Words like “Inverse Square” do frighten people. But the law is not that bad. In fact, it helps us tackle lighting and exposure difficulties with on camera flash. So readers can expect them at better position and knowledge to do flash photography!
Lighting Difficulties always arise when we try to photograph a group of people, standing or sitting at different distances from camera. These difficulties multiply when we are using camera's built-in or dedicated flash. No matter how powerful our flash is; subjects at different distances do get lit differently. Now this is common sense that as we go away from the light, the intensity decreases. So lets do case study.
So in this diagram, we can see 4 subjects at different distances from camera. So, to photograph all of them, we need to do focusing we can use higher aperture to keep all of them in focus. But It will be little harder for the flash to make them properly lit. more the distance between the subjects, poorer lit the scene. That, if we focus at subject 1, ONLY subject 1 will be properly lit. subject 4 may not be visible. Then, if we focus at subject 2, ONLY subject 2 will be properly lit and subject 1 will be overexposed! It is then obvious that subject 3 and 4 will be gradually underexposed!!
Essentially, we need to know how the intensity of light decreases with increasing distance. Studying physics may give us an answer. But hold on, we need to not to do a Ph.D on it. There is a simple law named “Inverse Square Law of Light”(See the diagram.). Actually speaking, this law is not only for light but for any point source which spreads its influence equally in all directions without a limit to its range. Gravitation, Electrostatics, Electromagnetic Radiation as well as light obey this law. We have got nothing to do with the former three things, so we better concentrate on how light obeys the law. From the simplest point of view, doubling the distance between the light and the subject results in one quarter of the light hitting the subject. Now there cannot be simpler interpretation than this.
Scientific statement of the law (relating to light)can be like, “The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source.” Big bad scientific sentences again! This is great but how to use it in daily life? Lets get back to that case study again(Refer to the right half of the diagram). Assume that the distance between camera and subject 1 is 2 feet and the distance between each if the subject is 1 foot. So the distance between camera and subject 4 is twice than that of between camera and subject 1. Hence, the incident light on subject 4 is a quarter of that incident on subject 1. In photographic terms, subject 5 is underexposed by more than 2 stops. Exposure difference of 2 stops is not acceptable at all. Now don't tell me you shoot in RAW and will later change the exposure; That is not real photography!
We need to be a bit innovative to use this law of Inverse Square of Light. Let us play around the law to understand it better. Go through the following 3 statements.
1.Focus and expose for subject 3.
2.Now the distance between light source and the subject 3 is 4 feet. So the distance between camera and subject 1 and between subject 3 and subject 5 is 2 feet(half of 4 feet).
3.Hence the incident light on subject 3 is one quarter of the incident light on subject 1. And incident light on subject 5 is further one quarter of that on subject 3.
Then what will happen if go away to 4 feet distance from subject 1?; keeping the distance between each subject constant at 1 foot. Refer to the diagram and read the statements.
1.Focus and expose for subject 1.
2.Now the distance between light source and the subject 1 is 4 feet. So the distance between camera and subject 1 and between subject 1 and subject 5 is 4 feet again.
3.Hence the incident light on subject 5 is one quarter of the incident light on subject 1. Or this makes subject 5 two stops underexposed than subject!
This worsens things. After careful analysis of the above said examples, we can arrive at a conclusion. Halving and doubling of the light depends upon the distance between light source and the subject. As we go near the subject, distance at which the light intensity doubles also lessens; and vice versa. Practically speaking, if we focus and expose for 2 feet distance, light intensity will halve at 2 feet distance.
I hope it isn't getting too much of science. As it can be fun doing this much of simple maths and visualizing different situations! The final solution for the problem from first diagram is exposing for subjects from the distance more than double of their range. That is for our problem, as our subjects are spread over 4 feet distance, we should expose from 8 to 12 feet distance!
Lighting Difficulties always arise when we try to photograph a group of people, standing or sitting at different distances from camera. These difficulties multiply when we are using camera's built-in or dedicated flash. No matter how powerful our flash is; subjects at different distances do get lit differently. Now this is common sense that as we go away from the light, the intensity decreases. So lets do case study.
So in this diagram, we can see 4 subjects at different distances from camera. So, to photograph all of them, we need to do focusing we can use higher aperture to keep all of them in focus. But It will be little harder for the flash to make them properly lit. more the distance between the subjects, poorer lit the scene. That, if we focus at subject 1, ONLY subject 1 will be properly lit. subject 4 may not be visible. Then, if we focus at subject 2, ONLY subject 2 will be properly lit and subject 1 will be overexposed! It is then obvious that subject 3 and 4 will be gradually underexposed!!
Essentially, we need to know how the intensity of light decreases with increasing distance. Studying physics may give us an answer. But hold on, we need to not to do a Ph.D on it. There is a simple law named “Inverse Square Law of Light”(See the diagram.). Actually speaking, this law is not only for light but for any point source which spreads its influence equally in all directions without a limit to its range. Gravitation, Electrostatics, Electromagnetic Radiation as well as light obey this law. We have got nothing to do with the former three things, so we better concentrate on how light obeys the law. From the simplest point of view, doubling the distance between the light and the subject results in one quarter of the light hitting the subject. Now there cannot be simpler interpretation than this.
Scientific statement of the law (relating to light)can be like, “The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source.” Big bad scientific sentences again! This is great but how to use it in daily life? Lets get back to that case study again(Refer to the right half of the diagram). Assume that the distance between camera and subject 1 is 2 feet and the distance between each if the subject is 1 foot. So the distance between camera and subject 4 is twice than that of between camera and subject 1. Hence, the incident light on subject 4 is a quarter of that incident on subject 1. In photographic terms, subject 5 is underexposed by more than 2 stops. Exposure difference of 2 stops is not acceptable at all. Now don't tell me you shoot in RAW and will later change the exposure; That is not real photography!
We need to be a bit innovative to use this law of Inverse Square of Light. Let us play around the law to understand it better. Go through the following 3 statements.
1.Focus and expose for subject 3.
2.Now the distance between light source and the subject 3 is 4 feet. So the distance between camera and subject 1 and between subject 3 and subject 5 is 2 feet(half of 4 feet).
3.Hence the incident light on subject 3 is one quarter of the incident light on subject 1. And incident light on subject 5 is further one quarter of that on subject 3.
Then what will happen if go away to 4 feet distance from subject 1?; keeping the distance between each subject constant at 1 foot. Refer to the diagram and read the statements.
1.Focus and expose for subject 1.
2.Now the distance between light source and the subject 1 is 4 feet. So the distance between camera and subject 1 and between subject 1 and subject 5 is 4 feet again.
3.Hence the incident light on subject 5 is one quarter of the incident light on subject 1. Or this makes subject 5 two stops underexposed than subject!
This worsens things. After careful analysis of the above said examples, we can arrive at a conclusion. Halving and doubling of the light depends upon the distance between light source and the subject. As we go near the subject, distance at which the light intensity doubles also lessens; and vice versa. Practically speaking, if we focus and expose for 2 feet distance, light intensity will halve at 2 feet distance.
I hope it isn't getting too much of science. As it can be fun doing this much of simple maths and visualizing different situations! The final solution for the problem from first diagram is exposing for subjects from the distance more than double of their range. That is for our problem, as our subjects are spread over 4 feet distance, we should expose from 8 to 12 feet distance!
