ضبط عاملى التعريض حين استخدام الضوء الخاطف كمصدر وحيد للاضاءة ١-a .. كتاب آلة التصوير

Adjust exposure factors when using flashlight
as the sole source of illumination

First, adjust the shutter speed:

Since the period during which the image is recorded is the period of the flash of light, and the speed of the flash varies according to the type of flash light source, so we differentiate between two cases:

(a) The case of using electronic flashlight:

The flash of the electronic light takes only a very short time, usually ranging between 1/1000 to 1/10000 of a second, which is much faster than the highest shutter speed in any camera. Completely neglecting the shutter speed when using electronic flashlight as the only source of illumination, so the shutter speed should be half a second or 1/500 of a second as long as the shutter speed is set to an x ​​sign, which indicates that the flicker reaches its peak when the shutter reaches its maximum width.

(b) The case of using normal flashlights:

The time it takes for flash bulbs to flash varies according to the type and characteristics of the lamp and its manufacture. This time ranges, as we know, between 1/25 to 1/250 of a second. Therefore, the time that the flash takes may be less than the appropriate shutter speed for the speed of movement. This calls for us to differentiate between two cases:

1- If the speed of the light flashing is greater than the shutter speed we estimated, then the shutter can be set to any lower speed that is less than the flashing speed (1/25 of a second, for example).

2- If the shutter speed needed to photograph motion is greater than the speed of the flash of light (as if the speed needed to photograph motion is 1/250 of a second, for example, and the time it takes for the flash = 1/50 of a second), then we set the shutter of the camera to the higher speed, which is 1/ 250 of a second, so what counts is always the higher speed, and it is the one we rely on. (This is assuming that the lamp and the press are suitable for this case: for example, a lamp of the M family and the M press as well).

Second: Estimating the focal number in the case of flash photography:

When photographing with flashlight, the estimation of the focal number is directly related to the distance between the object being photographed and the light source, which calls for an increase in the width of the diaphragm aperture as this distance increases, and vice versa. In this case, the inverse square law is applied, which states that the intensity of illumination is inversely proportional to the square of the distance. If the distance between the light source and the object increases to double, the intensity of illumination decreases to a quarter, which necessitates an increase in the aperture of the diaphragm by an amount equal to four times the first magnitude.

Flash Factor - Guide No.:

In order for there to be a link to estimate the appropriate focal number for each of the distance between the source and the object on the one hand, and the power of the flash light source on the other hand, numbers were set that were called the guide number. This number is determined when manufacturing the lamp or electronic flashlight device according to the power of the source. Whereas this guide number was set only to assist in estimating the correct exposure by determining the appropriate focal number for the distance of the object from the flashlight lamp, and given that the speed of the film’s sensitivity has a great effect in estimating the correct exposure, so this guide number is related to the speed of the film’s sensitivity. For example, it is said that the index number = 44 when using a film with a sensitivity of 32 shinners. If the speed of the film sensitivity decreases, the numerical value of the index number decreases. The focal number is calculated according to the following equation:

Focal number = the number indicating the distance (in meters or feet)

It may have been noted in the previous tables that the index number is reduced to half if the sensitivity of the film is reduced to a quarter, as we mentioned that:

If the film sensitivity speed is 32 shinners, the guide number becomes 44.

If the sensitivity of the film decreases to 29 shinners, the index number becomes 32.

If the film's sensitivity speed decreases to 26 shinners, the guide number becomes 22.

This is assuming that the power of the light source is the same and does not change when using the three different films in the speed of their sensitivity (1).

This is when the indicative number is estimated, it must be indicated if the distance measurement will be conducted in meters or feet. If the indicative number is 44 when measuring in meters, then the distance (calculated in meters) between the object and the flash light source must be measured. In this manner, we also proceed if the guide number is estimated based on the calculation of the distance in feet.

Third: The case of using flash light as an auxiliary source for another main light:

The purpose of using flash light in this case is to illuminate the areas of shadows resulting from the main strong source (such as the sun, for example). We summarize the following steps that the photographer follows to estimate the exposure factors:

1- Estimate the appropriate exposure factors for the intensity of illumination of the light areas or