Aperture Stops and Pupils Flashcards
o To investigate the properties and functions of aperture stops in various optical systems. o To examine how the size of the aperture stop specified. o To learn why aperture stops are important when two optical systems are cascaded together
What is an aperture stop?
A surface or component of the optical system that limits the size of a beam coming from an axial object point.
How does vignetting occur and does it appear on imagery?
Vignetting is the blocking of rays by a surface other than the aperture stop or field stop. It typically occurs at the edge of the field where beam from an off axis object point travels through the aperture stop but is not focussed on the image point. This results in a reduced image brightness at the periphery (fading into its background without a definite border)
What is a field stop?
A field stop is an aperture placed at the intermediate image plane and is used to control the field of view.
Provide examples of an aperture stop.
Iris of the eye
Diaphragm of a camera lens
Objective lens of a Keplerian telescope
List 3 factors that can be controlled by the size of the aperture stop (diameter)
Beam Width = image brightness
Image quality = aberrations and diffractions
Depth of Field
List a factor that can be controlled by the position of the aperture stop.
For a cascading system, the position of the aperture stop affects the level of aberrations.
Define real and virtual entrance pupil.
Entrance pupil is the image of the aperture stop as seen from object space. Real entrance pupil is located in front of the optical system. To locate the virtual entrance pupil, we trace the paraxial pupil ray from right to left/ turn the optical system upside down.
Where is the entrance pupil located if the aperture stop is the first surface/component?
The entrance pupil is located at the aperture stop and it is a real object (not merely an image).
Define real and virtual exit pupil.
Exit pupil is the image of the aperture stop as seem from image space. Real exit pupil is located at the back of the optical system.
Where is the exit pupil located if the aperture stop is the last surface/component?
The exit pupil is located at the aperture stop and it is a real object (not merely an image).
What is the relationship between the aperture stop and entrance and exit pupils?
They are conjugates. All light that passes through the system and forms an image must pass through the entrance and exit pupil.
How does the aperture stop affect the beam from an off axis object point?
Providing there is no vignetting, ray from an off axis object point that passes through the entrance and exit pupil must also pass through the aperture stop.
How can we locate the entrance and exit pupil using ray tracing?
The location of the entrance and exit pupils is where the central ray of the beam appears to cross the optical axis in object and image space.
What is the optical invariant?
The optical invariant is a invariant that is calculated using the refractive index, pupil ray height and aperture stop radius. To ensure that paraxial calculations are performed accurately, the optical invariant should be fixed and constant throughout the system.
What does the optical invariant depend on?
The optical invariant depends upon the aperture stop diameter, position of the conjugate planes and the nominated size of the field of view.
