Through-the-lens metering

Last updated

In photography, through-the-lens metering (TTL metering) refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light meter. In some cameras various TTL metering modes can be selected. This information can then be used to set the optimal film or image sensor exposure (average luminance), it can also be used to control the amount of light emitted by a flash unit connected to the camera.



Through-the-lens metering is most often associated with single-lens reflex (SLR) cameras.

In most film and digital SLRs, the light sensor(s) for exposure metering are incorporated into the pentaprism or pentamirror, the mechanism by which a SLR allows the viewfinder to see directly through the lens. As the mirror is flipped up, no light can reach there during exposure, the necessary amount of exposure needs to be determined before the actual exposure. Consequently, these light sensors could traditionally be used for ambient light TTL metering only. In newer SLRs as well as in almost all DSLRs, they can also be utilized for preflash TTL metering, where the metering is carried out before the mirror flips up using a small preflash of known intensity and the necessary amount of flash light is extrapolated from the reflected flash light measured by the metering cells in the roof of the camera and is then applied during the exposure without any possible real-time feedback.

There were a few particularly sophisticated film SLRs including the Olympus OM-2, the Pentax LX, the Nikon F3, and the Minolta 9000, where metering cells located at the bottom of the mirror box were used for ambient light metering, depending on model either instead or in addition to metering cells in the roof of the camera. Depending on model, the light was reflected down there either by a secondary mirror behind the half-transparent main mirror, a special reflective coating of the first shutter curtain, the surface of the film itself, or combinations thereof. One of the advantages of this approach is that the measuring result requires no adjustments when changing focusing screens or viewfinders. Also, some of the cameras using this configuration (e.g. the Minolta 9000) are virtually immune against measurement errors caused by light reaching the metering cells at larger angles, for example with tilt–shift lenses.

Metering cells located at the bottom of the mirror box using light reflected off the film are also used in all film SLRs supporting the classical form of real-time TTL flash metering.

Some early Pentax DSLRs could use this same configuration for TTL flash metering as well, but since the reflectance properties of image sensors differ significantly from those of film, this method proved to be unreliable in practice. Therefore, digital SLR cameras typically don't support any real-time TTL flash metering and must use preflash metering instead. The ambient and flash light metering is then carried out by a metering module located in the roof of the camera (see above).

Digital SLRs supporting live view or video will use the read out of the image sensor itself for exposure metering in these modes. This also applies to Sony's SLT digital cameras, which use the image sensor for exposure metering all the time. As of 2012, no digital SLR or SLT camera on the market supported any form of real-time TTL flash metering using the image sensor. However, it can be expected that such methods will be introduced as image sensor technology progresses, given the advantages of metering with real-time feedback and without preflash.

TTL metering systems have been incorporated into other types of cameras as well. Most digital "point-and-shoot cameras" use TTL metering, performed by the imaging sensor itself.

In many advanced modern cameras multiple 'segments' are used to acquire the amount of light in different places of the picture. Depending on the mode the photographer has selected, this information is then used to correctly set the exposure. With a simple spot meter, a single spot on the picture is selected. The camera sets the exposure in order to get that particular spot properly exposed. On some modern SLR systems the spot metering area or zone can be coupled to the actual focusing area selected offering more flexibility and less need to use exposure lock systems. With multiple segment metering (also known as matrix or honeycomb metering), the values of the different segments are combined and weighted to set the correct exposure. Implementations of these metering modes vary between cameras and manufacturers, making it difficult to predict how a scene will be exposed when switching cameras.


The first camera to feature through-the-lens light metering was by Japanese company Nikon, with a prototype rangefinder camera, the SPX. The camera used Nikon 'S' type rangefinder lenses. [1]

The Japanese company Pentax was the first manufacturer to show an early prototype 35 mm behind-the-lens metering SLR camera, which was named the Pentax Spotmatic. The camera was shown at the 1960 photokina show. The first TTL light metering SLR was the 1963 Topcon RE Super, which had the CdS metering cell placed behind the reflex mirror.

Off the film metering

In the 1970s Olympus marketed the OM-2 camera, which measured the exposure directly off the film (OTF). In OTF metering used by Olympus, metering was performed in one of two ways — or a combination of both — depending upon the shutter speed in use. [2]

In the OM-2's Auto Dynamic Metering (ADM) system the first shutter curtain had the lens-facing side coated with a computer generated pattern of white blocks to emulate an average scene. As the mirror flipped-up the metering cell in the base of the mirror box measured the light reflected from the subject bouncing off this pattern of blocks. The timing of the release of the second curtain was adjusted in real time during the actual exposure. As the shutter speed increased, the actual light reflecting off the film surface was measured and the timing of the second curtain's release adjusted accordingly. This gave cameras equipped with this system the ability to adjust to changes in lighting during the actual exposure which was useful for specialist applications such as photomicrography and astronomical photography.

Leica later used a variation of this system, as did Pentax with their Integrated Direct Metering (IDM) in the LX camera. A variation of this "OTF" system was used on early Olympus E-Series digital cameras to fine-tune the exposure just before the first curtain was released; for this to work, the first curtain was coated in a neutral grey colour.

Through the lens flash metering

The process of calculating the correct amount of flash light can also be done 'through the lens'. This is being done in a significantly different way than non-flash 'through the lens' metering. The actual metering itself happens in two different ways, depending on the medium. Digital TTL works differently than analog TTL.

The analog version of TTL works as follows: when the incoming light hits the film, a part of it is reflected towards a sensor. This sensor controls the flash. If enough light is captured, the flash is stopped. [2] During early testing of this system by Minolta and Olympus it was found that not all brands and types of film reflect the light to the same amount although the actual difference between brands was less than half a stop. The one exception was Polaroid's instant slide film which had a black surface and could not be used in TTL flash mode. Nevertheless, for most applications analogue TTL flash exposure metering was more advanced and accurate than systems used previously and permitted far more flexibility - with bounced flash exposures in particular being more accurate than manually calculated equivalents.

With digital, this way of direct reflection metering is not possible any more since a CMOS or CCD chip, used to collect the light, is not reflective enough. There are a few older digital cameras which still use the analog technique, but these are getting rare. The Fujifilm S1 and S3 are the most well known digital cameras to use this technique.

Digital TTL works as follows: Before the actual exposure one or more small flashes, called "preflashes", are emitted. The light returning through the lens is measured and this value is used to calculate the amount of light necessary for the actual exposure. Multiple pre-flashes can be used to improve the flash output. Canon referred to this as Evaluative TTL (E-TTL) and later improved the system with E-TTL II. The first form of digital TTL by Nikon, called "D-TTL", was used in a few early models. Since then, the superior "i-TTL" system has been used. [3] [4]

When using front-curtain flash (when the flash fires immediately after the shutter opens), the preflashes and main flash appear as one to the human eye, since there is very little time between them. When using rear-curtain flash (when the flash fires at the end of the exposure) and a slow shutter speed, the distinction between the main flash and the preflashes is more obvious. [5]

Some cameras and flash units take more information into account when calculating the necessary flash output, including the distance of the subject to the lens. This improves the lighting when a subject is placed in front of a background. If the lens is focused on the subject, the flash will be controlled to allow for proper exposure on the subject, thus leaving the background underexposed. Alternatively, if the lens is focused on the background, the background will be properly exposed, leaving the subject in the foreground typically overexposed. This technique requires both a camera capable of calculating the distance information, as well as the lens being capable of communicating the focal distance to the body. Nikon refers to this technique as "3D matrix metering", although different camera manufacturers use different terms for this technique. Canon incorporated this technique in E-TTL II.

More advanced TTL flash techniques include off-camera flash lighting, where one or more flash units are located at different locations around the subject. In this case a 'commander' unit (which can be integrated in the camera body) is used to control all of the remote units. The commander unit usually controls the remote flashes by using flashes of visible or infrared light, although TTL-capable radio triggering systems are available. The photographer can normally vary the light ratios between the different flashes. The technique of using preflashes to get a proper exposure is still used in automatic flash modes.

See also

Related Research Articles

<span class="mw-page-title-main">Single-lens reflex camera</span> Camera that typically uses a mirror and prism system

A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system that permits the photographer to view through the lens and see exactly what will be captured. With twin lens reflex and rangefinder cameras, the viewed image could be significantly different from the final image. When the shutter button is pressed on most SLRs, the mirror flips out of the light path, allowing light to pass through to the light receptor and the image to be captured.

<span class="mw-page-title-main">Camera</span> Optical device for recording images

A camera is an optical instrument used to capture and store images or videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photography and videography, cameras have played a significant role in the progression of visual arts, media, entertainment, surveillance, and scientific research. The invention of the camera dates back to the 19th century and has since evolved with advancements in technology, leading to a vast array of types and models in the 21st century.

<span class="mw-page-title-main">Focal-plane shutter</span> Mechanism that controls the exposure time in cameras

In camera design, a focal-plane shutter (FPS) is a type of photographic shutter that is positioned immediately in front of the focal plane of the camera, that is, right in front of the photographic film or image sensor.

<span class="mw-page-title-main">Autofocus</span> Optical system to focus on an automatically or manually selected point or area

An autofocus optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication. Autofocus methods are distinguished as active, passive or hybrid types.

<span class="mw-page-title-main">Digital single-lens reflex camera</span> Digital cameras combining the parts of a single-lens reflex camera and a digital camera back

A digital single-lens reflex camera is a digital camera that combines the optics and the mechanisms of a single-lens reflex camera with a solid-state image sensor and digitally records the images from the sensor.

<span class="mw-page-title-main">Nikon FA</span> Camera model

The Nikon FA was an advanced amateur-level, interchangeable lens, 35 mm film, single-lens reflex (SLR) camera. It was manufactured by the Japanese optics company Nippon Kogaku K. K. in Japan from 1983 to 1987. The FA used a titanium-bladed, vertical-travel Nikon-designed, Copal-made focal plane shutter with a speed range of 1 to 1/4000th second plus Bulb and flash X-sync of 1/250th second. It was available in two colors: black with chrome trim and all black. The introductory US list price for the chrome body only was $646. Note that SLRs usually sold for 30 to 40 percent below list price.

<span class="mw-page-title-main">Nikon FE</span> Camera model

The Nikon FE is an advanced semi-professional level, interchangeable lens, 35 mm film, single-lens reflex (SLR) camera. It was manufactured by Nikon in Japan from 1978 to 1983, and was available new from dealer stock until c. 1984. The FE uses a metal-bladed, vertical-travel focal plane shutter with a speed range of 8 to 1/1000 second, plus Bulb, and flash X-sync of 1/125th second. It had dimensions of 89.5 millimetres (3.52 in) height, 142 mm (5.6 in) width, 57.5 mm (2.26 in) depth and 590 grams (21 oz) weight. It was available in two colors: black with chrome trim and all black. As on the FM, its model designation did not appear on the front of the camera, but was engraved as a small "FE" preceding the serial number on the rear of the housing.

<span class="mw-page-title-main">Canon A-1</span>

The Canon A-1 is an advanced-level single-lens reflex (SLR) 35 mm film camera for use with interchangeable lenses. It was manufactured by Canon Camera K. K. in Japan from April 1978 to 1985. It employs a horizontal cloth-curtain focal-plane shutter with a speed range of 30 to 1/1000 second plus bulb and flash synchronization speed of 1/60 second. It has dimensions of 92 millimetres (3.6 in) height, 141 millimetres (5.6 in) width, 48 millimetres (1.9 in) depth and 620 grams (22 oz) weight. Unlike most SLRs of the time, it was available in only one color; all black. The introductory US list price for the body plus Canon FD 50 mm f/1.4 SSC lens was $625; the camera body was generally sold with a 30–40% discount.

<span class="mw-page-title-main">Nikon FG</span> Camera model

The Nikon FG is an interchangeable lens, 35 mm film, single-lens reflex (SLR) camera. It was manufactured by Nippon Kogaku K. K. in Japan from 1982 to 1986.

<span class="mw-page-title-main">Canon T90</span>

The Canon T90, introduced in 1986, was the top of the line in Canon's T series of 35 mm Single-lens reflex (SLR) cameras. It is the last professional-level manual-focus camera from Canon, and the last professional camera to use the Canon FD lens mount. Although it was overtaken by the autofocus revolution and Canon's new, incompatible EOS after only a year in production, the T90 pioneered many concepts seen in high-end Canon cameras up to the present day, particularly the user interface, industrial design, and the high level of automation.

The Olympus OM System was a line of 35mm single-lens reflex cameras, lenses and accessories sold by Olympus between 1972 and 2002. The system was introduced by Olympus in 1972. The range was designed by Yoshihisa Maitani, chief designer for Olympus, and his staff; OM stands for Olympus Maitani.

<span class="mw-page-title-main">Olympus OM-2</span>

The Olympus OM-2 is a single-lens reflex film system camera produced by Olympus of Japan.

<span class="mw-page-title-main">History of the single-lens reflex camera</span> Aspect of photography history

The history of the single-lens reflex camera (SLR) begins with the use of a reflex mirror in a camera obscura described in 1676, but it took a long time for the design to succeed for photographic cameras. The first patent was granted in 1861, and the first cameras were produced in 1884, but while elegantly simple in concept, they were very complex in practice. One by one these complexities were overcome as optical and mechanical technology advanced, and in the 1960s the SLR camera became the preferred design for many high-end camera formats.

<span class="mw-page-title-main">Pentax LX</span>

The Pentax LX is a 35 mm single-lens reflex camera produced by Pentax in Japan. It was introduced in 1980 to commemorate the 60th anniversary of Asahi Optical Co., and was produced until 2001. The LX uses the K mount, which is the Pentax proprietary bayonet lens mount, and has manual and aperture priority automatic exposure modes. It is the top-of-the-line professional, or "system", camera in the Pentax manual focus range, and has a large body of accessories. Compared with contemporary professional camera bodies from rival manufacturers, like the Canon New F-1 or Nikon F3, the LX body is smaller and lighter, weighing in at 570 grams with its standard FA-1 finder.

<span class="mw-page-title-main">Olympus OM-4</span> Interchangeable-lens, 35 mm film single lens reflex (SLR) camera

The Olympus OM-4 is an interchangeable-lens, 35 mm film, single lens reflex (SLR) camera; manufactured by Olympus Optical Co., Ltd. in Japan, and sold as OM-4 from 1983 to 1987 and as OM-4Ti from 1986 to 2002.

<span class="mw-page-title-main">Live preview</span>

Live preview is a feature that allows a digital camera's display screen to be used as a viewfinder. This provides a means of previewing framing and other exposure before taking the photograph. In most such cameras, the preview is generated by means of continuously and directly projecting the image formed by the lens onto the main image sensor. This in turn feeds the electronic screen with the live preview image. The electronic screen can be either a liquid crystal display (LCD) or an electronic viewfinder (EVF).

<span class="mw-page-title-main">Pentax ME F</span>

The Pentax ME F was an amateur level, interchangeable lens, 35 mm film, single-lens reflex (SLR) camera. It was manufactured by Asahi Optical Co., Ltd. of Japan from November 1981 to 1984. The ME F was a heavily modified version of the Pentax ME-Super, and a member of the Pentax M-series family of SLRs. It was the first mass-produced SLR camera to come with an autofocus system.

<span class="mw-page-title-main">Lenses for SLR and DSLR cameras</span>

This article is about photographic lenses for single-lens reflex film cameras (SLRs) and digital single-lens reflex cameras (DSLRs). Emphasis is on modern lenses for 35 mm film SLRs and for DSLRs with sensor sizes less than or equal to 35 mm ("full-frame").

This article discusses the cameras – mainly 35 mm SLRs – manufactured by Pentax Ricoh Imaging Corp. and its predecessors, Pentax Corporation and Asahi Optical Co., Ltd.. Pentax must not be confused with Pentax 6x7 or Pentax 67 which are 120 medium format 6x7cm film cameras.

<span class="mw-page-title-main">Canon EOS 500D</span> Digital single-lens reflex camera

The Canon EOS 500D is a 15-megapixel entry-level digital single-lens reflex camera, announced by Canon on 25 March 2009. It was released in May 2009. It is known as the EOS Kiss X3 in Japan, and as the EOS Rebel T1i in North America. It continues the Rebel line of mid-range DSLR cameras, is placed by Canon as the next model up from the EOS 450D, and has been superseded by the EOS 550D (T2i).


  1. Stephen, Gandy. "Nikon Shibata Book". Stephen Gandy's CameraQuest. Retrieved 2008-06-08.
  2. 1 2 "Olympus OM Flash Photography - Part 1". Retrieved 23 April 2018.
  3. The Nikon Creative Lighting System: Wireless, Remote, Through-the-Lens Metered (iTTL) Flash! Imaging Resource
  4. Guide to Nikon TTL Flashes
  5. "voorflitsen". Retrieved 23 April 2018.