آلات قياس الضوء ( Exposure meters ) .. كتاب فن التصوير الفوتوغرافي

Exposure meters

Photometric instruments are based on the principle that ordinary light can be converted into electric current by a photoelectric cell.

There are two types of these machines: the first is selenium as an active substance in the photovoltaic cell, which converts light into an electric current.

As for the second type, it depends on cadmium sulfide (Cdimium CSD) (sulfide). As an active substance in the photovoltaic cell. . This type requires a small dry battery, and it works by measuring the change in the resistance of this material to electric current under the influence of the force of light. The advantage of this type of light measuring instrument is its ability to measure a very small power of light, which is useful in indoor photography and in very low light conditions.

These instruments can be used to measure light in two ways: the method of measuring the incident light on the subject, and it is called the method of measuring the incident light (incident light) and the method of measuring the light reflected from the subject, and it is called the method of measuring the reflected light | (Reflected Light).
Most photometers are designed to measure reflected light.

- Incident Light Meters

The idea of ​​this type of machine is based on the fact that the strength of the light falling on the subject to be photographed determines the necessary exposure, and since this is true, all you have to do is measure this light.

The use of this type of instrument is very simple, as you only have to stand near the subject to be photographed, and point the scale towards the camera lens. In this way the meter receives the same illumination falling on the subject.

In the event that the subject is illuminated unevenly, as is the case in indoor photography, or in the case of subjects with very light parts and dark parts, this type of meter or this method of measuring light is not successful.

Reflected Light Meters

This type of photometer works on the principle that the camera sees and records the light reflected from the subject. Since this is true, the best method is to measure the reflected light and not to measure the light falling on the subject.

How to use the light meter:
In all light measuring machines there is a disk or a needle that must be placed on the number indicating the speed of the used film, and this is often in the ASA system, then you can rely on the reading of the scale.

Some machines allow the exposure to be read directly, and some require a computer disk to be rotated.
Some instruments give you a direct reading of the exposure values.
If your camera uses the EV system, you don't have to worry about the aperture value (f-numbers) or the shutter speed.

The quick and easy way to use a light meter:

In this method you only need to point the meter at the subject to get the average general illumination as can be seen in image 6 .
This method is suitable for outdoor photography, where the lighting is reasonably distributed, and where the shadow details are not important.

Be careful not to let your finger or the lid of the case cover the photocell. If the scene includes a part of the sky, point the scale slightly downward to reduce the amount of sky covered by the angle of view of the scale, because this gives a high reading due to the intense sky light, which results in an underexposed image.

If your camera is E.V labeled, simply set the exposure indicated by the meter on it.
If your camera does not use this system, you must choose the best combination of shutter speed and f-number for the exposure indicated by the meter.

Close-up method

This method is used in photographing people (portrait) or in pictures that contain a subject or one important element. Hold the scale at a distance of 15 centimeters from the subject. If the subject is smaller than 15 centimeters, approach the scale to a distance equal to the smallest dimensions of this subject, then point the scale towards Exact topic and watch out not to cast a shadow on it.
If you cannot reach, measure the similar light of similar objects nearby.
Remember here that one of the things similar to a human face is the palm of the hand, as shown in Figure 7.