mean if image screen is larger ?
larger image screen captures more of the object
what is the effect of a stop placed between the lens and image plane on objects with a small angle ?
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.
what happens as angle size gets larger when a stop is placed between the lens and image plane ?
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
what is the effect as the angular subtense of object increases ?
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
what is full illumination ?
all the light that goes through aperture stop also goes through to reach the image
what happens as the angular subtense of object increases further ?
How does the location of field stop affect illumination?
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.
what is the region of full illumination and what happens as object angle or field angle increases ?
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.
what happens when field of view is not located at image plane ?
Therefore when the field stop is not located at the image plane, the field of view falls off gradually.
what happens when field of stop is placed at the image plane ?
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
what does the point of half illumination indicate ?
is used to indicate the size of the field of view
what are aperture stops based on ?
based on the axial marginal rays
