Metering is in photography, the collective term for various methods to determine the appropriate combination of shutter speed and aperture, with a correctly exposed image can be generated.
All photographic recording and reproducing method, whether negative film, slide film, photo paper or digital cameras, have two characteristics: the light sensitivity of the film or the image sensor and the maximum displayable contrast range. Objective of metering is to find an optimal compromise between the possibilities of the material used on the one hand, the lighting conditions and the reflectivity and contrast characteristics of the subject on the other hand.
The exposure is the integral of illuminance ( in lx ) on the exposure time ( in s) ⇒. In zeitinvariantem continuous light thus applies
- 3.1 Application Examples
- 3.2 Adjusting the Exposure Compensation
There are different types of metering:
- Object measurement ( also: luminance measurement) with the camera - here is measured reflected by the object or emitted light. The object measurement is the most widely used method of measurement which is also found in most cameras. Is measured either by the lens with the method TTL ( TTL: through the lens ), or via a located on the camera body exposure meter.
Variants: metering, Spot metering, evaluative metering or matrix metering, detail measurement, two- and multi-point measurement.
- Object measurement by a computer located in the flash meter.
- Light measurement - here the light incident on the object light is measured. It sets usually requires specially equipped handheld exposure meter with the dome, but with suitable accessories with some cameras possible. Using a gray card known as a substitute measurement is also possible with cameras or simple hand exposure meters without Messkalotte.
Classic automatic exposure measurements are based on simple averages: It is assumed a Landscape on a clear, blue sky, lit largely frontal (at 35 ° to 55 ° of the sun) and reflects 18% of incident light towards the camera.
Modern computerized metering modes (Evaluative or matrix metering ) try the other hand, using a location database or the motif contrast to detect a given, also deviating from the norm, light situation - and thus can often provide surprisingly good results.
External exposure meter
For measuring light camera measured the light incident on the object light from the object / subject in direction. To do this, the meter, the meter, be specially equipped and feature dome or diffuser, which are pushed as white hemisphere or blinds over the cell volume. From the falling light on the subject can be close to the reflection of light towards the camera. This measurement method is independent of the motif and its contrast.
In the TTL metering (English through the Lens, through the lens ', also inside measurement ) is that amount of light is measured, which actually arrives behind the lens. The sensor ( measuring cell) may be within the camera at various points in the beam path, for SLR cameras, for example, above the focusing screen.
The term was introduced to distinguish between cameras with external measuring cell, the accuracy is limited because of not matching with the image section measuring base and due to possible interference. The first camera ever with TTL metering was the Topcon RE Super Topcon; the first SLR with TTL metering, the Pentax Spotmatic Asahi Pentax, came in 1964 on the market. In the measurement object towards its light emission (remission ) is determined by the camera or exposure meter from the object / subject. Helpful can here a spot diameter of 1-5 ° measuring angle be: This can be derived from several measurements of different brightnesses motif, a mean gain (multi- spot metering ). This method is primarily dependent on the reflectance of the subject and the subject contrast. The latter must also be analyzed and evaluated.
An alternative to photographic light value determination is the measurement value determination on a gray card, which is kept as far as possible within the optical axis parallel to the film plane, directly in front of the subject. This method is often used for artistic photography or studio photography. The applicable using the substitute measurement method targeted under-or over-exposure can be found in different form in the zone system.
Neutral gray and gray card
As a unit in the metering applies the light value - equivalent to an aperture setting of the lens, a time step of the camera shutter, or a sensitivity level of the film ( the sensor in digital cameras).
A light meter provides information on an existing amount of light that he puts on the light value and the set film (sensor ) sensitivity at a time and aperture combination in relationship, which is necessary to a subject ( in part ) corresponding to a mean gray value exposed.
The mean gray value is defined as an area with a light output of 18% - the average Remissonswert (or " Zone V " ), between drawing modern white or deep black; based on a reproducible dynamic range of five exposure levels.
As an aid to proper exposure metering is used the gray card that can be alternatively be appropriate if the subject is even above average contrast; thus consists of a variety of different gray surfaces. It is defined as neutral gray ( no color cast ) and an exact remission of 18 %, that radiates 18% of the incident light back. The same does receive the dome or the diffuser at hand exposure meters, as the white hemisphere (or blind) are pushed over the measuring cells of the equipment: Again, exactly 18 % of the light detected by measurement. What a so-called light measurement - a direct measurement from the back motif to the light source - can be realized.
In the measurement object, an exposure value is from the camera to the object ( subject) through determined which of those settings on the camera that would be needed to capture this motif in a medium brightness, a mean gray value to the film. As for motives, deviates whose remission of it, so predominantly white ( eg, snowfields ) or black ( eg coal pile ) represents corresponds to the determined exposure value is not the correct setting. Only one average illuminated and contrasting motif ( eg a landscape with sun in the back ), the average of all the light and dark areas are largely used as a valid measure for light. Otherwise, a value determined must be corrected.
In the light measurement, however, the calculated value is appropriate, a subject independent of contrast ( extensive ), including white as white and black as black scan. Here only caution is required when the exposure range of the scene is larger than that of the film.
Exposure compensation is a term used in photographic technology and is the process of deliberately departing from the exposure value which indicates the light meter of the camera. Due to this deviation or " correction " the image brighter or darker than the automatic exposure or the exposure meter is it claims. The unit of exposure is the exposure value (abbreviation: EV, German: light value ).
With 2 EV exposure compensation
Exposure compensation is always displayed when the brightness of a subject does not meet the standards for automatic exposure. Example of this are:
- High-key motifs: A bright object against a bright background ( White poodle in the snow ). The automatic exposure would reflect the white objects in the image gray. A correction must therefore be exposed longer than the automatic determined ( for example, " 2" ).
- Low-key motifs: Dark object against a dark background ( black poodle in the coal pile ). The automatic exposure would reflect the black objects in the image gray. A correction must be shorter than expose of the automatic determined ( for example, " -2 ").
- Against the light: light from a light source shining directly into the camera port ( portrait with sunset in the background ). This light " outshines " the object that is between the camera and light source. The automatic exposure would reflect too dark in this case the object. A correction must therefore be exposed longer than the automatic determined ( for example, " 2" )
Exposure compensation can also be useful if the entire film is to be exposed deviating from the manufacturer. Example of this are:
- One particular by photographer tried for his purposes emulsion differs from the printed ISO.
- The color reproduction of a color film or slide film is to be reinforced. This is done either to adjust in the negative range ( for example, " 0.5 " ) or the ISO sensitivity is raised ( for example, ISO 125 instead of ISO 100).
Adjusting the Exposure Compensation
There are several options:
- Exposure compensation function of the camera: Usually a small switch on the dial or a function in the camera menu. Is set, the change of the light value (Exposure Values, EV) recording, corresponding to a change of the aperture value while maintaining the same exposure time, for example " -2 ... -1 ... 0 ... 1 .. 2 ". If the diaphragm is no longer can be increased or decreased, the exposure time is extended or reduced as required.
- Adjust the film speed
- Manual setting of shutter speed and aperture
Features of cameras
Modern cameras often have multiple versions of the object measurement, between which the photographer can choose depending on the situation or preferences.
The most flexible metering is spot metering, especially if it measures out the smallest possible angle; a higher level of control is obtained by the photographer only with an external spot meter. For snapshots spot metering sometimes leads to unsatisfactory results, if by chance a particularly bright or a particularly dark part of the subject is appropriate.
The latest variant is the metering or matrix metering, which measures several fields in the frame and weighted by a set of complex algorithms, some of which lean against the zone system Ansel Adams. The results usually fall out well, but can also sometimes lead to unexpected exposure settings. Since the behavior of the measurement system by photographer is difficult to calculate, it is recommended to resort in difficult lighting situations on the spot metering.
A common variant is the average, which works mostly center weighted. It provides for complex lighting situations such as pages or backlight predictable bad, but it is calculable results that the photographer can be compensated by a manual exposure, with average shooting situations but mainly good to very good exposure results.
Many older cameras and professional cameras have no built-in exposure meter; Here the photographer has to rely on an external light meter, experience or estimates. The estimates symbols, exposure tables or exposure slices can help.
Only for Hasselblad V series, there are interchangeable viewfinder with integrated Lichtmesskalotte. However, there are some camera systems Messkalotten that can be scheduled instead of a lens, enabling a real light measurement. Because of the complicated handling and against a separate light meter limited options, such systems are not widespread.