Inverse Square Law
Another way to control or modify our light sources is using the Inverse Square Law.
And it works as follows: If you double the distance between the subject and the light source, it illuminates a surface area four times bigger than the one before, respectively, the light intensity decreases to a quarter.
The light intensity drops rather heavily when the subject is first moved further away from the light source. After that, it continuously decreases, but on a lower level. For example: If the distance between the light source and the subject is increased from 1 to 2 meters, 75% of light intensity is lost on the subject. But when we increase the distance from 4 to 10 meters, we only lose 5%.
Meaning that the light intensity close to the light source has particularly high values. But in the distance, this intensity only reaches a small value. In practice, the closer the subject is to the light source, at a constant shutter speed, the f-value increases. And vice versa, if the distance between the subject and the light source increases, the f-value decreases. In both cases, the shots would look practically the same because the amount of light that enters through the camera lens is the same. This is how theoretically the correct f-value for each combination of distance, light intensity and shutter speed formula is created.
When illuminating only one static subject, one fixed f-value is sufficient. With moving subjects, however, require flexible f-values, especially if very close to the light source. Because of the inverse-square law, a small change of the distance to the light source leads to an extreme change of lighting. But, for a long distance subject, one fixed f-value is sufficient – even if moving on a larger scale.
In this example, we can observe that if the light source is placed at a distance of 8 meters to the first model, the light fall-off to the 4th model is only about 2/3 stops. However, if you were to place the light source only 2 meters away, the difference in brightness between the 1st and 4th model would be a total of 2 1/3 stops (e.g., f-stop 8 -> 3.5).
To correctly illuminate backgrounds achieving an even illumination of both background and subject, the light source should be placed at a significant distance. However, if the objective is to produce the reverse, creating a higher contrast in the picture, by having a darker background, the subject should be placed closer to the light source.
And it works as follows: If you double the distance between the subject and the light source, it illuminates a surface area four times bigger than the one before, respectively, the light intensity decreases to a quarter.
The light intensity drops rather heavily when the subject is first moved further away from the light source. After that, it continuously decreases, but on a lower level. For example: If the distance between the light source and the subject is increased from 1 to 2 meters, 75% of light intensity is lost on the subject. But when we increase the distance from 4 to 10 meters, we only lose 5%.
Meaning that the light intensity close to the light source has particularly high values. But in the distance, this intensity only reaches a small value. In practice, the closer the subject is to the light source, at a constant shutter speed, the f-value increases. And vice versa, if the distance between the subject and the light source increases, the f-value decreases. In both cases, the shots would look practically the same because the amount of light that enters through the camera lens is the same. This is how theoretically the correct f-value for each combination of distance, light intensity and shutter speed formula is created.
When illuminating only one static subject, one fixed f-value is sufficient. With moving subjects, however, require flexible f-values, especially if very close to the light source. Because of the inverse-square law, a small change of the distance to the light source leads to an extreme change of lighting. But, for a long distance subject, one fixed f-value is sufficient – even if moving on a larger scale.
In this example, we can observe that if the light source is placed at a distance of 8 meters to the first model, the light fall-off to the 4th model is only about 2/3 stops. However, if you were to place the light source only 2 meters away, the difference in brightness between the 1st and 4th model would be a total of 2 1/3 stops (e.g., f-stop 8 -> 3.5).
To correctly illuminate backgrounds achieving an even illumination of both background and subject, the light source should be placed at a significant distance. However, if the objective is to produce the reverse, creating a higher contrast in the picture, by having a darker background, the subject should be placed closer to the light source.
Sources:
https://petapixel.com/2016/06/02/primer-inverse-square-law-light/
https://petapixel.com/2014/07/01/inverse-square-law-light-explained-simple-terms-photographers/
https://www.youtube.com/watch?v=373eg4BW-NM
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