OpticsRealm Tutorial - 12 - Stops and pupils

Summary

This tutorial from the YouTube channel 'optics' dives into the concept of aperture stops and pupils in optical systems. It explains the role of the aperture stop in limiting on-axis and off-axis rays, discusses the significance of pupils in imaging systems, and clarifies misconceptions about entrance and exit pupils. The video also covers how to locate pupils and the importance of pupil magnification in photography. It concludes with the concept of telecentric lenses and their impact on optical performance, providing a comprehensive guide for those interested in the fundamentals of optics.

Takeaways

• 🔍 The aperture stop is the single aperture in an optical system that limits the on-axis beam and also affects the off-axis rays to prevent vignetting.
• 🌀 Pupil aberrations can skew the rays and are an advanced topic in optical imaging aberrations.
• 👀 The entrance pupil is not necessarily at the front of the lens; it is located where the chief ray crosses the optical axis.
• 📏 To find the entrance pupil's size, trace the marginal rays backward to the point where they intersect with the optical axis.
• 🔬 The double Gauss lens system, common in camera lenses, has an aperture stop that is not near an optical surface and can be adjusted with an iris.
• 📦 Viewing an optical system as a 'black box' allows for the simulation of light bundles' movement by knowing the location of the entrance pupil.
• 👁 The exit pupil is the view of the aperture stop as seen from the image space and is important for understanding relative illumination.
• 🔄 Pupil magnification, or pupil ratio, is the diameter of the exit pupil divided by the diameter of the entrance pupil, useful in close-up photography.
• 🌟 Telecentric lenses are designed to place the exit pupil at infinity, which can affect the balance of spherical aberration and imaging quality.
• 🎓 Understanding pupils is crucial for aligning the eye's pupil with the exit pupil in devices like microscopes and rifle scopes for optimal viewing.
• 📝 The script provides a comprehensive tutorial on aperture stops, pupils, and their roles in an imaging system, with practical examples and definitions.

Q & A

What is the aperture stop in an optical system?

The aperture stop is the single aperture in the optical system that limits the on-axis beam of light. It can be a piece of metal with a hole, an iris, an optical surface, or a lens. It also limits the ray bundle for off-axis beams to prevent vignetting.

How does the aperture stop affect the imaging system?

The aperture stop determines the size of the beam of light that enters the optical system, which in turn affects the quality and characteristics of the image formed. It is crucial for controlling the amount of light and the depth of field in the image.

What are pupil aberrations and how do they relate to the aperture stop?

Pupil aberrations are optical imperfections that distort the path of light rays as they pass through the aperture stop. These aberrations can skew the rays, affecting the clarity and quality of the image.

How is the entrance pupil defined in an optical system?

The entrance pupil is the image of the aperture stop as seen from the object space. It is located wherever the chief ray, traced from the object, crosses the optical axis.

What is the significance of tracing the chief ray to find the entrance pupil?

Tracing the chief ray helps determine the location of the entrance pupil, which is crucial for understanding how light enters the optical system and how it is manipulated by the system to form an image.

How does the double Gauss lens system handle the aperture stop?

In a double Gauss lens system, the aperture stop is typically buried within the optical system, not near an optical surface. This allows for the use of an iris to vary the F number, which adjusts the amount of light entering the system.

What is the difference between the entrance pupil and the exit pupil?

The entrance pupil is the image of the aperture stop in the object space, while the exit pupil is the image of the aperture stop as seen from the image space. The entrance pupil determines how light enters the system, and the exit pupil determines how it exits.

Why is the location of the exit pupil important for relative illumination calculations?

The exit pupil's location is crucial for calculating the uniformity of illumination across the image plane. It helps determine how light is distributed, which can affect the brightness and contrast in different parts of the image.

What is pupil magnification and how is it calculated?

Pupil magnification is the ratio of the diameter of the exit pupil to the diameter of the entrance pupil. It is used in photography for close-up imaging and helps understand how the size of the pupils changes as light passes through the optical system.

What does it mean to telecentric a lens and why is it important?

Telecentricing a lens involves adding a lens to collimate the chief rays or to place the exit pupil at infinity. This ensures that the chief rays are parallel, which is important for applications like microscopy where uniform illumination and precise focusing are critical.

Outlines

00:00

📚 Understanding Aperture Stops and Pupils

This paragraph introduces the concept of aperture stops and pupils in an optical system. The aperture stop is defined as the single aperture that limits the on-axis beam of light. It also limits the ray bundle for off-axis beams to prevent vignetting. The paragraph explains that the aperture stop can be a piece of metal, an iris, an optical surface, or a lens. It further discusses pupil aberrations and how to determine the location of the entrance pupil by tracing the chief ray. The entrance pupil is crucial for understanding the optical system's behavior, and its location can be found by tracing the chief ray to where it crosses the optical axis. The paragraph also touches on the concept of a double Gauss optical system, where the aperture stop is buried within the system, and the importance of understanding the entrance pupil in such systems.

05:00

🔍 Locating Entrance and Exit Pupils

This paragraph delves deeper into the concept of pupils, specifically focusing on the entrance and exit pupils. The entrance pupil is the view of the aperture stop from object space, while the exit pupil is the view from image space. The location of the entrance pupil is determined by tracing the chief ray forward to where it crosses the optical axis, and the size is determined by tracing the marginal rays to this point. Similarly, the exit pupil's location and size are found by tracing the chief ray backward and the marginal rays accordingly. The paragraph emphasizes the importance of understanding the location and size of these pupils for optical system design and performance, such as in relative illumination calculations. It also introduces the concept of pupil magnification, which is the ratio of the exit pupil diameter to the entrance pupil diameter, and its relevance in photography and close-up imaging.

10:01

🌟 Telecentricity and Pupil Magnification

The final paragraph discusses the concept of telecentricity, which involves making the lens telecentric by placing the exit pupil at infinity. This is achieved by inserting a lens with a focal length equal to the distance to the exit pupil, effectively collimating the chief rays. The paragraph highlights the potential pitfalls of this approach, such as changes in the optical system's balance and spherical aberration. It also mentions the practical applications of pupils, such as in microscopes and binoculars, where matching the eye's pupil to the exit pupil of the device is crucial for optimal viewing. The homework for this topic is mentioned, encouraging viewers to engage with the content further on the provided website.

Keywords

💡Aperture Stop

The aperture stop is the single aperture within an optical system that limits the on-axis beam of light. It is crucial for determining the amount of light that enters the system and thus affects depth of field and image brightness. In the script, the aperture stop is described as not necessarily being a physical object like a lens but could be any feature that restricts the light, such as a piece of metal with a hole or an iris.

💡Pupils

Pupils in the context of optics refer to the image of the aperture stop in any given optical space. They are important for understanding how light is focused and manipulated within an optical system. The script discusses entrance and exit pupils, explaining that the entrance pupil is where the chief ray crosses the optical axis entering the system, while the exit pupil is the view of the aperture stop as seen from the image space.

💡Imaging System

An imaging system is a setup that captures or forms an image, often using a combination of lenses and other optical components. The video script explains how aperture stops and pupils relate to the performance and characteristics of an imaging system, such as how they affect the light bundles and potential issues like vignetting.

💡Vignetting

Vignetting refers to the gradual fading of an image towards the corners or edges, often caused by the aperture stop limiting the light that reaches the edges of the lens. The script mentions vignetting in the context of how the aperture stop limits the ray bundle for off-axis beams to prevent this effect.

💡Pupil Aberrations

Pupil aberrations are optical imperfections that distort the path of light rays as they pass through the optical system, affecting image quality. Although not extensively covered in the script, it is mentioned as an advanced topic related to the way pupils can skew the light rays in an optical system.

💡Entrance Pupil

The entrance pupil is the location where the chief ray crosses the optical axis entering the system. It is a key concept for understanding the light gathering capability of an optical system. The script explains how to locate the entrance pupil by tracing the chief ray and how it is used to determine the system's pupil size.

💡Exit Pupil

The exit pupil is the location where the chief ray crosses the optical axis as it leaves the system, seen from the image space. It is important for understanding the illumination uniformity and the light distribution on the image plane. The script describes the process of finding the exit pupil and its significance for relative illumination calculations.

💡Chief Ray

The chief ray is a specific ray that is traced through an optical system to determine the location of pupils and to analyze the system's performance. The script uses the chief ray to illustrate how to find both the entrance and exit pupils and discusses its importance in the context of optical system analysis.

💡Pupil Magnification

Pupil magnification, or pupil ratio, is the ratio of the exit pupil diameter to the entrance pupil diameter. It is used in photography and other optical applications to understand the magnification effect on the pupils. The script briefly introduces this concept and its relevance to close-up imaging.

💡Telecentric Lens

A telecentric lens is designed to place the exit pupil at infinity, which means that the chief rays are collimated or parallel. The script explains the concept of making a lens telecentric by inserting a lens to achieve this effect, and the potential pitfalls of such a design, like changes in spherical aberration.

💡Relative Illumination

Relative illumination refers to the uniformity of light distribution across the image plane, which can affect the brightness and quality of the image. The script mentions that the exit pupil's location is important for calculating relative illumination, which is often related to the cosine to the fourth power of the angle defined by the chief ray.

Highlights

The aperture stop is the single aperture in the optical system that limits the on-axis beam.

The aperture stop also limits the ray bundle for off-axis beams to prevent vignetting.

Pupil aberrations can skew the rays and are an advanced topic in imaging aberrations.

The entrance pupil's location can be determined by tracing the chief ray to where it crosses the optical axis.

The size of the entrance pupil is found by tracing marginal rays to the location where the chief ray crosses the optical axis.

In a double Gauss optical system, the aperture stop is buried and can be varied with an iris.

The exit pupil is the view of the aperture stop as seen from image space and can be located by tracing the chief ray backward.

The size of the exit pupil is determined by tracing marginal rays backward to the exit pupil location.

Pupil magnification is the ratio of the exit pupil diameter to the entrance pupil diameter and is crucial for close-up imaging in photography.

Telecentric lenses are designed to place the exit pupil at infinity, which can affect the balance of spherical aberration.

Understanding the location and size of pupils is essential for calculating relative illumination and uniformity in an optical system.

The concept of pupils is exemplified in devices like microscopes and rifle scopes, where matching the eye's pupil to the exit pupil is important.

Homework is provided to reinforce the understanding of pupils and their practical applications in optical systems.

The tutorial emphasizes the importance of understanding aperture stops and pupils in the design and function of imaging systems.

The video serves as an educational resource for those interested in the intricacies of optical systems and their components.

Engagement with the community is encouraged through comments and additional channels for further discussion on the topic.

Transcripts

hi this is the YouTube channel optics

realm and optics blog or video blog and

today we're going to do tutorial optics

tutorial twelve stops and pupils the

goal of this video is to understand what

aperture stops and how they relate to

pupils and how they relate to an imaging

system so first off yet what is the

aperture stop the aperture stop is the

one aperture in the optical system that

limits the on axis beam now just looking

at this one rate race here you can kind

of tell that the aperture stop is here

because all these other apertures are

larger than the the footprint the blue

rays and this could be this aperture

stop could be anything it could be a

piece of metal with a hole in it it

could be an iris it could be an optical

surface or a lens so the aperture stop

also limits the Ray bundle for the off

axis beams for no vignetting so here's a

case where the this lens does not

vignette at all and you can see if I

blow nup the edge of this aperture stop

here all the different fields which are

different colors they all go through the

edge of the aperture stop now an

advanced topic is pupil aberrations

we've not even talked about a lot of

imaging aberrations yet but there will

be pupil aberrations that skew these

these rays here there's some examples of

an entrance pupil a lot of people want

to say well I'll look at the front of

the lens and that front lens is the

entrance pupil and that's not really the

case the top example is a singlet and in

the case of a singlet the aperture stop

is generally where that lens is and so

you can judge the entrance the optical

systems entrance pupil diameter by the

diameter that lens in this case as

opposed to the bottom case where you've

got a fisheye lens the front lens is

huge but the entrance pupil is not huge

it's tiny you've got

this is essentially an inverse telephoto

and the entrance pupil is buried within

the system and I'll tell you what a an

entrance pupil is here coming up the key

concept of understanding pupils is you

trace the chief ray

wherever that chief ray equals zero that

is where the stop or the pupil is

located now that just because you're

looking at a chief ray and it doesn't

appear to cross the access you can trace

that chief ray forwards or backwards to

see where it crosses the optical axis

and that is the location of the pupil

and this occurs in all spaces where the

stop is objects based image space in

between lenses inside of lenses even so

let's go ahead and talk about that the

entrance pupil and a double Gauss and I

like the double Gauss because most

camera lenses are a derivative of it the

aperture stop is buried in the optical

system and it's not Nera usually not

near an optical surface so you can make

it an iris to vary your F number so what

we're going to do and again this is a

the Buried varied aperture stop

here's object space here is image space

and I've also drawn the chief ray this

chief ray you can see where it's zero it

well by definition it goes through the

optical axes where the stop is but we're

going to look where the entrance pupil

is so I said wherever the chief ray

crosses the axes is the location of the

pupil so this may appear it doesn't

cross the axes well if we trace this Ray

forward this is the - green line we

trace this ray forward where it crosses

this optical axes that is where your

entrance pupil is located now to get the

size of the entrance pupil you take your

marginal rays and you trace them you

trace virtual raised in this case ritual

Ray's to that location now since we're

collimated the size of the interest the

interest pupil diameter is the size same

size as the diameter out here so that is

how you find your entrance pupil

from a blackbox standpoint if you don't

know what's going on inside your optical

system you you can view the system as a

black box

so this black box you've got who knows

how many lenses in it and you can

simulate how the the bundles of bundle

vrai is move versus filled by knowing

where your entrance pupil is located in

this case the interest pupil is inside

the optical system and it's virtual now

I'm also going to show I'm showing here

these input fields these shaded input

fields on top of the ray trace showing

where the entrance pupil is located with

your calculating some perspectives you

need to know the location of the

entrance pupil not necessarily your

front principal plane the exit pupil

we're going to repeat the process the

exit pupil is the view of the aperture

stop as seen from image space so we've

got a trip we got a chief right here

we're going to backwards retrace it like

this here's the dashed line and where it

crosses the axes that is your exit pupil

and again to find the size of the exit

pupil we take the marginal rays and we

trace the marginal rays in this case

we're backwards ray tracing and they're

going to be virtual Ray's they trait and

we trace them back to the exit pupil

location and we get the size of the exit

pupil and again from a blackbox

standpoint you can view the exit pupil

as just some point out here where your

cones of light versus field come to

originate from and come to focus on your

on your image plane and I'll overlay

this these same shaded cones on top of

the ray trace you can see your entrance

pupil comes back here sorry the exit

pupil is back here the exit pupil is

important important location to

reference your relative illumination

calculations if you want to know how

uniform the illumination is in other

words if you've got you're looking at a

white background the corners may appear

darker on

your image plane and that's called

relative illumination so you use this

we'll cover later the radiometric

calculation is usually cosine to the

fourth and that cosine to the fourth is

the angle defined by this chief ray well

so the location of the exit pupil is

very important for that let's put it all

together here's a black box you've got a

virtual entrance pupil a virtual exit

pupil and you can see how the input

beams come and how the output beams come

characterises fully your opticals well

characterizes the footprints in your

optical system and we'll put this on top

of the array trace and you can see that

it's where the entrance and entrance and

exit pupils are located now I want to

talk quickly about pupil magnification

or pupil ratio and that's simply the

diameter of the exit pupil - the

diameter the entrance pupil not going to

get into this in great detail yet but

this is used in photography for close-up

imaging but put the definitions at the

end of the presentation today so we can

understand what I'm talking about again

just in review the aperture stop is the

one aperture single aperture in the

optical system that limits the on axis

bundle of rays a pupil is the image of

the aperture spa aperture stop in any

given optical space if it's in the iya

object space is called the exit pupil if

it's in the image space it's called the

exit pupil and pupil magnification is

defined by the exit pupil to entrance

pupil diameter now I want to talk about

telecenter ating the lens or make it

telecentric or telecenter the lens I've

heard a lot of different nomenclature

for this but the concept is we're going

to insert a lens right here to collimate

these chief rays or to place the exit

pupil at infinity so here's the concept

I just overlaid a red lens and how do

you do this well what happens is this

lens if you make this lens focal length

the distance to where the exit pupil is

you're essentially collimating your

collimating your chief ray now I've also

shown the cones come to focus a little

bit shorter because this is adding power

to the optical system so when there are

some pitfalls for for doing this not

only does it change the optical system

but it's going to change the balance of

the spherical aberration we've not

discussed spherical aberration but

needless to say well you know we'll talk

about that later but what that's going

to do is reduce your imaging the ability

to image well and one final note on

pupils if you've ever looked in a

microscope you you look into the optics

into the glass where the eyepiece is and

you can see this black field and if you

move your head around you'll see like a

little window pane and you bring your

eye close to that window pane and what

you're doing is you're matching the

pupil of your eye to the exit pupil of

the microscope so that that is a

hardcore example of what a pupil is and

we also see them in binoculars they're

quite noticeable in a rifle scope

because the exit pupil is pulled way

back so when you fire your rifle you're

not going to get hit in the eye with the

scope here's the homework I will also

place this on the my webpage optics

rom-com under the video topics thanks

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in have a good one