OpticsRealm Tutorial - 12 - Stops and pupils
Summary
TLDRThis 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
📚 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.
🔍 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.
🌟 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.
Mindmap
Keywords
💡Aperture Stop
💡Pupils
💡Imaging System
💡Vignetting
💡Pupil Aberrations
💡Entrance Pupil
💡Exit Pupil
💡Chief Ray
💡Pupil Magnification
💡Telecentric Lens
💡Relative Illumination
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
for tuning in if you have anything
relevant to this topic please post down
below you can also get a hold of me at
the site these other channels I've been
swamped I'm not keeping up with it thank
you so much and I appreciate you tuning
in have a good one
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