In an ideal world, exposure metering would be unnecessary. If an image converter could register all brightness gradations in the scenes we are focusing on. We would not need to determine how the tonal values in the image should be displayed until after the recording. The dynamic range that a sensor can cope with is as limited as the movie’s earlier, So we need to decide which part of the brightness spectrum should be captured in the image before shooting. That is the task of the exposure measurement.
Operation of a digital counter
A digital counter does not have the same functioning as an analog meter since they have their characteristics, and the user has to know them beforehand. The measuring device has a red light that indicates the function you are performing at that moment:
The activity of the red light can mean any of these possibilities:
The customer knows at all times if the digital counter is working correctly, but if not, you can call the distributor to send a technician and check that everything is correct at no cost.
Reading with a digital light meter.
The digital counter raises another way of reading the consumption since it appears a five-digit code where spending is detailed, and the most advisable is to make the reading on the same day of each month. The process for reading the meter depends on the contracted electricity tariff:
- The analog meter details the consumption without defining periods.
- The digital counter specifies how much consumption is done in each time slot.
- The consumer, with time discrimination, has a digital counter.
- The smart meter can be installed in the user’s home.
- The rental of new measuring equipment can be between € 0.80 and € 1.15 approximately.
- If it is off, no consumption is being made.
- When the light flashes quickly, electricity consumption occurs.
- If the blinking takes place very quickly, the light expenditure has been skipped.
The measure of light
A light meter is a particular version of the lux meter, the device that measures merely ambient light.
Ambient light is usually expressed in lux, but photographers use a unit of measurement called EV (Exposure Value). The EV scale is not linear: each increase of one unit (from EV 0 to EV 1) corresponds to a doubling of the luminous intensity.
The light meters make it possible to perform two types of measurement:
• measurement of the incident light (the one that “falls” on the subject independently of its reflexivity)
• analysis of the reflected light (which depends on the reflexivity of the photographed subject).
To note: the measurement of the reflected light is the only possible measure when the Photographer can not approach the subject: in animal photography or for telephoto shots.
The most common exposure meters have a wide measurement angle (of the order of 30 °) and can be used for the measurement of incident light or reflected light.
Other more specialized devices, called spot meters, have a very narrow measurement angle (from 1 ° to 10 °) and are used only for the measurement of reflected light.
Today, most cameras are equipped with an integrated exposure measurement system. This system measures the reflected light, usually through the lens.
For all current cameras, this is based on a measurement of the light that passes through the lens. But the concept of exposure measurement leads us astray because the value you’re looking for can not be determined this way. The task of an exposure measurement would be to compensate for the influence of the differently bright ambient light; the brighter the scene is illuminated, the tighter the camera should expose so that always the same amount of light falls on the sensor. Through the lens, however, you can only measure the reflected light from the subject, which allows no conclusions about the ambient light. A certain subject brightness may be because a dark object is very brightly lit or a bright purpose is only dimly lit;
Using a separate light meter (see illustration), you could instead measure the light by measuring from the subject toward the light source. This type of exposure measurement is very reliable, but not always applicable in practice, because the item can be inaccessible or no time for the light analysis remains. You could also put a standard theme like a gray card next to the main subject and measure it accurately. Since the gray card reflects an equal amount of light (typically 18 percent), its measured brightness only depends on the ambient light. But even the placement of a gray card often lacks the time or opportunity. So it usually remains only to try the impossible and to rely on the exposure measurement through the lens.
The three most common methods of exposure metering, spot metering, integral metering, and multi-field metering, differ not only in their metering characteristics but rather quite fundamentally in the way they try to guess the optimal exposure – and more than guessing is not because the required value cannot be measured.
Spot metering, which measures brightness in a small, usually one to two percent, part of the image field, assumes that the appropriate portion of the subject has average reflectivity; this is defined as throwing back 18 percent of the incoming light. The camera then selects shutter speed and aperture such that a matter of this brightness produces an average amount of charge in a sensor pixel. It does not necessarily have to be a midrange tone (128 if the tonal values range from 0 to 255), as it also depends on the gradation curve, which can shift it up or down. The precision of the spot exposure measurement depends crucially on whether the photographer measures a motif detail with 18 percent reflectivity. In predominantly light or predominantly dark objects, this is often not possible, and one must then try to find a replacement motif with the desired reflection behavior, which is illuminated just like the actual subject. Whatever one measures, is considered by the camera as a subject of medium brightness and be imaged accordingly; therefore, the potential for error in the careless application is high. An image detail that is optimally suited for spot metering will not necessarily be in the center of the image, so you first have to save the measured value and then select the image section. Some cameras also offer the option of linking the spot metering area to the AF point, which is not very useful:
Spot metering does not always have to be based on a medium brightness value. Alternatively, Olympus’ DSLRs offer a measurement on the brightest or darkest subject details that should still reveal a structure. The camera then exposes such that a subject with the measured brightness is just within the dynamic range that can be handled by the sensor. One can also simulate this behavior with ordinary spot metering by setting a positive or negative exposure compensation value, the size of which must be experimentally determined. However, this can fail because the required value is outside the limits of the exposure compensation (usually ± 2.0 EV).
While the spot metering demands a lot of attention from the photographer and can lead to glaring incorrect exposures if misused, the integral measurement is much more robust and uncomplicated to handle. The basic idea is that even if there is no motif detail with a medium object brightness, the brighter and darker parts within the image usually compensate each other, and the average brightness again corresponds to an average value to which the exposure can be tuned. The averaging usually weight the image parts differently and makes the center, in which the main motif is more likely to be located, more heavily involved than the image edge in the calculation.
The integral measurement is by no means immune to mistakes: a snowy landscape with a snowman is on average brighter than a medium gray value and would, therefore, be underexposed as seen in the example on the right – from the white snow, the integral measurement makes an unsightly medium gray. One can not avoid correcting the exposure manually. Still, just when one suspects that a correction will be necessary anyway, the integral measurement provides a reasonable basis – on the one hand, because its behavior is transparent and can be predicted well, but on the other hand, because their result remains insensitive to small changes in the image detail.
The multi-field or matrix measurement in this regard is the exact opposite because their behavior is complex, not always transparent, and can change even with a small pan of the camera. Therefore, the multi-field measuring methods of the various camera manufacturers have a higher success rate than an integral measurement and, unlike the spot measurement, does not require the assistance of the photographer. Therefore, the multi-field analysis is generally selected as standard, and if a camera supports only one measurement method, then it is almost always this.
A multi-field measurement measures the brightness in several, between 16 and several hundred fields. In multi-field analysis with a small number of measurement fields, these are sometimes irregular in shape. Still, usually, it is a matrix of predominantly equal rectangles in which the image is divided. One could now weight the results of the exposure measurement in the individual fields individually and then calculate an average, but that would make the multi-field measurement merely a more flexible variant of the center-weighted integral measurement. Averaging, however, would destroy the decisive advantage of the multi-field measurement in that it can analyze the tonal distribution in the image. A typical multi-field measurement, therefore, does not rely on surmise about the object brightness, like the other measuring methods. But the exposure problem is more direct – it illuminates in such a way that the brightness values represented in the picture can be registered as completely as possible and well resolved by the sensor so that as little details as possible are lost in the lights and shadows despite the limited dynamic range. As long as the subject is not too rich in contrast, this task can be easily mastered; it is only essential to illuminate in such a way that the lowest and highest brightness values are approximately equidistant from the limits of the dynamic range that the sensor can handle. Although this rendering tone reproduction can go astray if dominated in the photographed scene predominantly light or predominantly dark subjects, which would then be too dark or too bright. Even then, the multi-field measurement would have ensured.
More difficult is the exposure in cases where the subject contrast overtaxes the dynamic range of the sensor. This happens quite often but does not mean that good results would not be achieved, provided that the automatic exposure control receives the most critical part of the tonal spectrum. The task of the multi-field measurement can be compared with the zone system developed by Ansel Adams: The different brightness ranges must be distinguished, among other things, whether they are lights and shadows, the tonal values are still differentiated, or highlights and deep shadows, the undifferentiated can be represented by pure white or black. The zone-based photographer classifies brightness levels according to the intended image effect, and the like must also perform the multi-field measurement. For this purpose, it must recognize as many typical shooting situations and motifs as possible. Besides the exposure measurement itself, other sources of information can also be included: Using multi-field autofocus, the position of the foreground and background can be determined, and the orientation of the image can be recognized with an orientation sensor.
What motives are recognized and what the criteria are is different from model to model, and no camera manufacturer discloses the details of its measurement process. For example, a camera could detect that a darker foreground is located in front of a very bright background, especially in the upper area, and conclude that there is a backlight situation here. If the total amount of contrast is too large for the sensor to reproduce fully, it would then adjust the exposure to the presumably critical dark foreground and tolerate overexposure of the bright sky for its high-contrast rendering. Conversely, if the background is dark and low-contrast, but the front appears much brighter and more contrasty, this speaks for targeted illumination of the foreground, for example, a musician on stage who is illuminated by spotlights. In such a case, the camera should illuminate the bright foreground and accept undifferentiated shadow areas in the background as a result.
Depending on what the camera has detected for a photo situation, the multi-field measurement will weight the individual measurement fields quite differently, and not infrequently more significant parts of the image are assumed to be unimportant and, therefore, largely ignored. In this way, the sections of the tonal spectrum recognized as necessary can be reproduced in a high-contrast and finely resolved manner, even if the entire richness of the tonal values cannot be preserved. This property makes the behavior of a multi-field measurement but also challenging to predict because even a slight swivel can cause the camera to reclassify the scene, for example, because a subject in the foreground is now shifted to the edge and no longer considered the main issue. Although the multi-field measurement will usually lead to good results, more common than the comparatively coarse center-weighted integral measure, which always assumes that lighter and darker subjects balance each other, occasionally, however, it will also produce surprising results. Those who have bought a new camera should familiarize themselves with the peculiarities of their multi-field measurement to better anticipate problematic situations. For example, if the camera tends to overexpose the bright background of backlit subjects in favor of the foreground, but the photographer prefers a correct exposure of the sky, he must correct the exposure accordingly and, in turn, lighten the dark front in an image editor. That lighter and darker designs balance each other. Occasionally, however, it will also produce surprising results, and those who have bought a new camera should familiarize themselves with the peculiarities of their multi-field measurement to better anticipate problematic situations. For example, if the camera tends to overexpose the bright background of backlit subjects in favor of the foreground, but the photographer prefers a correct exposure of the sky, he must correct the exposure accordingly and, in turn, lighten the dark front in an image editor. That lighter and darker designs balance each other. Occasionally, however, it will also produce surprising results, and those who have bought a new camera should familiarize themselves with the peculiarities of their multi-field measurement to better anticipate problematic situations. For example, if the camera tends to overexpose the bright background of backlit subjects in favor of the foreground, but the photographer prefers a correct exposure of the sky, he must correct the exposure accordingly and, in turn, lighten the dark front in an image editor. You should familiarize yourself with the peculiarities of their multi-field measurement to better anticipate problematic situations. For example, if the camera tends to overexpose the bright background of backlit subjects in favor of the foreground, but the photographer prefers a correct exposure of the sky, he must correct the exposure accordingly and, in turn, lighten the dark front in an image editor. You should familiarize yourself with the peculiarities of their multi-field measurement to better anticipate problematic situations.