Exposure meter

An instrument that can calculate the combination of aperture and shutter speed according to the brightness of a scene when taking a photograph. The word "exposure" generally used in photography refers to the process of giving the film the most suitable amount of light to expose it when taking a photograph. To take a good photograph, the brightness of the subject must be judged correctly and the film must be given the appropriate exposure. However, this brightness can change dramatically depending on many conditions, such as the season, time, weather, and shooting distance, making it very difficult. For this reason, various tools have been devised since ancient times to determine exposure, but today, electric exposure meters that measure brightness electrically using light-receiving elements such as silicon photocells are widely used, and the term "exposure meter" has come to refer to these electric exposure meters. Currently, most cameras have these built in, and they are set up for automatic exposure.

In the early days, selenium photocells, which did not require a power source, were used as the light receptors in exposure meters, but later CdS cells became mainstream, which have the advantages of being highly sensitive and allowing the light receiving angle and light receptor to be made smaller. However, currently, silicon photocells, which have even faster and more responsive reactions, are widely used as light receptors in exposure meters, AE cameras, automatic light control strobes, and other devices.

Exposure meters built into cameras are of the reflected light type, which measures the light reflected from the subject to determine the exposure, and the majority have the light receiving part located on the front of the camera. Meters that place the light receiving part behind the lens and measure the light that passes through the lens to determine the exposure are called TTL (through the taking lens) types, and are widely used in exposure meters for single-lens reflex cameras. In addition to the reflected light type mentioned above, there is also an incident light type, which measures the brightness of the light source to determine the exposure, as a metering method for standalone exposure meters. When exposing color reversal film, it is often better to use the incident light type to obtain better results.

[Poetry by Ito]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

写真撮影に際し、その場の明るさに応じた絞りとシャッター速度の組合せを算出することのできる計器。一般に写真で使われる「露出」ということばは、撮影のときフィルムが感光するのに、もっとも適した光量をフィルムに与える操作をさす。そしてよい写真を撮るためには、被写体の明るさを正しく判断し、適切な露出をフィルムに与えなければならない。しかしこの明るさは、季節、時間、天候、撮影距離など、数多くの条件によって激しく変化するので非常にむずかしい。そのため古くから露出を決める道具が種々考案されてきたが、現在ではシリコンフォトセルなどの受光素子を使って、電気的に明るさを測る電気露出計が広く使われ、単に露出計といえば、この電気露出計をさすようになった。なお現在ほとんどのカメラにはこれが組み込まれ、自動露出化されている。

　露出計の受光体には、初期は電源の要らないセレン光電池が使われたが、その後、感度が高く、受光角や受光体を小さくつくれるなどの利点があるCdSセルが主流となった。しかし現在では、さらにより反応速度が速く応答性のよいシリコンフォトセルなどが、露出計をはじめ、AEカメラや自動調光ストロボなどの受光体として広く使われている。

　カメラに組み込まれた露出計は、被写体からの反射光を測定して露出を決める反射光式といわれるもので、大多数は受光部がカメラの前面にある。受光体をレンズの後方に置き、レンズを透過してきた光を測って露出を決めるものをとくにTTL（through the taking lens）方式とよび、一眼レフなどの露出計として広く採用されている。単体露出計の測光方式には、前述の反射光式のほかに、光源の明るさを測って露出を決める入射光式とよばれるものもある。カラーリバーサルフィルムへの露出は、この入射光式を使うほうがよい結果を得られることが多い。

［伊藤詩唱］

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