A single-lens reflex camera or SR camera for short is a camera with a fold-away mirror between the lens and the image plane . Before taking the picture, the picture is reversed on a screen that is usually horizontal . With older cameras it is viewed from above either directly or with the help of a magnifying glass ( light shaft finder ). Later, prism viewfinders came into use, with the help of which the image can be viewed through an eyepiece, right side up and horizontally or parallel to the optical axis of the camera .
A variant of the single-lens reflex ( English single-lens reflex , SLR ) Standard form is the twin-lens ( English twin-lens reflex , TLR ) camera. It has its own viewfinder beam path through a second lens ("second eye") attached above the main lens, followed by the mirror, focusing screen and usually a light shaft. Both cameras are only comparable due to the viewfinder image on a horizontal screen.
SLR cameras with digital recording sensor are usually short as DSLR ( English for digital single-lens reflex ) or DSR (digital SLR) referred. Sometimes this term is mistakenly used for all digital system cameras, although there are also mirrorless models .
History and Development
The first SLR camera was designed by Thomas Sutton in 1861 . In 1893 an interchangeable magazine for the reflex camera was patented. The first SLR camera manufactured in Germany was the Zeus mirror camera and came from Richard Hüttig's factory in Dresden .
One of the first reflex cameras with a folding mechanism was produced by Fritz Kricheldorff (* 1865, † 1933) from Berlin (see Julius Kricheldorff ): around 1895 he developed the first folding mirror reflex camera . He applied for a patent for his mirror reflex folding camera model 1910 .
The first 35mm SLR camera in the world was the Kine Exakta from Ihagee in Dresden, presented at the Leipzig spring fair in 1936. Its designer was Karl Nüchterlein (1904–1945). Like all SLR cameras with a light shaft viewfinder, these cameras have the disadvantage that the viewfinder image is reversed ( axis reflection ). Kurt Staudinger compensated for this in August 1931 with the invention of the side-reversing roof prism . However, this was not installed in the Contax S ( Zeiss Ikon ) and Rectaflex (Italy) until 1949 (series production) .
Jenő Dulovits patented the first SLR viewfinder for viewing at eye level with a laterally correct, upright image in Hungary on August 23, 1943 - with the Duflex he also designed the first 35mm SLR camera for this viewfinder, which is common today - however he did not use a roof prism, but individual mirrors. This camera also had the first swing-back mirror , with the help of which the viewfinder image becomes visible again immediately after the picture is taken.
The light passing through the lenses of the objective (1) is reflected upwards in a single-lens reflex camera from the inclined oscillating mirror (2) and reaches the focusing screen (5) of the viewfinder system. With a converging lens ( field lens ) (6) and the reflection within the roof prism (7), the image is finally visible in the viewfinder (8). In this way, exactly the same image appears in the viewfinder that is recorded when the shutter button is pressed.
Some SLR cameras use a light shaft finder or a Porro mirror finder instead of a prism finder with a roof prism (7) . However, the principle remains the same. A diopter compensation in the eyepiece permits adaptation to vision defects of the user.
In addition to the focusing screen, which shows the photographer the image to be taken, the mirror system also sends the image to the sensors for exposure metering and the autofocus system . These can be integrated in the viewfinder or, supplied via an additional auxiliary mirror, sit in the bottom of the camera (e.g. with the Nikon F3 ).
When you press the shutter release button, the mirror system must first be swiveled out of the light path (indicated by an arrow in the picture). Then the closure (3) opens ; the image now reaches the film plane (4) or the film or image sensor . Therefore, no image can be seen in the viewfinder during the exposure.
The necessary swiveling up of the mirror results in a small time delay in recording. In order to eliminate this, a permanently mounted, partially transparent mirror or a prism is used in some special designs ( e.g. Canon Pellix ) instead of the oscillating mirror. This enables significantly faster exposure sequences, especially with motor-driven cameras, but also provides a less bright viewfinder image and lets less light through to the sensor or film, since the mirror splits the light both in viewfinder mode and during exposure. Usually around a third of the light is reflected in the viewfinder and two thirds let through to the sensor or film.
Since the early 1980s, auto focus systems have increasingly been built into single lens reflex cameras. The first camera housing built in series was the Pentax ME F model . The main mirror is made semitransparent and a second auxiliary mirror, coupled to the main mirror, directs the light transmitted through the main mirror to one or more separate autofocus sensors in the bottom of the camera. The error signal obtained from the sensors is transmitted mechanically or electrically from the camera housing to the lens, which moves the lenses responsible for focusing.
To take the picture, the mirrors have to be folded away so that no further autofocus measurements can be made via the autofocus sensors in the last tens of milliseconds before the picture is taken. In the case of newer digital single-lens reflex cameras, an additional autofocus measurement is increasingly being carried out via the image sensor so that, for example, the sharpness can be measured while using the live view or during a video recording.
Basically, a distinction is made between two types of single-lens reflex cameras : single and double-lens reflex cameras .
Two-lens reflex camera
The two-lens reflex camera ( twin lens reflex , TLR) always has two lenses of the same focal length on its front . Here the film is exposed through the first (lower) lens . This taking lens always has a central shutter . The second (upper) lens projects a reversed image onto a focusing screen via a mirror. Often the viewfinder lens has a simpler design for reasons of cost, but has a higher light intensity than the taking lens in order to ensure the brightest possible viewfinder image and to simplify focusing. Both lenses are moved in parallel via the distance adjustment mechanism so that the focus can be set using the focusing screen.
Typical representatives are Rolleiflex and Mamiya C , with only the Rolleiflex being produced in three variants for medium format and one for Minox small format.
This type of camera has a number of advantages:
- The viewfinder image is always visible and is not darkened by the working aperture;
- the recording noise is very quiet and
- the triggering of the camera shutter causes practically no vibrations.
There are some disadvantages to this:
- Complex lenses are not implemented for reasons of cost, since they would be required twice;
- a parallax error occurs , which is particularly noticeable in close-up or macro shots, as the optical axes of the two lenses are shifted against each other.
Today, two-lens cameras only play a subordinate role, primarily for nostalgics and collectors. Single-lens reflex cameras have established themselves in practical photography. However, some models of two-lens reflex cameras with very high quality lenses are available on the second-hand market at prices that allow an affordable entry into medium format photography.
Single lens reflex camera
The single lens reflex camera ( SLR) has a folding mirror (swing-back mirror) and usually a roof prism , more rarely a light shaft, above the focusing screen as a viewfinder. Before and after the picture is taken, the image is projected via the mirror onto the focusing screen and can be viewed correctly and upright via the roof prism. The mirror is only folded up or to the side at the moment the picture is taken so that it is no longer in the way of the film level and the film can be exposed when the shutter is released.
The main advantage of the single-lens reflex camera is the ability to use interchangeable lenses (e.g. wide-angle and telephoto lenses). In most cases , the shutter is a focal plane shutter that is located directly in front of the film plane, so that the interchangeability of the lenses is guaranteed. Exceptions in the area of medium format cameras (such as Hasselblad ) use a combination of focal plane shutter and central shutter , which is contained in the lens.
Due to the oscillating mirror, there is a fairly large minimum distance between the film plane and the rear lens of the objective. The retrofocus design is used for short focal lengths (for small images below about 40 mm), which makes the lenses more complex and expensive. The image quality can also suffer from the additional lens elements.
Another disadvantage is that the swing mirror causes the camera to vibrate. The mirror also darkens the viewfinder image for the duration of the exposure.
Since it is difficult to evaluate the image on the focusing screen when the aperture is darkened , the open aperture measurement was developed, with which the working aperture selected on the lens is automatically set shortly before the shutter is released (so-called automatic spring aperture, or ASB for short). During the light measurement, the correction of the aperture is transferred to the exposure meter in the housing via special electronics, or a measurement is carried out with the working aperture. To assess the depth of field , the aperture of some devices can be closed manually to the working aperture value. In contrast, the open aperture is optimal for assessing the distance setting, since the depth of field is minimal with it.
In the small picture format 24 mm x 36 mm only SLR cameras are in use. Even in the medium format from 45 mm × 60 mm, despite their significantly higher prices, they have largely displaced the two-lens models because they are mainly used in the professional sector and the absence of parallax errors as well as a more free choice of lenses and accessories are decisive here.
The modern film camera is a single-lens reflex camera. Instead of the roof prism, it has correction optics so that the ground glass image remains correct and upright even when the finder tube is swiveled. A mirrored wraparound fastener corresponds to the oscillating mirror.
Digital SLR cameras
Digital camera systems with single-lens reflex cameras are also referred to as DSLR or D-SLR ( digital single lens reflex ). DSLRs are very similar to their analog counterparts in terms of their mechanical structure, but instead of a film they house an image sensor ( CCD , CMOS or active pixel sensor ).
The main advantage of digital cameras compared to their film-based predecessors is the direct availability of image data, as the time-consuming development of film material is no longer necessary. The built-in display makes it possible to view the photos immediately after they have been taken, which means that a failed or incorrectly exposed photo can be recognized within the scope of the possibilities that the size and quality of the display allow. Most cameras can also show a histogram (frequency distribution) of the brightness, which makes it easier to examine the image for underexposure or overexposure and is independent of the display's rendering properties. Further help are over- and underexposure warnings, in which the incorrectly exposed areas of the image are highlighted flashing.
As with traditional SLR cameras, most manufacturers use their own lens and accessory systems, which is why DSLR users are largely confined to this system after choosing a particular make. In some cases, lenses from one supplier can also be used on DSLRs from other manufacturers. There is often the option of using bayonet locks from other manufacturers via adapter rings, although various automatic functions may only be partially supported or not supported at all. Some manufacturers do not use a specially developed bayonet system, but license an existing one, so that optics can also be used on other cameras. For example, Fujifilm installs the F bayonet developed by Nikon on its own DSLR .
Compared to 35mm film, many DSLRs use a smaller image sensor, which means that a smaller angle of view is used for a given focal length . In order to achieve the same image section on such a camera as with a 35mm camera, a lens must have a focal length that is shorter by the format factor (also known as “ to crop ”). Typical values for this format factor, which is often incorrectly referred to as the “focal length extension factor”, are the APS-C sensors, × 1.5 (Nikon, Sony / Minolta, Pentax, Samsung), × 1.6 (Canon) or × 2 (Olympus, Panasonic ). This means that with a 50 mm lens on a camera with a format factor of 1.5, the image section is as large as that of a 75 mm lens on a 35mm SLR camera.
Basic problems of digital SLR cameras are dust and other soiling of the image sensor. When changing lenses, dust can get into the mirror box, which can be deposited on the exposure sensor during subsequent shots. Mechanical abrasion or the finest droplets of lubrication from the mirror and locking mechanism can also be reflected. While in analog cameras, the impurities are removed sooner or later via the film transport, they remain as deposits on the image sensor and, with small apertures, are more or less clearly visible shadows on all subsequent images. DSLR manufacturers offer various technical processes to reduce this problem.
Some photographers see DSLRs only as a compromise solution, as originally, due to the design, no live preview ( live view ) of the image on the display was possible. As early as the late 1990s, SLRs were available with a permanently attached lens and a semi-transparent mirror, which enabled the viewfinder image to be viewed both in the optical viewfinder and on the display on the back of the camera. After a break, the concept was continued by Sony in 2010, but here with an electronic viewfinder.
Since around 2009, almost every DSLR manufacturer has had cameras in their range that enable a live preview. Limited by their small size, relatively low resolution and display delays, most preview displays cannot currently (as of 2012) be regarded as a full replacement for the SLR viewfinder, but they do complement it in some shooting situations. If the actual recording sensor is also used for the live preview, it will heat up because it is continuously supplied with power and not only during the recording, which leads to higher noise.
The "Electro-Optic Camera" from Kodak from 1987 is considered the first DSLR .
A video DSLR (also: VDSLR, HDSLR) is understood to be a DSLR (sometimes corresponding mirrorless system cameras are also incorrectly referred to as that) that is able to record videos in addition to photographs.
The starting point for the development of video DSLRs was the integration of so-called " live view " screens in SLR cameras, which began around 2005 and in which the image could also be examined on an LC display instead of looking into the reflex viewfinder. This required closures that could be opened permanently in order to receive an image on the image sensor , which was then continuously read out and displayed on the screen. With the D90 , Nikon presented the first video-capable DSLR at photokina on August 27, 2008 , Canon followed with the EOS 5D II. Most DSLRs now have a video mode, but not all parameters can be controlled manually on all models. The recording time of the DSLRs is usually limited to 4 GiB due to the FAT file system , which corresponds to about 5 to 30 minutes depending on the resolution, image sequence and codec used . DSLRs heat up in video mode similarly to live view mode, which increases the noise from the image sensor.
Video DSLRs are also used for commercials and short films, and sound is usually recorded using an external audio recorder . The way it works can be compared to a classic film camera. The additional equipment of a film camera such as focus tracking and matte box is also used. On May 17, 2010, an episode of the series Dr. Greg Yaitanes shot completely with DSLRs by director Greg Yaitanes was released. House , the sixth season finale.
Advantages of SLR cameras
One advantage over rangefinder cameras or cameras with separate viewfinders is the correspondence of the viewing and recording axes, i.e. what the photographer sees in the viewfinder is shown from the same viewing angle and with the same perspective on the film or image sensor without parallax shift .
Disadvantages of SLR cameras
Immediately noticeable disadvantages are the significantly higher weight and size, especially with fast lenses. The mirror mechanism makes additional noise. This is a hindrance when taking photos in places where absolute silence is required, for example in the theater. In addition, swiveling the mirror can cause the camera to vibrate and the resulting blurring of the image. This can be counteracted with a mirror lock-up.
Using the mirror may result in a focusing error when setting the distance , since the image focus is not determined in the image plane of the film or the image sensor , but with the aid of a focusing screen or a separate focus sensor. In the case of image field curvature of the lens used, manufacturing tolerances, misalignments or in the case of inferior production, these adjustment levels do not represent the image plane geometrically with sufficient accuracy , so that the images or image areas are recorded blurred. In the case of a single-lens reflex camera with a 24-megapixel image sensor in full format , the pixel pitch is 6 micrometers, for example. If a highly opened lens with a focal number of 1.2 is used, the image widths of the focusing screen, autofocus sensor and image sensor would have to be adhered to within 5 micrometers in all image points so that the focusing error does not become larger than the pixel size.
As long as the mirror covers the image sensor, a digital single lens reflex camera cannot be operated in live view . Then no corresponding exposure histograms can be determined and displayed. However, these two restrictions do not apply to some of today's DSLRs that have a second image sensor for this purpose, as built into some Sony DSLRs. Furthermore, it is also not possible to adjust the sharpness based on a contrast measurement with the image sensor, for example in face recognition , with focus peaking or the sharpness tracking of defined patterns in the image. Since single-lens reflex cameras with fixed, partially transparent mirrors (English pellicle mirror ) were introduced in the 1960s , which replaced the oscillating mirror mechanism, the use of the two optical paths to the sensor or to the film plane or to the viewfinder was no longer restricted to an "either-or". but both paths could be used at the same time.
Due to the comparatively large back focus , retrofocus lenses must always be used for larger angles of view . The lenses designed for the large focal length also have a small opening angle on the image side , so that the beam path on the image side can generally not be designed to be telecentric . When exposing image sensors, this leads to linear aberrations in the corners of the image, where the main rays are incident relatively obliquely, due to infrared filters , optical low-pass filters , microlens arrays and color mosaics , which lead to blurred pixels.
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- Advertisement from Fritz Kricheldorff
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