Possibilities of exposure measurement – Object metering

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We have the option to perform object metering using either an external handheld exposure meter or the built-in metering unit in the camera. We almost always choose the latter method, as the camera viewfinder provides the most accurate determination of the metering range and we are only interested in the amount of light reflected from the subject. It is also known as TTL (Through-The-Lens) metering and automatically takes into account any lengthening factors caused by any filters or extension of the lens.

To perform the measurement, we point the camera at the subject from the selected location in such a way that the measurement area marked in the viewfinder captures only the light reflected from the subject, if possible. The determined time-aperture combination then tells us something about the subject’s brightness. And there is a catch. Because the light meter interprets everything as a medium gray with 18% remission, we actually have to use an area of this brightness for the measurement. But the subject’s brightness depends on its illuminance and reflectance properties. Thus, it is easy to understand that a highly reflective subject at low illuminance will result in the same subject brightness as a weakly reflective one at high illuminance. In both cases, the light meter will show us the same value. I will use a commonly quoted example to illustrate this point. Let’s assume that we want to take a picture of a black cat in front of a pile of coals and a white rabbit on a snow field, one after the other. In the first case we have a subject black in black, in the second one white in white. Depending on the illumination level, both reflect a certain amount of light to the light meter. If direct sunlight falls on the cat and the pile of coal, they may reflect just as much light as the rabbit and the snowfield at dusk. The light meter would naturally perceive both motifs as having the same brightness, and it would display the same measured values in both cases. – After all, how would it know whether the subject itself is white or black? However, based on the knowledge we’ve gained thus far, we can already predict that the same exposure setting in both cases will only result in an undesirable image. Every subject location we measure automatically aligns with point D of the density curve, resulting in medium brightness images. The subjects in our example, however, are not of medium brightness but predominantly black or white. Conclusion: Without the incorruptible reference to medium brightness or medium gray, we cannot trust any exposure measurement! Let’s now take a closer look at what this means for the various methods of object metering.

Integrative measuring methods

Integrative metering methods take the entire image field in the camera viewfinder into account more or less evenly. Integral metering has been the most commonly used variant in the past. The basis for this method is the fact that the total brightness of numerous subjects adds up to an average value of 18%. Since all parts of the subject contribute to the measurement result according to their size and brightness, it is easy to see that a dominant sky or shadow area or extremely bright or dark objects, such as the sun or its reflections, run counter to the requirement for overall average brightness. If they appear in the image, position them outside the measurement section and record the measured value using the save button. The previously selected section can then be selected again for the exposure.

But let’s make it practical again and exchange the little animals from our example mentioned at the beginning with each other. The black cat now sits in the snow, while the white rabbit stands in front of the coals. In both motifs, we observe individual shades of gray ranging from black to white, but overall, a distribution emerges that never matches the required brightness of the middle gray. In the first case, the large white snow field dominates the measurement, so that its brightness, which is far above the required 18%, comes to rest on point D of the density curve. Accordingly, the exposure meter will reduce the exposure to such an extent that the cat is barely visible. In the second scenario, the situation is reversed. Here the dark coal pile dominates, and now its brightness, which is far below 18%, lands on point D. This of course leads to a relatively strong exposure, which leaves „no good hair“, i.e., no details at all, left in the white rabbit.

So the moral of the story is: If we depend solely on integral metering and are dealing with a subject situation in which above-average or below-average brightness values dominate, we must correct the exposure manually. – Those with extensive experience benefit from this approach, as the numerous deviations prevent any definitive conclusions. As a general rule, predominantly bright subjects must be exposed more, predominantly dark subjects less. A good way of gaining experience and determining the correct exposure – even subjectively – is the exposure series. We therefore use the value supplied by the light meter as a starting point and expose one, two or three additional exposures with a deviation of ½ or 1 f/stops each. We can also reduce the interval to 1/3 f/stops in particularly critical situations.

In order to eliminate the most serious shortcomings of integral metering, the designers created a variant that no longer weighted the image field 100% evenly but divided it into strongly and less strongly weighted areas. We call this center-weighted integral metering because it accommodates the habitual placement of the main subject in the center of the image. In many cases, this stronger weighting of the image’s center leads to a more precise exposure than pure integral metering, which has consequently displaced it. Returning to the previously outlined example (cat in the snow, rabbit in front of the coals), center-weighted integral metering would certainly correctly expose the two small animals if they were located fairly precisely in the center of the image.

However, even the center-weighted variant is not free of disadvantages and shortcomings. Naturally, if extremely bright or dark objects are located in the center of the image within the strongly considered measuring range, the system will also fail. Furthermore, „center-weighted“ is not the same as „center-weighted“, because depending on the philosophy followed by the camera manufacturer, the weighting ratio from center to edge can vary between 80% – 20% and 60% – 40%. And even if the camera description illustrates the distribution of sensitivity zones, they are usually not marked on the focusing screen. Another uncertainty factor is that the size of the measurement zones is affected by the lens focal length. Finally, only a few high-quality professional cameras with asymmetrically distributed measuring sensitivities can respond to switching to portrait format. At the end of the day, center-weighted integral metering is only a crutch for a few photographers. If you’re looking for precise guidance in challenging situations, center-weighted integral metering won’t be of assistance.

The multi-field metering divides the image field into several sectors and determines their brightness completely independently of each other. These measured values are analyzed based on general brightness, contrast range, the location of particularly bright and dark parts of the image, and the distance information provided by the autofocus. This lets the electronics figure out what kind of subject it is, like landscapes, sunsets, backlit subjects, or subjects at night, and change the exposure to match.

In practice, it works like this: If the camera electronics suspect a twilight subject due to the low general brightness and the high contrast, it will automatically set the exposure to the bright parts of the lights. If, on the other hand, the analysis results in a backlight situation characterized by a dark central part with a bright peripheral zone and a large contrast, it will orient the exposure metering to the dark central part. Since the repertoire of stored subject situations is now quite extensive and the analysis algorithms are very sophisticated, the multi-field metering in automatic mode has a very high exposure reliability.

However, don’t let this statement fool you. The camera only informs you about the selected time/aperture combination. It does not indicate which reference points it used to classify the subject. Therefore, even with multi-field metering, there is a risk of incorrect evaluation and exposure of a unique motif, such as delicate reflections on a water surface or fine gradations of brightness in a light-filled forest scene. Therefore, the manual exposure determination from A to Z continues to be the sole reliable method that accurately identifies what needs correction and how.

Selective measuring methods

The selective metering methods, unlike any automated method, leave the decisive step of exposure determination completely in the hands of the photographer: He determines what the measurement is based on. So here we are dealing with wonderfully genuine craftsmanship! Who among us does not feel envious of the craftsmen’s abilities?

But slowly. First of all, let’s familiarize ourselves with the situation. Under the term selective, I want to summarize here all variants, which evaluate only individual, clearly defined motive areas. These include selective metering with measuring angles between 5 and 30 degrees and spot metering with measuring angles between 1 and 7.5 degrees. Since such narrow measuring ranges require particularly good optical control, only the TTL variant integrated in the camera or external spot meters with their own viewfinder can be used. For practical implementation, we approach the subject so that the measuring range only covers the truly important spot. Which one is that? Naturally, it’s the one whose brightness aligns with our frequently mentioned middle gray. Now this is easy to say and write, but many motifs either do not let us recognize the middle gray easily or do not show any at all. Nevertheless, we can also master them with this measuring method.

If the middle gray is difficult to recognize, the contrast measurement we performed at the very beginning is of great help. We determined the brightness difference between the brightest and darkest part of the subject visible by spot metering. At the beginning, we assumed that the contrast of the subject would not exceed the exposure range of our image carrier, and we want to keep this assumption. Currently, neither the measured values of the highlights nor the shadows are suitable for orienting the exposure to the subject, as they do not correspond to the desired average brightness. But the average value of both will give us a precise exposure. But be careful: because the aperture scale is logarithmic and not linear (each increase of one stop doubles the amount of light), you can’t simply take the arithmetic mean. So the result of f/4 and f/16 is not 10, but 8. The easiest way to derive this is to count the f-stops on your lens. You can achieve even greater exposure reliability if you select two points in the highlights and shadows instead of just one and determine the average of these four values. You can also use this technique, known as multi-point metering, to more reliably determine the average gray alone. To do this, you also measure two, three, or four subject points that you assume correspond to the average brightness and then determine the average value for the final exposure.

In the second scenario, when the subject lacks an average brightness, we can utilize a technique known as substitute measurement. To do this, we measure a detail that does not belong to the subject but whose brightness we either know or can at least estimate accurately enough. The most incorruptible reference for this measurement is the gray card. Manufacturers such as Kodak or Fotowand use them to produce the medium gray in the form of variously sized plastic cards „to go“, as it were. They feature a neutral gray coating on one side, which accurately reflects 18% of the incident light, and a white color on the other side. We use this white, which has a remission of 90%, when the light is so weak that the exposure meter no longer reacts to the gray side. However, using it requires a correction of +2 1/3 stops to the metering result. In any case, its handling is very simple. We align it for spatial subjects so that the imaginary extension of its center bisects the angle between the main light source (the sun when shooting outdoors) and the camera. Under diffuse lighting, when no particular direction of light prevails, the card may be parallel to the camera. In repro shots, on the other hand, it must be parallel to the original, since the angle of illumination in this case affects the brightness. Now, we don’t have to constantly walk around, carrying the gray card from subject to subject. If the lighting conditions at the shooting location match, we can perform the measurement at the camera location without any issues. Other proxies for the medium gray include an already somewhat worn asphalted road and a clear blue sky that is appropriately oriented in the opposite direction of the sun. On the other hand, a matte black surface for metering necessitates a 2 stop reduction in exposure, while a green meadow requires a 1 stop reduction. Finally, to ensure you’re ready for any situation, consider this frequently described tip: compare the measured value of the inner surfaces of your hands with that of a gray card. With this correction value in mind, you will then always have a „handle“ for determining the exposure, regardless of the subject environment and your equipment.

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