البوزومترات المدموجة بالآلة .. طريقة عمل آلة التصوير .. كتاب التصوير الضوئي

The built-in positron and meters ..
How the camera works .. The book of photography

The built-in positron and meters

Many cameras are equipped with an internal positron that measures behind the lens. Some are semi-automatic and others are fully automatic. It is very important to know what method is used to measure the light in the image field. The three main methods are explained below:

The total method:

A single cell in the pentagonal prism of the camera measures only a limited part of the subject. When there are two cells, the measurement works on the entire image, but often with greater sensitivity to the two areas indicated on the right.

The relative method:

To reduce the poor total measurement results, the cells converge towards a central area in the image where the main subject is most often located. The measurement improves this central area and is less sensitive to the secondary image areas.

The selective or spot method:

This method uses only a single cell placed
Behind the mirror so that it reads only a small part of the image. This reading directed towards a lit area of ​​the subject gives a direct measurement. It is also possible to measure high lights and then shadows in succession to obtain an average exposure value: a complex method but one that gives very accurate results.

Reflected light metering

The usual way to measure the amount of light in a scene is to measure the light reflected from the subject towards the instrument. The metering is concerned with the amount of light reflected by the subject as well as the amount of light that illuminates the subject. A dark subject requires a stronger exposure than a bright subject even if both subjects are illuminated by the same amount of light. For this reason, direct the metering towards the most important or most expressive part of the scene you want to photograph.

Incident light metering

This metering is more accurate than the previous one since it only cares about the light that is directed at the subject. This metering is done by placing the bosometer in front of the subject to be photographed and directing it - covered by the cone - towards the instrument lens. The metering here does not care about the bright or dark value of the subject because the latter is translated or interpreted by an image density that matches the ability to reflect light.

How to make an average exposure metering

Many scenes contain specialized light values ​​ranging from deep shadows to bright lights. If these areas are evenly distributed over the entire field, the measurement will undoubtedly be correct using any type of positron. However, if the dark or light areas dominate, the exposure value will only be acceptable using a positron with a large angle for a standard field (total measurement), as is the case with the independent positron that works on selenium and in most of the positrons integrated with the machine. It is better to take two consecutive measurements. The first measurement is for strong lights and the other for shadows. Then the average exposure value is taken, which usually suits the most important part of the subject. This value can be slightly tilted in the direction that suits the nature of the subject: towards the shadows or towards the lights.

To obtain the correct exposure value, take a measurement of the sky and then another measurement of the water and adopt the average value between them.

Take the measurement based on the most illuminated part - the sky - the image will be very dense.

Take the measurement based on the dark part - the water -: The result of the first level, which is the water, was good. As for the other parts, they are bright and not detailed.