Exposure metering

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In photography, exposure measurement is the collective term for various methods for determining the appropriate combination of shutter speed and aperture with which a correctly exposed image can be created.

All photographic recording and reproduction processes, regardless of whether they are negative film , slide film , photo paper or digital cameras , have two characteristic properties: the light sensitivity of the film or the image sensor and the maximum range of contrast that can be displayed . The aim of exposure measurement is to find an optimal compromise between the possibilities of the material used on the one hand and the lighting conditions and the reflection and contrast properties of the subject on the other.

The exposure is the integral of the illuminance (in lx) over the exposure time (in s) ⇒ . In the case of time-invariant continuous light, the following applies ${\ displaystyle H}$ ${\ displaystyle E}$ ${\ displaystyle t}$ ${\ displaystyle H = \ int E (t) dt}$ ${\ displaystyle H = E \ cdot t}$

Measurement method

There are different forms of exposure measurement:

Object measurement (also: luminance measurement ) with the camera - the light reflected or emitted by the object is measured here. Object measurement is the most widely used measurement method that can also be found in most cameras. Measurements are taken either through the lens using the TTL method (TTL: through the lens), or using a light meter on the camera housing.

Variations: metering , spot metering , evaluative metering and matrix metering , detailed measurement , two and multi-point measurement .

Object measurement by a light meter in the computer flash.

Light measurement - the light incident on the object is measured here. It usually requires specially equipped hand-held exposure meters with a spherical cap , but with suitable accessories it is also possible with some cameras. With the help of a gray card , a so-called substitute measurement is also possible with cameras or simpler hand-held exposure meters without a measuring cap .

variants

Classic automatic exposure measurements are based on simple average values: A landscape subject with a clear, blue sky is assumed, which is largely illuminated from the front (at 35 to 55 ° of the sun) and reflects 18% of the incident light in the direction of the camera.

Modern computer-aided exposure metering methods ( multi-field or matrix metering ), on the other hand, attempt to use a motif database or the motif contrast to identify a given light situation that also deviates from the norm - and can often deliver surprisingly good results.

A light meter. At the front the push-in diffuser cap for light measurements.

External light meter

For light measurement , the light falling on the object is measured from the object / picture in the direction of the camera. For this purpose, the measuring device, the exposure meter , must be specially equipped and have a spherical cap or diffuser , which is pushed over the measuring cell as a white hemisphere or blind. From the light falling on the subject, conclusions can be drawn about the light reflection in the direction of the camera. This measuring method is independent of the subject and its contrast.

TTL

In the TTL -Belichtungsmessung (English through the lens , through the lens' also inside measurement ) is measured that amount of light which actually arrives behind the lens. The sensor (measuring cell) can be located at different points in the beam path within the camera. B. above the screen.

The term was introduced to differentiate between cameras with an external measuring cell, the accuracy of which is limited because of the measurement base that does not match the image section and because of possible interference. The first camera with internal measurement was the Topcon RE Super from Topcon; the first SLR cameras with TTL exposure metering, the Pentax Spotmatic from Asahi Pentax, came onto the market in 1964.

When measuring an object , the camera or a light meter determines its light emission ( remission ) towards the object / motif . A spot meter with a measuring angle of 1–5 ° can be helpful here: This allows a mean value to be obtained from several measurements of different subject brightness ( multi- spot measurement ). This procedure is primarily dependent on the remission of the subject and the subject contrast. The latter must also be analyzed and assessed.

Substitute measurement

method

An alternative to the photographic determination of the light value is the determination of the measured value on a gray card , which , if possible, is held within the optical axis, parallel to the film plane , directly in front of the subject. This method is often used in artistic photography or studio photography . The method of targeted underexposure or overexposure, which can be used with the help of the substitute measurement, is also found in another form in the zone system .

Neutral gray and gray card

The unit for exposure measurement is the light value - equivalent to an aperture level of the lens, a time level of the camera shutter, or a sensitivity level of the film (of the sensor, with digital cameras).

A light meter provides information on an existing amount of light it on the light value -aperture combination time sets and the set film (sensor) Sensitivity to a related, which is necessary to a subject (in part) a mean gray value according to expose.

The mean gray value is defined as an area with a light emission of 18% - the mean reflectance value (or "Zone V"), between unmarked white and deep black; based on a reproducible contrast range of five exposure levels.

The gray card serves as an aid for correct exposure measurement, which can be used as a substitute if the subject itself is rich in contrast above average; thus consists of a large number of different gray areas. It is defined as neutral gray (without color cast) and an exact remission of 18%, i.e. 18% of the incident light is reflected back. The same thing is done by the dome or the diffuser on hand-held exposure meters , which are slid over the measuring cells of the devices as a white hemisphere (or blinds): Here, too, exactly 18% of the light is measured. With which a so-called light measurement - a direct measurement from the subject to the light source - can be realized.

Exposure compensation

When measuring the object, an exposure value is determined from the camera to the object (subject), which corresponds to the settings on the camera that would be necessary to capture this subject on the film with a medium brightness, a medium gray value . For subjects whose remission deviates from this, i.e. predominantly white (e.g. snowy landscapes) or black (e.g. coal heaps), the exposure value determined does not correspond to the correct setting. Only in the case of an averagely illuminated and contrasting motif (e.g. a landscape with the sun behind you) can the mean value of all light and dark areas be used as a largely valid standard for exposure measurement. Otherwise a determined value must be corrected.

When measuring light, on the other hand, the determined value is suitable for photographing a motif regardless of the contrast (scope), i.e. also white as white and black as black. Caution should be exercised here if the exposure range of the subject is greater than that of the film.

The exposure compensation is a term used in photographic technology and is the process of consciously departing from the exposure value that the light meter of the still camera displays. This deviation or "correction" makes the image lighter or darker than the automatic exposure control or the light meter dictates. The unit of measurement for exposure correction is the exposure value (abbreviation: EV, German: light value ).

Application examples

Without exposure compensation
With + 2EV exposure compensation

An exposure correction is always indicated if the subject brightness does not correspond to the standardization of the automatic exposure . Examples are:

High-key motifs : A bright object stands in front of a light background (white poodle in the snow). The automatic exposure would render the white objects in the image gray. A correction must therefore expose longer than determined by the automatic (for example "+2").
Low-key motifs : Dark object in front of a dark background (black poodle in a pile of coals). The automatic exposure would render the black objects in the image gray. A correction must therefore expose for a shorter time than determined by the automatic (for example "-2").
Backlight : Light from a light source falls directly into the camera opening (portrait with sunset in the background). This light "outshines" the object that stands between the camera and the light source. In this case, the automatic exposure would render the object too dark. A correction must therefore expose longer than determined by the automatic (for example "+2")

An exposure correction can also be useful if the entire film is to be exposed differently from the manufacturer's instructions. Examples are:

A certain emulsion tested by the photographer for his purposes differs from the printed ISO value.
The color rendering of a color film or slide film is to be enhanced. Either an exposure correction in the negative range (for example "-0.5") or the film speed is set high (for example ISO 125 instead of ISO 100).

Setting the exposure compensation

There are several options here:

Exposure correction function of the camera: Usually a small switch on the setting wheel or a function in the camera menu. The change in the exposure value (EV) of the exposure is set, which corresponds to a change in the aperture value with the same exposure time, for example "-2 ...- 1 ... 0 ... + 1 ... + 2" . If the aperture can no longer be enlarged or reduced, the exposure time is extended or reduced accordingly.
Adjusting the film speed
Manual setting of time and aperture

Equipment of cameras

Modern cameras often have several variants of object measurement that the photographer can choose between depending on the situation or preferences.

The most flexible exposure metering is spot metering , especially when it measures the smallest possible angle of view. With an integrated spot exposure meter, the range of contrast of the subject can be approximately determined. The photographer can only get a higher degree of control with an external spot light meter . For snapshots, spot metering can lead to unsatisfactory results if a particularly bright or a particularly dark part of the subject happens to be appropriate.

The most modern variant is the multi-field measurement or matrix measurement , which measures several fields in the image section and weighted them according to a set of complex algorithms, some of which are based on the Ansel Adams zone system. The results are mostly good, but can occasionally lead to completely unexpected exposure settings. Since the behavior of the measuring system is difficult for the photographer to predict, it is advisable to use spot metering in difficult lighting situations. Some matrix measuring systems can be configured so that, for example, overexposed highlights are avoided in any case or only the "recognized" face is taken into account in recording modes such as portrait programs.

A widespread variant is the integral measurement , which is mostly center-weighted. It delivers good exposure results in average shooting situations. Complicated lighting situations such as backlighting often result in poor results, which, however, are usually easy to estimate and which the photographer can compensate for using manual exposure correction based on empirical values . Many older viewfinder cameras with built-in light meters also measure integrally.

Many older cameras and view cameras do not have an integrated exposure meter; here the photographer is dependent on an external exposure meter, empirical values or estimates. Symbols, exposure tables or exposure disks can help with the estimates.

Interchangeable finders with an integrated light measuring dome are only available for the Hasselblad V series. However, for some camera systems there are measuring calottes that can be attached instead of a lens and thus enable real light measurement. Such systems are not very common because of the complicated handling and the limited possibilities compared to a separate hand-held exposure meter.

literature

Adrian Bircher. Exposure metering. Measure correctly, expose correctly . 96 pages. Gilching: Verlag Photographie 2002. ISBN 3-933131-59-6
Martin C. Sigrist and Matthias Stolt: Designing with light in photography . 95 pages. Knaur 2002. ISBN 3-8043-5145-X
Ansel Adams: Das Negativ , Munich 2000 (Christian)

Web links

CCInfo: Exposure measurement - elaboration by Chr. Caspari
Exposure with a pinhole camera - by J. Reincke