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Hi! John Hess from Filmmaker IQ.com and today we'll dive into the history and science
of lenses and how these little pieces of glass make filmmaking possible.
People have been fascinated by the properties of translucent crystals and glass since antiquity
long before we understood any about light.
The first lens or oldest artifact that resembled a lens is the Nimrud lens - dating back 750
to 710 BC Assyria. The intended use of this piece of
polished crystal is a bit of a mystery - perhaps it was just a decorative stone, perhaps it was
used as a magnifying glass for making intricate engraving or
perhaps it was used as a fire starter.
The Ancient Greeks and Romans give us the first recorded mention of a lens in Aristophanes'
play “The Clouds” from 424 BC mentioning a burning-glass -
a fire starting magnifying glass made out of water filled glass sphere. In fact our
word “lens” comes from the Latin for Lentil which is shaped like a
double convex lens.
But these first lenses were either polished crystals or water filled glass vessels - the
idea of producing a lens purely out of glass didn't come
about until the middle ages.
It began with this man: Abu Ali Hasan Ibn Al-Haitham, also known as Alhazen. Born in
Basrah in 945AD in what is now present day Iraq, he settled in
Spain where his ideas would found the basics of the scientific revolution including theories
on vision, optics, physics, astronomy and mathematics. He
was the first to accurately describe the eye as a receiver of light rather an emitter of
rays that the Greek scholars Ptolemy and Euclid believed. He
was the first to describe the camera obscura - a pinhole camera that had been known to
the Chinese but never written down.
But for our story today, Alhazen was key for his theories on glass lenses. Based on his
works, European monks began to fashion reading stones,
hemispherical pieces of polished glass that could be placed on top manuscripts to make
them easier to read.
This, as you could imagine, was a godsend for monks with aging eyes…But why stop there? As glass making became
more sophisticated, Italian glassmakers began making reading
stones thinner and even light enough to wear. The first spectacles appeared in Venice between
1268 and 1300 AD. This mid-14th-century frescos by
Tommaso da Modena, featured monks donning the trendiest and most sophisticated wearable
technology of the time.
But Lenses weren't just for utility and fashion - they were about to be used for important
scientific study - that is being able to see things really
far away and really close up. The first refracting telescopes for astronomy were built by Dutch
spectacle makers in 1608 and refined by Galileo in
1609. A few years later Galileo would alter a few elements on the telescope and create
the world's first microscope.
From opening up the vast cosmos, with Galileo observing the moons of Jupiter to inner space
revealing to Robert Hooke the microscopic cells furthering
our understanding of biology - the lens has been both a literal and metaphorical fire
starter for humanity's scientific understanding.
This is a good time in our story to stop and look at the science of how lenses work. It's
always a little tricky when looking at the history of
science because a lot of the basic understandings we take for granted today were total mysteries
to scientists back then..
Having said that though, let's cheat and apply some 20th century understanding to the
discoveries being made by people like Willebrord Snellius,
Christiaan Huygens, and Isaac Newton.
Let's start with a 20th century understanding of light. We now know that Light is a form
of electromagnetic radiation which also includes radio,
microwaves, infrared, ultraviolet, X-rays and gamma waves.
All electromagnetic radiation travels at the speed of light in a vacuum - a constant 299,792,458
meters per second (approximately 186,282 miles per
second). That's regardless of who the observer is.
But that is the speed of light in a vacuum.
When light travels through a medium, the electrons inside the medium disrupt the path light ray
- slowing it down. The amount of slowing down is
described by the material's “index of refraction” - the larger the index of refraction
- the slower light travels through that medium.
Air has a miniscule index of refraction: 1.000293 - so for anything that's not on a planetary
scale, it's negligible. Water has an index of 1.33. If
we shine a laser through an aquarium at an angle, we can see how the slowing down of
light bends the light beam as it travels through the water, once
it reaches the end of of the aquarium, the light beam continues along it's original angle.
But if we curve the surfaces of the entrance and exit points we can bend the light and
direct light beams along a different path.
To demonstrate I've created a few homemade lenses using gel wax. Using a square piece
of gel wax and a protractor we can determine the index of
refraction using Snell's law.
Now if we curve the surface - into a convex shape - here a double convex lens - we can
redirect a light beam. Convex lens will bend light inwards
where as a concave lens - here a double concave lens, will diverge light.
There's only so much you can do with homemade lenses. To explore the properties of lenses
we stepped up our experiments with real glass lenses and a
visit to YouTube Space in Los Angeles.
The first and most important part of a single lens is the focal length. The focal length
is the distance from the lens to the point where collimated
light rays, that's parallel light rays, converge. You can think of collimated light
as light coming from a very far away point in space - like the
sun. Using a pair of laser pointers and some fog, we can see that this convex lens has
a focal length of 130mm.
For determining the focal length of a concave lens - we would continue our diverging lines
backwards - this double concave lens has a focal length of about 170mm
So now let's talk about how we get a real focused image using a single lens. When the
object we're trying to focus on is very far away - we're dealing
with collimated light rays. So in order to get a focused image, we would need to our
imaging sensor at the focal length.
But not all light rays are collimated - light radiates from objects in a spherical fashion
and closer you are to something the more divergent the
light rays. So how does a lens focus the light from an object that's close? To solve this
question we have the thin lens equation 1/Distance from the
Object + 1/Distance to the image plane = 1/focal length.
First off let's talk about a really far away object. As the Distance to the Object
approaches infinity, the 1/distance to object approaches zero -
leaving us a little algebra and the distance to the image plane equals the focal length.
So far so good - but let's try to focus on something closer. Don't worry, we won't
get too crazy with the math - in fact it's easier to visualize with
a lenses and a couple of laser pointers and do some lens ray tracing.
We'll fire our first laser perpendicular to the lens. Once it hits the lens, it will
bend toward the focal point on the far side of the lens Now we'll
fire our second laser - this time aiming toward the focal point in front of the lens, so that
when it hits the lens it will be bent and exit in lens
at perpendicular angle. Where the lasers first meet in front of the lens is at our object
distance and where the lasers converge behind the lens is
where the focused image will be.
So in this first example, our object distance is 260mm and our focused image will be also
at 260mm using this 130mm lens. Notice how both distances
are exactly 2 times the focal length - and the math checks out.
Let's try another example this time with the object distance at 340mm. Using the same
130mm lens the focused image will be at 210mm.
Notice how the beams converge beneath their origins, this means the image created will
be upside down. Laser ray tracing can be a bit abstract - let's
try it with a light bulb as our object and a piece of paper as our image sensor. Putting
the lightbulb at 340mm in front of the lens and paper at 210
does indeed yield a real focused image. Notice how shape of the light bulb filament is upside
down in the projected image and if we move our imaging
plane closer or further away we'll see the image go in and out of focus.
Now this works for a single thin lens with an object distance of over say about 2 times
the focal length. As the object distance gets closer to 1
times the focal length - the imaging distance approaches infinity - sort of the reverse
of what happened when we talked about collimated light. But
what happens when the object is inside the focal length?
Well the answer is we'll have a negative image distance. It's hard to simulate with
my experiment design but what you'll notice is the light never
comes to a focus past the lens - instead it looks like it's even more divergent. Let's
use a diagram to make it easier to see - again one ray
perpendicular to lens which bends to the focal point on the far side of the lens - and the
second ray coming from the focal point through the lens to
create a perpendicular ray.
The key here is our eyes and brain don't know that light is being bent by the lens
- we assume that all light rays are straight and continuous. So if
we follow the light rays back from the lens we end up constructing a virtual image behind
the object - right side up and magnified- this is how a
magnifying glass works.
The real fun occurs when we bring more than one lens into the mix. Telescopes use an objective
lens with a long focal length and an eyepiece lens for
focusing - the lenses have to be placed inside their focal lengths in order to work. A microscope
switches out the objective lens with its long focal
length for a lens with a very short focal length.
Combining multiple lenses also allows us to change the magnification - here is a model
of an afocal zoom lens - using two convex lenses with a concave
lens in between. As we move the concave lens we change the distance of the beams entering
the lens system. Focus on the bright green beams, the others
are reflections created by inferior glass. Notice how the beams change distance as I
move the middle double concave lens. Using a light bulb in place
of the lasers and an aperture on the final focusing lens to increase the sharpness, we
can see this zoom lens in action.
The demand for better and better lens systems for scientific discovery kept lensmakers busy
throughout the 17th and 18th century, but the coming age
of photography would bring a whole new game to town.
THE INFANCY OF PHOTOGRAPHY
The very first lenses used for photography in the 19th century were single element pieces
of glass just like in our science demonstration. But the
problem is, there's a lot of photographic issues from using just one lens including
Chromatic Aberration - that's where light of different
wavelengths get bent differently as they pass through a lens. Anyone who wears glasses can
see this effect when they look at a neon sign that has blue
and red lights, Spherical Aberration - where not all light rays are converging at the focal
point, and Coma Aberration - where off axis light smears
creating a comet like tail. These are just a few of the problems image makers have to deal with.
The first widespread photography process - the French originated Daguerreotype used a lens
by French lensmaker Charles Chevalier in 1839. This lens
was an achromatic doublet, cementing a biconvex element of crown glass with a biconcave element
of flint glass. These two types of glass have
different properties and combined these lens greatly reduced chromatic aberration leading
to sharper images. This early lens used an aperture, a small
hole that reduces the angle of the light rays coming in which further increases sharpness
but reduces the amount of light available for the film. With
an aperture of f16 - f stop is the ratio of the the lens' focal length to the diameter
of the aperture, this lens was very slow - taking twenty to
thirty minutes for an outdoor daguerreotype exposure. Because of this limitation, this
lens became known as the French Landscape Lens.
For portraits, especially indoor portraits, a new type of lens configuration was needed.
In 1840 the French Society for the Encouragement of National
Industry offered an international prize for just such a thing. Joseph Petzval a Slovakia
mathematics professor with no background in optics with the
help of several human computers from the Austro-Hungarian army took up the challenge and submitted his
design in 1840 - the Petzval Portrait lens.
This was a four element lens which had an aperture of f3.6 - much faster than the Landscape
lens - a shaded outdoor sitting would only take a minute
or two and with the new wet collodion process for photography and this lens could even expose
an indoor portrait in about a minute.
But Petzval didn't win the prize… mainly because he wasn't French, but his lens would
go on to be a dominant design for nearly a century - it was
sharp in the middle but fell out of focus quickly on the sides which gave those portraits
from the 1800s that soft edge halo focusing effect.
And although Petzval lens was a mathematically devised lens, lensmaking would resort to trial
and error for the next 50 years which included the first
wide angle Harrison & Schnitzer Globe lens of 1862 and the Dallmeyer Rapid-Rectilinear
(UK) and Steinheil Aplanat from 1866.
These four lenses, The French Landscape, the Petzval Portrait, the Globe and the Rapid-Rectilinear/Aplanat
were the four go-to lenses of the found in
the 19th century photographer's bag.
Heading into the 20th century, the story of lenses simply explode - we'll take a look
at a few notable examples and historical.
Lens technology took at huge leap forward in 1890 with the release of the Zeiss Protar.
For the first time since the Petzval Portrait we have a lens
designed based on scientific formulas to reduce all lens aberrations including astigmatism.
Part of the key to success is the use of a new Barium
Oxide Crown glass developed by Carl Zeiss' Jena Glass Works by Ernst Abbe and Otto Schott.
This new “Schott” glass had a higher index of refraction
making it key the development of better optics.
Now with better materials the cat was out of the bag and new designs for lenses flooded
the marketplace.
In 1893, Dennis Taylor who was employed as chief engineer by T. Cooke & Sons of York
patented the Cooke Triplet as a result of the new designs made
possible by the invention of Schott Crown glass. The Triplet featured three elements,
the center element being flint glass while the other two being
crown glass. The Cooke Triplet came to dominate the low end industry - even used in modern
projector lenses, binoculars, as well as some of the early
motion picture lens of the 20th century.
But the folks at Zeiss weren't done just yet - Paul Rudolph working at Zeiss patented
the Tessar in 1902. Similar to the Cooke Triplet, it added a
fourth glass element greatly improving performance. The Tessar design is still used on a lot of
pancake style lenses.
As aberrations came under control with these new lens designs, attention turned to increasing
the aperture size to allow for faster shooting.
Ernemann Ernostar in 1923 opened up the aperture of a 85mm up to an f2.0 and later to f1.8
in 1924 leading to a new era of photo journalism as less
light was needed to expose a photograph.
In 1926 Ernemann was absorbed by Zeiss and the Ernostar design was reworked and renamed
Sonnar - by 1932, a 50mm f1.5 was available.
Another notable style of lens design was the Double Gauss lens. Named after the mathematician
Carl Friedrich Gauss. The double Gauss took what was
originally an objective lens for a telescope and doubled it… the resulting lens has become
the most intensely studied lens formula of the 20th
century. The Gauss design greatly reduced optical flaws in almost every way and these
lenses could be made with really wide apertures and relatively
inexpensively. Although the first commercially successful double gauss - the Taylor, Taylor
and Hobson Series 0 was released in 1920, there was a
problem that prevented the Double Gauss from really taking off… and that was reflection
- Double Gauss needed at least four elements to work, most
modern designs have up to 8 elements to control aberration. Reflection, like the reflections
we saw on the zoom lens laser demo, cuts down on the
amount of light that travels through the glass - reducing its performance.
The solution would come in anti reflective coating. Back in 1896 Dennis Taylor working
at Cooke noticed something peculiar about older lenses - glass
that had been sitting around for a long time took brighter images. This was due to an oxidation
layer that had built up through time that suppressed
reflection due to dispersion. By 1939, an artificial coating was developed at Zeiss
to cut down reflections as much as 66%. With this improvement, the
Double Gauss lens began to surpass the Sonnar in terms of popularity. Hundreds of variations
have been produced and millions of these types of lenses
sold. The common “nifty fifty” Canon and Nikon 50mm lense are based on the double Gauss
design.
Now up to this time we've been talking about strictly prime lenses - lens with only one
focal length. The Variable focal length lenses - a zoom lens
was first patented in 1902 by Clile C. Allen. Called Travelling, Vario or Varo lens, they
didn't see production for motion picture camera until the
late 20s, the first use of a zoom shot was this one from 1927's “It” starring Clara Bow.
Motion picture film required less resolving power than stills film. An acceptably sharp
zoom lens for still photography didn't come around until 1959
with the Voigtländer Zoomar, 36–82 mm
So with all these lenses being designed and experimented with in the first half of the
20th century, an interesting shift occurred at the close of
World War II. So far we've been talking about European lens manufacturers starting
with the French, English and finally German lenses which include
the powerhouse brand Zeiss. But in 1954, as part of the post war economic recovery campaign,
Japan began to seriously push quality lens production
with manufacturing organizations Japan Machine Design Center (JMDC) and Japan Camera Inspection
Institute (JCII) banning the practice of copying of
foreign designs and the export of low quality photographic equipment. They enforced it with
a rigorous testing program that had to be passed before
companies could ship orders. By the 1960s through a major industry push by the government,
Japan's lens industry began eclipse that of Germany in
terms of quality - with many German brands closing up shop and licensing their name to
products to be manufactured in South East Asia. That also marks
the end of naming lenses like Sonnar or Tessar as the Japanese much prefered using brand
names and feature codes to label their lenses. The quality
control organizations ended in 1989 having completed their function but as a result when
we talk about camera technology and lenses today we almost
exclusively speak about Japanese companies.
I feel like we've only gotten a taste of the world of lenses. In the next video on
lenses we will focus on the properties of modern day lenses - the
basics of what you are looking when you put a lens on to a camera. There's been a lot
of history and lot of science to get us to today - so go out
there, use it and make something great. I'm John Hess and I'll see you on FilmmakerIQ.com
コツ:単語をクリックしてすぐ意味を調べられます!

読み込み中…

The History and Science of Lenses

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Yan Xue 2019 年 10 月 8 日 に公開
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