The History and Science of Lenses

The History and Science of Lenses


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