Lighting ratios

Common rules

when sunlight passes through greater amounts of particles in the atmosphere at dawn or sunset, exposure times increase compared to a reading taken at noon.Exposure times will obviously be shorter on a clear day. Applying these to a studio situation, the greater the impedance to the light(diffusion, reflection, filtration) the longer  the exposure. Direct light(no diffusion, reflection, filtration) the shorter the exposure. The amount of light falling on the subject decreases to 1/4 of it's original intensity when the light to subject distance is doubled, and increases by 4x when the light to subject distance is halved. For example, if a reading of F16 is obtained when the light to the subject distance is one metre, at two metres the reading would be F8, at 4 meters F4. It is also important to remember contrast in a studio situation is created not only by the reflectance level of the subject matter(SBR) but also by lighting ratios. When referring to lighting ratios the photographer is also referring to lighting contrast.

What are lighting ratios?

Light metres are often incorrectly called exposure meters. Exposure is only one part of its function. It can also be used for measuring lighting ratios. This is achieved by taking an incident reading of the light source from the subject. The metre is pointed at a specific light source to measure the amount of light falling on the subject. If there is more than one light source each light can be measured independently by ensuring only one light source is on at any one time. Lighting ratios and their relationship to film/sensor latitude is best demonstrated and understood at a practical level. Take, for example, a photographer using a film/sensor known to have a latitude of five stops. To make use of this information the photographer would allow a photographer to use a maximum lighting ratio of 32:1(five stops). This ratio would retain detail in the highlights and the shadows.

Example 1

In a darkened studio a person is lit with a single light source from the right- hand side at 90 degrees to the subject. A incident light metre reading is taken from the right- hand side of the person's face directly towards the light source. The aperture is F45 IS 1 second. An incident light metre reading is taken from the left-hand side of the person's face directly towards the opposite side of the studio to where the light is placed. The aperture if F4 at a shutter speed of 1 second. This is a lighting ratio of 128:1(seven stops). To reduce this ratio another light or a reflector(fill) is placed on the left- hand side of the subject. The fill is moved towards or away from the subject until an aperture reading no more than three stops lower in number than that from the main light source(F16) is obtained. This is now a lighting ratio of 8:1.

 

Stops Difference	Lighting Ratio	Stops Difference	Lighting Ratio
2/3	1.5:1	2	4:1
1	2:1	2-1/3	5:1
1-1/3	2.5:1	2-2/3	6:1
1-2/3	3:1	3	8:1