OP - Apertures, Stops & Pupils - Week 4

Question 1

Define how vignetting occurs, and how it appears on imagery.

Answer 1

Consider a beam passing through an aperture, through a lens, onto a surface.
If the angle of the beam through the aperture is such that some light falls off the periphery of the lens, it isnt refracted by the lens, and thus the light is lost.
Vignetting appears as as an image with progressively fuzzy peripheries, fading into black.

Question 2

Name 3 things that an aperture affects.

Answer 2

Brightness of an image
Quality of the image
Depth of field

Question 3

What is the aperture stop of a keplerian telescope?

Answer 3

Objective lens

Question 4

Define entrance and exit pupils.

Answer 4

Entrance - looking into the system object side

Exit - looking into the system image side

Question 5

Define the paraxial marginal ray.

Answer 5

Ray from an axial object point, touching the aperture stop edge, as well as the edge of the entrance and exit pupils.

Question 6

Define the paraxial pupil ray.

Answer 6

Ray from a point Q at the edge of an object field passing through the centre of an aperture stop, as well as the centre of the entrance and exit pupils.

Question 7

Describe the equation for F-number.

Answer 7

Equivalent focal length / entrance pupil diameter.

Question 8

Describe the equation for numerical aperture.

Answer 8

NA = n sin (a)

a is the angle of the ray to the object axis