Understanding light in photography Part 1: Inverse Square law


When you are lighting a subject it is important to understand how moving your light source will affect your subject. A while ago I wrote a bout the relative size of a light source in comparison to the subject i.e. a house hold lamp next to a model car compared to the same lamp next to a real car. When you move that light source it also changes its relative size in relation to the subject. The next few days I want to write a bit more about how light works in relation to photography (any feed back is appreciated).

Basic light stuff

The intensity of a light source (its luminescence) is measured in LUX and travels in a ray (straight line from a single source). As the rays of light get further always from a the original source the intensity of a light source weakens, as the rays are distributed in different directions


The lines coming from the source (S) represent rays of light. The total number of rays depends on the strength of the source. A greater density of light rays (lines in each area), the brighter an area will be. The further back you go from the source (S) the dimmer the light source. But the total number of rays wont actually change you just see less of them as they get distributed over a wider area. There is a mathematical formula called “the inverse square law” that will actually let you work all this out. Rather than doing the maths each time it good just to understand the concept.

Inverse-square law

It’s useful to know a little about the inverse square law especially when using flash or studio lights. Basically all the inverse square law says is that an object that is twice the distance from a point source of light will receive a quarter of the illumination. So what it means to us photographers is that if you move your subject from 3 meters away to six meters away, you will need four times the amount of light for the same exposure. This can most easily be achieved by opening the lens aperture two f-stops (or using a flashgun that is four times as powerful.


About shotbyscott

Just another photography blog.. I also like all kinds of accessories
This entry was posted in get it right in camera, I have seen the light and the dark side, Low Light, wow thats flash and tagged , , , , , , , , , . Bookmark the permalink.

2 Responses to Understanding light in photography Part 1: Inverse Square law

  1. shotbyscott says:

    All we really need to know

    An automatic camera will do all the maths for you so, unless you are using manual exposure, you don’t need to worry too much about the details. It is very useful though to have some understanding of what is going on so that it doesn’t come as a surprise when you see the effects of all this in under or over exposed photos. Just remember ‘at twice the distance, a quarter of the light reaches the subject’.

  2. Pingback: Understanding light in photography Part 2: Bounce and reflection. | shot by Scott's Blog

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