In
photography
Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed ...
, through-the-lens metering (TTL metering) refers to a feature of
camera
A camera is an Optics, optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), ...
s whereby the intensity of light reflected from the scene is measured through the
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
; as opposed to using a separate metering window or external hand-held
light meter
A light meter is a device used to measure the amount of light. In photography, a light meter (more correctly an exposure meter) is used to determine the proper exposure for a photograph. The meter will include either a digital or analog calcul ...
. In some cameras various TTL
metering mode
In photography, the metering mode refers to the way in which a camera determines exposure. Cameras generally allow the user to select between ''spot'', ''center-weighted average'', or ''multi-zone'' metering modes. The different metering modes all ...
s can be selected. This information can then be used to set the optimal
film
A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
or
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curr ...
exposure (
average luminance), it can also be used to control the amount of light emitted by a
flash unit
A flash is a device used in photography that produces a brief burst of light (typically lasting 1/1000 to 1/200 of a second) at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a ...
connected to the camera.
Description
Through-the-lens metering is most often associated with
single-lens reflex
A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin le ...
(SLR) cameras.
In most film and digital SLRs, the light sensor(s) for exposure metering are incorporated into the
pentaprism
A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism.
The beam reflects inside the prism ''twice'', allowing the transmission of an image through a r ...
or
pentamirror
A pentamirror is an optical device used in the viewfinder systems of various single-lens reflex cameras instead of the pentaprism. It is used to reverse again the laterally reversed image coming from the reflex mirror.
Instead of the solid block o ...
, 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 Olympus OM-2 is a single-lens reflex film system camera produced by Olympus of Japan.
Main features
The Olympus OM-2 is an aperture-priority automatic-exposure camera (with full manual operation selected via switch), based on the earlier, su ...
, the
Pentax LX
The Pentax LX is a 35 mm single-lens reflex camera produced by Pentax in Japan. It was introduced in 1980 and produced until 2001. The LX uses the K mount which is the Pentax proprietary bayonet lens mount. It has manual and aperture pri ...
, the
Nikon F3
The Nikon F3 was Nikon's third professional single-lens reflex camera
A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to ...
, and the
Minolta 9000
The Minolta 9000 AF is a professional Single-lens reflex autofocus camera, introduced by Minolta in August 1985.Durniak, 25 August 1985 It was both Minolta's and the world's first professional autofocus SLR. It was called Minolta Maxxum 9000 in t ...
, 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. 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 camera
A point-and-shoot camera, also known as a compact camera and sometimes abbreviated to P&S, is a still camera designed primarily for simple operation. Most use focus free lenses or autofocus for focusing, automatic systems for setting the exposu ...
s" 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.
History
The first camera to feature through-the-lens light metering was by Japanese company
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
, with a prototype rangefinder camera, the SPX. The camera used Nikon 'S' type rangefinder lenses.
The Japanese company
Pentax
is a brand name used primarily by the Japanese multinational imaging and electronics company Ricoh for DSLR cameras, lenses, sport optics (including binoculars and rifle scopes), and CCTV optics. The Pentax brand is also used by Hoya Corporation ...
was the first manufacturer to show an early prototype 35 mm behind-the-lens metering
SLR camera
A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin l ...
, which was named the
Pentax Spotmatic
The Pentax Spotmatic refers to a family of 35mm single-lens reflex cameras manufactured by the Asahi Optical Co. Ltd., later known as Pentax Corporation, between 1964 and 1976.
All Pentax Spotmatics used the M42 screw-thread lens mount which w ...
. The camera was shown at the
1960 photokina show. The first TTL
light meter
A light meter is a device used to measure the amount of light. In photography, a light meter (more correctly an exposure meter) is used to determine the proper exposure for a photograph. The meter will include either a digital or analog calcul ...
ing SLR was the 1963
Topcon RE Super
The Topcon RE Super, or Beseler Topcon Super D in USA, was launched by Tokyo Kogaku KK in 1963 and manufactured until 1971, at which point it was upgraded to the ''Super D'' and again to ''Super DM'' the following year. General sale continued for ...
, 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.
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
is a brand name used primarily by the Japanese multinational imaging and electronics company Ricoh for DSLR cameras, lenses, sport optics (including binoculars and rifle scopes), and CCTV optics. The Pentax brand is also used by Hoya Corporation ...
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.
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
, trading as Fujifilm, or simply Fuji, is a Japanese multinational conglomerate headquartered in Tokyo, Japan, operating in the realms of photography, optics, office and medical electronics, biotechnology, and chemicals.
The offerings from th ...
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.
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.
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
*
Canon Pellix
The Canon Pellix is a manual-focus single-lens reflex (SLR) camera released in 1965 that uses a stationary half-silvered mirror behind which a metering cell is raised during light level metering.
The First Canon with TTL
Canon suddenly switche ...
* Nikon
Speedlight
Speedlight is the brand name used by Nikon Corporation for their photographic flash units, used since the company's introduction of strobe flashes in the 1960s. Nikon's standalone Speedlights (those not built into the company's cameras) have the ...
*
Pre-flashing
In cinematography and photography, pre-flashing is the exposure of the film or other photosensor to uniform light prior to exposing it to the scene to be imaged. This adds a bias to the overall light input recorded by the sensor.
It is sometim ...
References
{{Reflist
External links
How TTL metering works from "Flash Photography with Canon EOS Cameras" by N. K. Guy
by Moose Petersen
Japanese inventions
Photographic lighting
Photography equipment