science

Through-the-lens metering

Through-the-lens metering is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Through-the-lens metering rather than just read about it. In short: 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.

Key takeaways

Through-the-lens metering belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Through-the-lens metering to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Through-the-lens metering from memory before moving on to harder problems.

Reference excerpt

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.

Description 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.

History 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. 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.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Through-the-lens metering

Start with the simplest possible case. Write down what Through-the-lens metering claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Through-the-lens metering before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Through-the-lens metering ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Through-the-lens metering

In research: Through-the-lens metering appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Through-the-lens metering in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Through-the-lens metering is common in secondary-school and first-year university syllabi. It links to neighbouring topics Japanese inventions, Photographic lighting, Photography equipment, so understanding it makes those chapters shorter.
In everyday life: Look for Through-the-lens metering outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Through-the-lens metering in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Through-the-lens metering means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Through-the-lens metering out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Through-the-lens metering in simple terms?

Why does Through-the-lens metering matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Through-the-lens metering?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Through-the-lens metering.