Through-the-lens
Encyclopedia
Through-the-lens metering is a photographic
term describing a feature of camera
s capable of measuring light
levels in a scene through their taking lens
es, as opposed to a separate metering window. This information can then be used to select a proper exposure
(average luminance
), and control the amount of light emitted by a flash
connected to the camera.
(SLR) cameras. A light sensor can be incorporated into the pentaprism
or pentamirror
, the mechanism by which an SLR allows the viewfinder to see directly out the lens. However, TTL metering systems have been incorporated into other types of cameras. All digital "point-and-shoot cameras" use TTL metering, performed by the imaging sensor itself. With digital SLR cameras, the metering is typically carried out by a metering/autofocus module located at the bottom of the mirror chamber, underneath the mirror, however other methods exist as well. Sony's
'SLT' cameras use a fixed semi-transparent mirror to bounce a small part of the light towards the autofocus and light metering module, located at the top of the camera.
In the 1970s Olympus marketed a camera (OM-2) which measured the exposure directly off the film (OTF metering). Leica later used a variation of this system as well. 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 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.
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 and the first curtain was coated in a neutral grey colour.
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 digital 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 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 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 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 known 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 refers to this technique as "E-TTL" and has 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.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
term describing a feature of camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...
s capable of measuring light
Light meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...
levels in a scene through their taking lens
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...
es, as opposed to a separate metering window. This information can then be used to select a proper exposure
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
(average luminance
Zone system
The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer. Adams described how the Zone System was developed: "I take this opportunity to restate that the Zone System is not an invention of mine; it is a codification...
), and control the amount of light emitted by a flash
Flash (photography)
A flash is a device used in photography producing a flash of artificial light at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a dark scene. Other uses are capturing quickly moving objects or changing the quality of light...
connected to the camera.
Description
Through-the-lens metering is most often associated with single-lens reflexSingle-lens reflex camera
A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...
(SLR) cameras. A light sensor can be incorporated into the pentaprism
Pentaprism
A pentaprism is a five-sided reflecting prism used to deviate a beam of light by 90°. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it as an ordinary right-angle prism or mirror would.The reflections inside the prism are not...
or pentamirror
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 upside-down and laterally reversed image coming from the reflex mirror....
, the mechanism by which an SLR allows the viewfinder to see directly out the lens. However, TTL metering systems have been incorporated into other types of cameras. All digital "point-and-shoot cameras" use TTL metering, performed by the imaging sensor itself. With digital SLR cameras, the metering is typically carried out by a metering/autofocus module located at the bottom of the mirror chamber, underneath the mirror, however other methods exist as well. Sony's
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....
'SLT' cameras use a fixed semi-transparent mirror to bounce a small part of the light towards the autofocus and light metering module, located at the top of the camera.
In the 1970s Olympus marketed a camera (OM-2) which measured the exposure directly off the film (OTF metering). Leica later used a variation of this system as well. 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 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.
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 and the first curtain was coated in a neutral grey colour.
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 digital 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 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.
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 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
Fujifilm
is a multinational photography and imaging company headquartered in Tokyo, Japan.Fujifilm's principal activities are the development, production, sale and servicing of color photographic film, digital cameras, photofinishing equipment, color paper, photofinishing chemicals, medical imaging...
S1 and S3 are the most known 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 refers to this technique as "E-TTL" and has 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.
External links
- How TTL metering works, from "Flash Photography with Canon EOS Cameras" by N. K. Guy
- The TTL Flash System, by Moose Petersen