# Origins of the camera
## Someone's god allegedly said...

"Let there be light!" Most philosophies, religious or otherwise, offer some sort of creation story. Many of these involve light, which for humans, with our highly evolved eyesight, has always been of fundamental importance. People have tried to capture what they have seen for millennia, first with their hands, and then with more advanced technology. Both the traditional arts of painting and sculpture and the modern arts of still and motion photography arose as a result of this drive. The main difference is in the tools, and the primary tool of photography is the camera.

## Raw vision

From an evolutionary standpoint, let us suggest for a moment that our first cameras were our eyes. Our evolutionary ancestors used information from their eye-cameras to avoid predators (big scary sabre-toothed tigers) or to find food (colorful fruit falling from trees, for instance). Our brains, by many measures the most sophisticated on the Earth today, spent millions of years evolving extremely well to efficiently process sophisticated images to detect both movement and objects of interest within a still frame. Back in those days, imagery was all about danger, food, and other serious business: the stuff of life that keeps you standing ... or gets you eaten. Incidentally, scientists now know, that when we are born our brains only see random sensory input, they don't have a presupposition that these-nerve-endings-over-here are to do with vision, and those-nerve-endings-over-there are to do with touch. It's all just random input, and in fact in recent years multiple adults have re-trained their brains to receive balance or sight from their tongues.

## Primordial aesthetics

Though there was probably very little art in those early times, our aesthetic senses had long since begun to evolve from innate responses to natural stimuli such as commanding views (safety), darkness (danger), light (warmth), and so on. These associations would carry forward in to art as we began to discover ways to record our perceptions of the world: in physical art such as cave paintings, through our sophisticated and relatively unique command of language, and through social modes of communication such as dance and ritual.

## Sitting still and going far

Still later, yet far before true cameras appeared, humans used these powers of observation to realise the fundamental principles of light. They knew, for example, that observation of the movement of shadows cast by standing objects over time could be reliably predicted and even associated with the time of day: if you stood at the end of the valley when the shadows had grown long, then you had better run back to the cave or face the evening hunt of the local tigers! This led, as humans began slowly to ponder such mysteries to good effect, to various sorts of technological innovations: chiefly chronological and architectural, but also mathematic, for the line of a shadow is pure and its relationship to the source intrinsic.

## Imperfect representation

When copying something, or the image of something, it's usually impossible to do so accurately. This is true fundamentally for the process of depicting our three-dimensional world (four if you count time!) on any two dimensional artwork: including approximately two dimensional cave walls, modern photographs, and early pictures. There are many reasons for this, but perspective is an important factor: our brains cannot perceive depth and therefore anything approaching realism on a flat piece of paper unless given other clues. The first such clue was size, or perspective.
The Grotte Chauvet cave in France includes some of the world's earliest known cave painting, estimated at about 32,000 years old. It is unclear whether these paintings specifically included the device of relative scale, however much of the global body of neolithic art did, ie. the artists sized objects and characters hierarchically according to their spiritual or thematic importance, not their distance from the viewer. It could be argued that theirs was a symbolic perspective, rather than a physical one.
The second such clue was overlapping to suggest relative depth. This was certainly well in use by around the beginning of the common era, with global examples plentiful, for instance in early ancient Egyptian art and Chinese Han Dynasty tombs.
The third such clue was tone. It took humans a great deal of time to comprehend fully the expressiveness of tone for the illusion of depth. Given the relative tonal limitations of naturally occurring rock outcrops and other early media such as pottery, it is perhaps not surprising to realise that the exploration of tone perhaps naturally almost co-incided with the development of that most powerful of mediums: paper! Paper appeared in China at least as early as the early second century BC. According to textual evidence, by the fifth century some of the earliest artwork exploring tones - the layered ink work of the Chinese 山水画 or "mountain and water painting" - had already developed to prominence.
Probably at the same time as the above developments, investigation in to visual perspective began, for instance around the fifth century BC in Greece where the philosophers Anaxagoras and Democritus worked out geometric theories of perspective. Euclid's Optics, a mathematical treatment of perspective, soon followed in around 300 BC.

## Then man said...

"Let there be lenses!" Eventually, a distant ancestor of ours had the bright idea of ending all this enjoyable evolutionary fun by discovering naturally occurring crystals that were capable of acting as lenses. Modern evidence of ancient lenses is partial, with some direct finds and some extremely fine workmanship that is held as evidence of lens-work.
In the former category, as recently as the last few years, concepts of ancient navigation in the north Atlantic are being reevaluated following the discovery of formerly near-mythical convex-lens sunstones made from a transparent calcite crystal known as 'Iceland spar' that allowed sailors to determine the direction of the sun even on very cloudy days, and after nightfall in northern latitudes. While the 16th century shipwreck it was found on is fairly late, it is thought to have been an established device by this era, having been referenced in 12th century literature as existing at least as early as the 10th century. The story is compounded by the Visby lenses, a collection of lens-shaped manufactured objects made of rock crystal (quartz) found in several Viking graves on the island of Gotland, Sweden, and dating from the 11th or 12th century. But that's nothing! An 1858 excavation at Niniveh in Babylon also unearthed an ancient Assyrian lens, now known as the Nimrud lens or Layard lens, dating from 750–710 BC, now held in the British Museum and thought to be the oldest in the world.
In the latter category, extremely fine workmanship of a kind considered unattainable with the naked eye (less than 0.1mm) qualifies a 1.3mm wide ivory carving from Abydos, Egypt, recently discovered by German archaeologist Gunter Dreyer that dates from 3300 BC. Other later but still early objects such as the Isopata gold ring from Crete, dated 15th century BC and with workmanship below 0.5mm and approaching 0.1mm and a jasper carving from second century Rome with 0.1-0.2mm details challenge alternate explanation.
Later, a friendly Italian fellow known as Giambattista Della Porta said let there be lenses on cameras! .. or words roughly to that effect. (He was in fact a failed dramatist with a flair for science, blessed with proximity to Venice, a major contemporary center of glass work, who also produced written works on refraction - the bending of light that is the primary science behind basic lens design.)

## The coming of the Camera obscura

The lensless camera, camera obscura or pinhole camera, is essentially an opaque box or room with a hole in it. The first surviving mention of some of the principles behind the camera obscura belongs to 墨子 (Mozi; 470-390 BCE), a Chinese philosopher and the founder of Mohism. Mozi correctly asserted that the image in a camera obscura is flipped upside down because light travels in straight lines from its source. His disciples developed this into a minor theory of optics.
In the western world, the camera has been in use in principle since the Renaissance. It was known as the camera obscura, Latin for 'dark chamber', and consisted of a darkened room with a pinhole in the wall facing the subject, which would be outside the room. An inverted image would fall on the opposite wall, which would then be traced manually.

# Cameras
## What is a camera?

At its most basic, a camera is a system for projecting light onto a surface, typically but not exclusively for the purposes of recording the image. This broadest definition includes microsocopes, camera obscura, digital cameras, video cameras (previously known as cine cameras), cell phone cameras and other such devices that are related to conventional cameras but do not necessarily include all of the same features.
Various dictionaries offer a surprising variety of generic but dated definitions for 'camera', most of which predate digital cameras and exclude both lensless devices (pinhole cameras) and video cameras. Here are some examples:
* Collins English Dictionary (United Kingdom; 2012): an optical device consisting of a lens system set in a light-proof construction inside which a light-sensitive film or plate can be positioned
* Macquarie Dictionary (Australia; 2014): a photographic apparatus in which a sensitive plate or film is exposed, the image being formed by means of a lens.
* Merriam-Webster (United States; 2016): a device that consists of a lightproof chamber with an aperture fitted with a lens and a shutter through which the image of an object is projected onto a surface for recording (as on film) or for translation into electrical impulses (as for television broadcast)
The three basic components of a camera are:
* A device to record the image, which is generally either traditional film or a digital sensor, but can also be various alternative forms of chemical recording surface, glass plates, or similar flat surfaces with the objective of recording an image.
* A lens that focuses light onto the recording device, and
* A dark box, or the camera proper, which keeps other light from interfering with the image being recorded.
These three elements may take various forms as required by the type of photography being performed. For instance, the pinhole camera may have a simple opening instead of a lens, and the dark box may in fact be a complex system of flexible, light-tight bellows or a tiny space behind the lens within a cell phone. Also, these basic elements are often accompanied by other equipment, such as shutters and adjustable apertures to control the amount of light entering the camera, viewfinders to aid in selecting and composing the image, as well as lens shades, carrying straps, tripods, etc., that help in creating images with specified characteristic for particular purposes.

## Some examples of cameras

Let's look at some examples.
* In a pinhole camera, one of the simplest types of cameras that it is possible to make, the direction of the image is controlled simply by punching a small hole in the camera box which functions to project an inverted image of the subject on to the surface (eg. recording device) inside.
* In a view camera, the image is generally captured through a lens (though pinholes can be used) attached to the front of a flexible system of frames and bellows that allows for extensive control of focus and perspective.
* In a mobile phone camera, a lens is typically fixed in to the device by the manufacturer and emphasis is placed on obtaining maximum utility in a broad range of common personal photographic scenarios, automatic operation, a user-friendly interface, and ease of sharing.
* In the case of a modern SLR (single lens reflex) or DSLR (digital SLR) camera, the image is almost always a captured through a glass lens, using a precise shutter to control the length of exposure.
| | Pinhole Camera | View Camera | Mobile Phone | SLR/DSLR |
| Lens | Pinhole | High-quality lens | Single general-purpose lens | High-quality lens |
| Shutter type | None/optional | Lens-mounted (usually between or behind the lens) | Electronic | Focal-plane |
| Aperture control | None | Precise mechanical | Precise electronic | Precise mechanical or electronic |
| Body | Fixed box (typically) | Simple/flexible | Incorporated into phone body | Metal/composite material, usually ergonomic |
| Recording device | Any | Commercial film or digital sensor (typically) | Digital sensor | Film or digital sensor |
Today, when we discuss cameras we are almost always discussing modern cameras, those incorporating an opaque camera body, precise shutter speed and aperture control, and a proper lens.

## Camera Controls
### Five basic controls

As the purpose of a camera is to project light onto a surface that will record an image, most cameras have the same basic controls and these controls affect how the image is recorded. These five basic controls are:
* Location and orientation of the camera: The most basic variables you can alter, and perhaps the most important. While distance from some subjects may sometimes be an asset (wildlife, covert photography, capturing natural expressions during event photography), often getting in close to a human social distance offers the most engaging angles. Similarly, shots from far above or below can lend incredibly different feelings to the subject, and suggest different reactions to the viewer. Generally, shots from far above may inspire feelings of safety or distance from the subject, encouraging the subject to be considered in a more distant or philosophical mindset by the viewer. Shots from below may make the subject appear large and dominating, exaggerating its presence.
* Focus: All but the simplest cameras allow the photographer to select the distance at which the image is the sharpest, or its "focus". This can take various forms, from manually turning a ring on a lens to bring the image into focus, to simply pointing to an object on a mobile phone's screen to select the corresponding distance automatically. In conjunction with the aperture setting below determines which objects in the final image will be sharp, and which blurry.
* Shutter speed ("how long the image is exposed for"): The amount of time that light will be allowed to pass through the lens during the exposure (ie. between the beginning and the end of the photograph). The longer the shutter is open, the brighter an image will become, however the image will also be more sensitive to motion blur as a result of movement in the camera or the subject it is capturing. Sports or action photography therefore depends upon fast shutter speeds (ie. shorter exposures), whereas careful and slow photography of relatively dark or still subjects (mountains, stars in the sky, etc.) generally depends upon slower shutter speeds.
* Aperture ("how big the hole is"): The size of the (usually roughly circular) opening behind the lens. A larger aperture (or opening) will allow more light to pass through the lens than a smaller one. A larger opening will create a brighter image, but the depth of focus (or "depth of field") becomes shallower, allowing fewer objects to remain sharp in the image. Correspondingly, small openings allow subjects within a greater range of distances from the camera to be crisply delineated, but allow less light in and therefore require longer shutter speeds for proper exposure. Many photographers use this to great artistic advantage by either bringing the image into crisp focus or just the opposite, focusing on a single element and letting the rest of the image blur out so as not to distract the viewer's attention. Apertures are measured in 'f-stops', which are written f/number. Note that these numbers are 'backwards': since they are fractions of the lens focal length, noted as "f", smaller numbers such as f/0.95) mean bigger openings and more light entering the camera, whereas larger numbers (such as f/6.3) mean smaller openings and less light.
* Sensitivity ("how fast the image is recorded") (often when expressed casually, ISO, which actually refers to the ISO 5800 standards documents originally published for film speed by the International Organization for Standardization): In any event, the sensitivity of the film or digital sensor to light. In traditional film cameras, you had to swap film to change this factor because it was a property of the type of film being used. In modern digital cameras, the ISO rating describes film-speed equivalency and is almost always possible to change in the camera, either manually or automatically. The scale is arithmetic, which means that a film with a rating of ISO 800, for example, will be eight times more sensitive to light than one of ISO 100. A higher ISO is useful in low light environments, however increasing the ISO will affect the quality of the image: in film the images become grainy, and in digital the image becomes noisier, with more undesirable speckles. Some of these noise articles can be removed after the fact, however, and low light performance is an area in which digital sensors have been making rapid improvements in recent years.
Changing any of the settings will affect how the image looks and will be discussed further later. For now let's briefly examine some additional controls available to photographers, then look at different cameras and where these controls can be found.

### Additional controls

In addition to the basic controls of shutter speed, aperture and sensitivity, some cameras provide or may be fitted with additional controls, including:
* Lens and zoom selection: Many cameras allow you to change lenses to achieve different optical effects, predominantly related to depth of field, minimum focal distance, what percentage of a scene will be captured within the frame, available light (faster lenses provide mode light) and whether or not zoom is possible to adjust without changing lenses again.
* Tilt/shift or lens/image plane alignment. Built in to some types of cameras, particularly old analog view cameras, but available as aftermarket lens features even on modern DSLR camera systems, such controls enable the photographer to carefully manage which portions of a subject remain in focus and distort or correct their perspective optically, prior to recording of the image.
* Lighting: Many cameras have built-in or optional 'flash' lighting. In addition, some cameras allow external lights to be connected by wire or wireless. Careful control of the lighting in a scene is most often made in studio photography, such as portraiture, still life and commercial product and advertising photography.
* Optical filters have long been used by photographers to achieve additional control of their images. There are many types available:
** Polarization (control of light wave orientation and therefore reduction of overall light entering the camera; these are further divided in to circular and linear polarization filters - the former are more expensive and are designed to allow modern camera metering and auto-focus systems to continue to function normally)
** Wavelength (for instance infra-red or ultra-violet wavelength filters may block these specific wavelengths, and individual colour filters may for example block blues, reds or greens. There are also warmth filters that enhance or reduce a broad range of colors over a greater breadth of the visible spectrum simultaneously, for example with the objective of countering the unnatural colour effects of certain types of artificial lights such as fluorescent bulbs.)
** Neutral density (blocking all wavelengths of light equally but only partially, in order to effect slower exposures of the same scene, for example to accentuate motion blur in moving subjects such as running water. These may be complete or graduated, ie. blocking more light on one side of the image than another, often in a clean gradient.)
** Blur, softening or diffusion (reducing focus from certain portions of an image, often to create a dream-like effect)
** Diopter, close-up or macro (allows a given lens to focus closer to a subject than it is usually capable)
** Bokeh (apply specific geometric shapes to the brightest portions of the bokeh - or highly unfocused image regions)
** Novelty (the classic example are star filters, which alter highly directional lights - also known as point lights - to obtain a cheesy, starry, visual effect; another example are also multivision filters which can provide multiple copies of the subject within the frame. Such filters are generally remnants of the analog era and are now being replaced by software post-processing.)
* Compound imaging modes are modes in which multiple exposures may be combined in to a single image. There are two main types of compound imaging:
** Multiple exposure is a classic analog-era technique in which two or more exposures are made to the same piece of film, often resulting in a dream-like tangle of tones or surreal combinations of subjects.
** High Dynamic Range (HDR) images may be produced in the digital-era by either manual or automated exposure bracketing, a process in which multiple exposures are taken of the same subject with different shutter speeds. These higher and lower exposures are then combined in to a single image, enhancing the effective dynamic range of the image (ie. capturing a broader range of shadow and highlight detail than would otherwise be possible). Some cameras have this feature built-in, others allow the automation of capture but leave the combination of the multiple resulting images to computer-based post-processing.
* Digital-only controls' are those which are artifacts of the era of digital photography.
** Effects are pre-built options on digital cameras allowing for certain types of post-processing within the camera itself. While these may be set before taking an image, much like the basic controls, they are in fact generally only applied by the camera in software after the image has already been captured, and thus are not true image-taking controls.
** Color depth is the amount of information to be stored for each individual pixel (square) of a digital image. It may be expressed in two ways, either:
*** For each color-channel (red, green and blue), or 'bits per channel'. This is the standard means of description in the photographic world.
*** In terms of the total 'bits per pixel' (combining all colour channels), which is a common means of description in the computer imaging world.
       For example, an 8 bit color depth per channel image would have 24 bits per pixel, a 16 bit color depth per channel image would have 48 bits per pixel, and so on. Modern digital image sensors provide at least 16 or 24 bits per channel, though 8 bits per channel is still adequate quality for many purposes and is very common online.
   * Resolution is the amount of pixels (square, component points within an image) to store when taking a photograph. Higher resolution generally creates a higher quality image, though poor lenses, motion or subject blur, long exposures or a constrained publishing resolution may all provide good reasons to reduce resolution at the time of capture. Reducing resolution increases camera image storage speed (increasing the number of images per second that may be stored, for example in burst mode photography of action such as sporting events) as well as overall storage space requirements, image transmission times, etc. It is generally expressed in pixels as width x height, though some cameras provide a simplified set of options based on the conventions "RAW, LARGE, MEDIUM, SMALL" and many support the dual storage of both RAW and processed raster files (usually JPEG) of a selected size.

## Types of cameras

The following terms have been historically used to describe various types of still cameras. These terms are not entirely exclusive (for example, you can have a Single lens reflex or twin lens reflex pinhole studio camera), nor are they necessarily the only terms around. They are included here for reference purposes.

### By primary use
#### Consumer/Prosumer

Consumer cameras are mass produced, mass market cameras designed for a broad range of common use by the general public. Once a significantly distinct form of camera from professional cameras, the combination of the popularization of digital camera technology and the rise of the 'prosumer' (ie. high end consumer) concept has tended to erode the distinction between professional and consumer cameras. In reality, many modern consumer/prosumer cameras are essentially capable of professional output.

#### Professional

Professional camera systems are essentially those not positioned for consumer use. This category includes expensive or specialist cameras utilized for artistic, industrial, or studio uses.

#### Industrial

Industrial cameras are those engineered for repeat utilization, generally as a part of a larger, automated, electronic system. This may include applications such as manufacturing quality control, satellite telescopes, microscopy, or surveillance. In general, industrial applications place a greater emphasis on reliability and a reduced emphasis on breadth of application. They may require extensive knowledge of physics, observed processes or optics to initially configure. They tend to be relatively expensive.

#### Studio

Studio cameras are those optimized for non-mobile applications. Once a relatively separate category of camera, today most studios utilize professional SLR cameras from major manufacturers which may incorporate connections to studio lighting (eg. flash rigs, reflectors) and positioning equipment (eg. tripods), so this term is perhaps falling out of use.

### By lens type
#### Pinhole

The pinhole camera is relatively rare today, but is enjoying a resurgence of casual interest due to its simplicity. It is one of the simplest camera designs possible and has three major components: a light-proof box, a light sensitive material (such as a traditional analog film, or a digital sensor) and opposite the material a hole that light passes through carrying the external image. There is no lens; the aperture is created by punching a small hole opposite where the film is mounted and is very small; and the 'shutter' in more advanced cameras is emulated manually by uncovering and covering the opening. Despite its simplicity, it still has many enthusiasts because of the unique pictures it creates and the imaginative ways of turning ordinary objects into pinhole cameras.
Analog pinhole cameras are very easy to make from scratch for exposing traditional film: the principle is identical to the pioneering camera obscura experiment. Typically, a prefabricated, light-sealed container like a biscuit tin or a match-box can be used. Most digital cameras with changeable lenses can be converted in to pinhole cameras by replacing the lens with a sheet of opaque material with a hole punched in it.
Note that a method exists for calculating the optimum pinhole size: too small or too large and the image will lack definition.

#### Fixed lens

These include most "point-and-shoot" cameras. While the lens on these cameras is not removeable, the focus is often adjustable, whether manually or automatically. These cameras are generally not considered high quality equipment, though several outliers, such as the Rollei 35 are prized for their high-grade optics.

#### Interchangeable lens

Most cameras for professional or advanced amateur use today have the ability to change lenses, depending on the photographer's need. The need for this is largely mooted by the advent of zoom lenses with adjustable focal lengths, but more advanced applications may still require the use of a specialized lens.

### By focal method

Focus is fundamental to photography, a fact that has determined the development of the different broad types of camera. Focus is dependent upon a number of relationships, distance of the subject from the camera being the most important.

#### No focus

Some cameras do not offer the photographer any means to adjust focus. These cameras would typically be of the following types:
* Very simple (lensless), such as pinhole cameras
* Very early, simple or less sophisticated amateur cameras, from the early development of modern photography
* Non-interchangeable, simple, fixed prime lens special purpose cameras (for example, certain early analog spy cameras or those designed to be operated from aerial balloons)
Today these cameras are usually made to simplify construction and lower costs, especially for applications where the subject-to-camera distance is likely to remain constant, such as fixed security cameras or in some technical applications. For general photography applications they are of only passing interest, though a number of artists have worked with them to great effect.

#### Fully manual focus

Many field or view cameras (the sorts of things you see people taking photographs under blankets with in early 20th century or late 19th century movies, and their spiritual successors) provide no automated means for focusing, instead relying on the photographer to manually adjust the focal ring on the lens based upon comparing an estimate distance to numbers marked or engraved there for that purpose. Eventually, separate devices for estimating subject distance became available, known as rangefinders.

#### Rangefinder cameras

Prior to the widespread development of electronic autofocus systems, the dominant focusing technique of the late 20th century was the analog rangefinder, sometimes shortened to RF. In the most common configuration of such a system, the photographer manually aligns two images within a viewport. Once the images are aligned, the camera is said to be in focus, and a subject distance may be displayed or derived. Earlier and cheaper systems including the initial, portable, off-camera systems required the photographer to manually transfer the resulting distance to the configuration of the focus ring on their camera, which would be marked with various distances in feet or meters. Later systems, such as those still produced by companies such as the German manufacturer Leica Camera AG, couple the results to the focusing mechanism of the camera and are known as coupled rangefinder cameras.
Historically, the major advantages of the rangefinder designs are for certain applications. Since there is no moving mirror, as used in SLRs, there is no momentary blackout of the subject being photographed. The camera is therefore often quieter, particularly with leaf shutters, and usually smaller and less obtrusive. These qualities make rangefinders more attractive for theater photography, some portrait photography, candid and street photography, and any application where an SLR is too large or obtrusive. The absence of a mirror allows the rear element of lenses to project deep into the camera body, making high-quality wide-angle lenses easier to design. However, it is important to note that these advantages are now shared by many types of digital cameras and cellphones, which usually do not require manual focus or exposure: for example, 'silent mode' on the Sony α7R II.

#### Autofocus (AF)

The majority of camera systems today provide some means of electronic autofocus (AF), though there are still other camera types produced. Electronic autofocus systems are very complex and can provide unrivaled support for certain photographic situations, for example:
* Action telephoto photography with occasional intermediate obstructions
* Normally moving subjects such as sports people running or walking
* Highly erratically moving subjects, such as darting animals or insects
Autofocus systems are based upon various technologies, a current example of which is 'phase-difference', presently in use (2016) by high-end Canon DSLRs such as the 50.4 Megapixel Canon 5DS.

### By method of optical projection
#### Twin lens reflex

The exact origins of the twin lens reflex (TLR) camera are obscure. Double-lens cameras were around from about 1870, when someone realised that having a second viewing lens alongside the taking lens meant that one could focus without having to keep swapping a ground glass screen for the plate afterwards, reducing the time delay in actually taking the shot.
Where the TLR came into its own was with the idea of using a reflex mirror to allow viewing from above, thus allowing the camera to be held much more steadily if handheld. The same principle of course applied to the single lens reflex, but early SLRs caused delays and inconvenience through the need to move the mirror out of the focal plane to allow light to the plate behind it. When this process was automated, the movement of the mirror could cause shake in the camera and blur the shot.
One of the earliest documented TLRs was made by the firm of R & J Beck of Cornhill, London in 1880 for Mr G M Whipple, a scientist and Superintendent of the Royal Observatory at Kew. It seems the design concept was his - to build a mirror reflex camera for cloud photography. The aim was to have a camera with lenses pointing upwards, but also to be able to compose the picture whilst looking horizontally. It seems this camera also used geared linking to synchronise the lenses, thus having many of the features of later mass-marketed TLRs .
There were a number of other types of TLR marketed between about 1890 and 1910, but these were gradually overtaken as more effective SLRs became available and cured the problem of parallax which bedevilled the TLR. The ability to see and compose the subject exactly in the taking lens outweighed the disadvantage of the moving mirror as SLR mechanisms improved.

#### Single lens reflex

As already discussed, focus is fundamental to photography, both in terms of what is and what is not in focus. A rangefinder camera, which allows one to determine the focusing distance, determines what should be in focus, but without actually demonstrating the degree. The TLR (Twin-Lens Reflex camera) goes one step further, by using a second viewing lens.
However, it is the SLR (Single Lens Reflex camera) that solves the problem fully. In this type of camera a mirror intercepts the light that passes through the lens and projects it onto a ground glass screen where it forms an erect (upright) but mirrored image. Now the photographer is truly viewing through the lens and able to accurately determine both the focus and depth of field. When the photograph is ready to be taken the mirror is retracted allowing the light to pass directly to the film, when the shutter is opened. The earliest models required the mirror to be retracted manually (this disappeared with the Speed Reflex in the mid-1920s), did not have the familiar prism of today, and demanded the viewer to inspect the image through a leather tlunnel to the ground glass screen. Another common feature of SLRs necessitated by their construction was the need for the light to pass through the lens to the reflex mirror unhindered. This lead to the focal plane shutter, where the mechanism is placed just in front of the film.
This is how most people perceive the SLR with the distinctive prism housing on top that first appeared on a Contax camera in 1948.
The prism serves to reflect and flip the mirrored image from the ground glass screen to the viewfinder, resulting in an erect and true image which is bright and often magnified by the viewfinder optics. The use of 35mm film allows these cameras to be relatively compact which removed one of the SLRs drawbacks. With the shutter positioned just ahead of the film within the camera's body, it is possible to change lenses without exposing the film, making the design very flexible. The principal shortcoming is that the focal plane shutter uses a variable gap to vary the shutter speed and that only a longish exposure time will synchronise with flash.

#### View camera

The view camera is of either a monorail design or what is called a flat bed or field camera. The flat bed being an older design and dating back to the middle of the 19th century. In both designs a flexible bellows separates the lens and film. The lens is affixed to a front standard and the film positioned in the rear standard. Both front and rear standards can move horizontally along the rail of a monorail or on tracks in the bed in the case of the flatbed design. In most designs the front and rear standards are equipped to pivot in both the x and y axis independent of each other. These are called "swings" and "tilts". There is usually some allowance for the rise and fall of both front and rear standards along the vertical plane. All of these movements allows for great flexibility in the control of the image.

# Notes

* ↑ See w:The_Brain_That_Changes_Itself, a great book on these recent developments in science.
* ↑ The Australian aboriginal people, whose culture still exists, have paintings made 60,000 and 40,000 years ago.
* ↑ Stated differently, they did not use foreshortening.
* ↑ Physical fragments dated 179-41 BC have been recovered.
* ↑ No pun intended.
* ↑ camera. Collins English Dictionary - Complete & Unabridged 2012 Digital Edition.
* ↑ camera. Macquarie Dictionary, Macmillan Publishers Group Australia, 2014.
* ↑ camera. Merriam-Webster Dictionary, 2016.
* ↑ Even plastic lenses in toy cameras, cheap webcams or mobile phones are superior to lens-less pinhole cameras in terms of the focus and overall quality they can produce.
* ↑ Lenses, which are a swappable component on many cameras, are generally described using their minimum f/ rating (ie. the maximum amount of light which they permit). Lenses with small numbers (such as f/1.0) are described as fast, whereas lenses with big numbers (such as f/6.3) are described as slow.
* ↑ Alpha Dog: Hands on with Sony A7R-II. DPReview.com, 2015.