عدادات التعريض الضوئي .. كتاب فن التصوير الفوتوغرافي

Exposure meters: In the early days of photography, determining the amount of light needed for a shot was done by guesswork without any measurements. Today, there are meters to measure the required degree of illumination that are sensitive to the slightest amount of illumination, and are essential for any self-respecting hobbyist. This does not necessarily mean that you are completely bound by the meter and not working

Without it, you must be an expert enough to work without the meter in urgent cases, using your inner intuition.

There are two main sections of the optical meters: a section that uses selenium cells, and another section that uses cadmium sulphide cells, or what is called for short, the Cds counters. Both types do the same thing, but in a different way. The selenium counter works by an electric current generated by light. The light enters the selenium cells and turns into a weak electric current that affects the needle of the meter and deflects it according to its intensity

This needle is the one that determines the aperture and speed required for a correct exposure, according to a scale placed at the bottom of it, on which the aperture numbers and speeds are written. Unlike the selenium counter, the cds counter works with the electric current that is supplied by special batteries. Otherwise, they both work in the same way, where the photocell measures the level of light in a scene and translates it into a suitable aperture and speed. This type usually contains an on/off button to conserve the battery. However, some of the newer models may not contain such a button, claiming that it is not needed, because when the meter is in its leather bag without use, its photocells do not sense the lack of light reaching them, and therefore this button is not needed. The CDS counter is gradually replacing selenium counters among the amateur masses these days, although both types are available in the market.

Most modern amateur cameras are equipped with an implicit CDS meter. The reason for the strong demand for this type of meter is due to its extreme sensitivity to light, as it gives an accurate reading of the light in bright sun as well as in poor light conditions. Even some of the advertisers for counters of this type guarantee the buyer that he can determine the intensity of the moonlight due to its sensitivity. These meters are equipped with a high and low key to facilitate handling of their wide range

Different lighting conditions, the low position is for reading lighting in weak conditions. This key is not available in selenium counters, which makes them unable to register weak lights. Despite the many advantages of CDS meters, they remain less than the ideal level required to solve all dilemmas of lighting measurement. One of the most important dilemmas that you suffer from is what is known as the memory effect.” The CDS cells have a memory that retains the light reading for a period of time, which makes taking a second reading immediately after the first reading fraught with errors resulting from the first reading, and for this you have to wait a little after the first reading and before taking the second reading Until the effect of memory gradually remains. It is worth noting that selenium cells do not contain such a dilemma, as the second reading comes completely separate from any effect of the first reading. As for the batteries used in the CDS counters, they last for several years, and their life ends suddenly, unlike other types of batteries whose strength gradually weakens.