STOPS PUPILS AND PORTS

INTRODUCTION

• When an optical instrument is used to study an object,the light reflected from the object enters the instrument through various lenses.

• Here Apertures control the amount of light that gets into an optical system and hence the intensity of the image formed by the system.

• stops are physical apertures that limit the rays through a system

Aperture stops

• The limiting diameter which determines the amount of light which reaches the imaging area is called the aperture stop

• Usually located within the lens (single element) or within the multi element system.

• In any real optical system there are multiple apertures, but only one, the aperture stop,determines which rays traverse the system and, as result, the intensity of images.

• Adjustable stops are commonly created from thin metallic leaf structures to determine the amount of light which reaches a film

PUPIL

•  Formally, a pupil is an image of the aperture stop.

• There are 2 types of pupils– 1.Entrance pupil– 2.Exit pupil

ENTRANCE PUPIL

• The entrance pupil of a system is the image of the aperture stop as seen from a point on the optic axis in the object plane.

• The entrance pupil takes over the role of the lens rim in the simple camera

• In the illustration, the physical aperture is behind the lens. You can see that more light will get through the physical aperture behind the lens than would have if the lens were not present. The extreme ray is one that is refracted by the lens so that it just passes through the aperture. Tracing the projected path of that extreme ray without the lens, you can see that the entrance pupil is the size aperture that would be required to pass that extreme ray in the absence of the lens

EXIT PUPIL

• The exit pupil of a system is the image of the aperture stop as seen from a point on the optic axis in the image plane.

•  In a telescope or compound microscope, this image is the image of the objective element(s) as produced by the eyepiece.

•  It is a virtual aperture in an optical system.

EXIT PUPIL

In this case where the physical aperture stop is in front of the lens, the exit pupil is the image of that aperture stop as seen from the optic axis in the image plane, looking back through the lens. In this case the entrance pupil is just the physical aperture since there is no lens intervening from the standpoint of the object.

FIELD OF VIEW QUANTITIES

Components in the optical system that limits the amount or size of the object that can actually be imaged and is relevent when you are concerned with off axis or extented object.

Field of view quantities

• Entrance port (object space)

• Field stop (device)

• Exit port (image space)

FIELD STOPS

• The element that limits the size or angular breadth of an object that can be imaged by the system is called the field stop.

• For cameras, the size of the film or CCD detector determines the maximum image size and serves as the field stop.

• Usually located at the focal

plane

Entrance port

• -The image of the F.S. formed by any lens in front of it

• -If no lens is in front of it, then the F.S. and the Entrance Port are the same device

• -Object space quantity that makes the smallest angle relative to the entrance pupil

Exit port

• The image of the F.S. formed by any lens behind it

• -If no lens is behind it, then the F.S. and the Exit port are the same device

• -Image space quantity that makes the smallest angle relative to the entrance pupil

Vignetting

• Rays that don’t make it through an optical system are said to be vignetted (shadowed)– maybe a lens isn’t big enough– maybe your eye’s pupil isn’t big enough, or is

improperly placed

• Often appears as a gradual darkening as a function of distance from the field center– the farther out you go, the bigger your lenses need to

be– every optical system has a limited (unvignetted) field of

view– beyond this, throughput goes down

The depth of field

• A camera can only focus its lens at a single point, but there will be an area that stretches in front of and behind this focus point that still appears sharp.

• This zone is known as the depth of field. It’s not a fixed distance, it changes in size and can be described as either ‘shallow’ (where only a narrow zone appears sharp) or deep (where more of the picture appears sharp).

Depth of focus

• It is the depth inside the camera, at the sensor position, within which the image can be considered acceptably sharp.

• Almost always, Depth of Field is the thing photographers are interested in. Depth of Focus can be considered an advanced or specialist topic. 

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