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Light, distance, and the inverse square law

We wonder a lot about the distance at which the flash unit can produce a good result in terms of exposure for distant subjects.
The inverse square law facilitates the process of measurement required in this field. We can also benefit from our knowledge of this law in the fields of exposure and magnification inside black rooms, and when working with a lens on a bellows.

Those who deal with flash in the field of photography are theoretically aware that they can only work at a specific distance, and the farther the subject is from them, the more difficult it becomes to get an appropriate exposure, and the reason is very simple. With the spread of light upon its exit from the flash, it begins to cover the areas in front of it little by little, from the adjacent area to an area farther and farther away until it spreads to very large areas, and the larger the area it covers, the weaker the light becomes, so that it is no longer sufficient to expose the film and record the image with the subject that you are looking for. Far more flash power this case looks more vivid
When filming outside at night, the results can be seen if we are working on photographing a group of people, separated by different distances from the camera. If the exposure is adjusted in harmony with those in the center of the image, we will see those who are close to the camera and have appeared with an exaggerated exposure, but if some of them are at a very close distance to the camera, then their faces will appear devoid of features and completely white, while the people who are relatively far from the center will have few faces. Exposure, while the faces of those far from the center completely disappear to be covered by blackness.

And even while working with an automatic flash to take a picture outside, we will find the process fraught with risks, as long as it is difficult to know how black the subjects far from the camera will be, compared to other subjects close to it. Fortunately here, the light is always within one state, and if We understand his condition and we can predict its results, even in unfamiliar circumstances.
In order to understand first how the light waves work, we can follow the ripples of another kind, such as the ripples that result from throwing a pebble in a stagnant water lake.
Until it disappears completely, at a distance far from the point of fall or - the center. ...
This is what happens with light with a slight change, as light spreads out in three dimensions. And from one point, with circles one within the other, similar to the process of inflating a balloon.
With the transmission of the light and the spread of its brilliance over a wider area, its strength gradually decreases, so that we find that the strength of the light is related to the distance of the subject from the source of this light, and whenever the distance is doubled, the light has to cover four times the area it covers, while the strength shrinks to a quarter of what it was. And if it is tripled, the light will cover nine times the area.
While his strength shrinks to 1/9 of what it was.
This rule is known as the inverse square law (*), where the intensity of light is related to the inverse square of the distance of the particular point and the source of this light, and this law is very useful as a means that helps to understand the nature of light.
In the field of practical application of this law, modern technology has removed many of the troubles that used to burden old photographers, which has turned complex calculations of exposure into a thing of the past, and shooting at night, with a computer-equipped flash, has become a very easy matter, without the need for calculations. that were necessary with the reference number. But it is necessary to know the way things go in their context so that the photographer can secure more freedom of his movement, by using the instructions that are now written on modern devices and the role of the inverse square law is always tilted in any place where we need to measure the required exposure, even if the subject is a snapshot A face illuminated by a full photographic lamp (*), so bringing the subject closer to the lamp will have an effect on exposure that far exceeds expectations, because shortening the distance between the subject and the light source by half will not double the illumination power on the subject’s face only twice, but four times, but with its distance away from the light source at a rate of four times the original distance, the power of the light will drop to 1/16 of what it was originally.
And inside the dark room, we find that the use of the inverse square law will save us time, trouble and more expense, and it will be necessary to make adjustments to the scale

edition. Let's make new editions enlarged for what edition. Familiarity with this law is enough to calculate the required exposure. Directly before moving the head of the magnifier, we take a measure of the distance from the lens to the base plate, then we move the head so that it includes the scale of the new edition with its frame on the base plate, and then we work again to measure the distance from the lens of the magnifier to the base plate, and here . The increase required by the new exposure is the square of the change in the height of the magnifier head. If, for example, the first height of the head is 8 cm. Then it increased to 16 cm. This means that the change in height is 8/16, i.e. two times, and the required change in exposure, then, is the square of the two times, i.e. 4. Considering that one stop is equal to 2, then the exposure needs to be increased.
Two stops in the exposure, but if the changes in the print scale come with complex numbers, it will be easier to use a small calculator to calculate the necessary changes in the exposure. It is also possible to benefit from the inverse square law, when using a lens on a bellows, as the magnification is related to the change in exposure, and when the lens focuses on the point of infinity, the film is then equal to its focal length. For most subjects, we do not find it necessary to distance them much more from the film, but if we want to form an image on the same scale as the subject. The lens has to be moved out twice and the light strength is only a quarter of what it would be in the case of working on distant subjects, which means that we need two extra stops to get a life-size image.
This and the same law allows calculating the amount of light that will fall at a different distance from
flash unit And according to what we mentioned at the beginning of the topic about the dispersion of light waves. The farther its circles are from us. maybe . With the inverse square law, see the difference in exposure between subjects
The near and far ones, and make sure if the result will be acceptable in terms of exposure. When capturing these subjects at different distances and distances from the light source -