|
Lighting
Ratios | Background Light Intensity
Subject-to-Background
Distance
Lighting
Ratios
Unless each of the four
lights we've discussed is at the proper intensity, the formula lighting approach
will not work.
 Since
the key light is the dominant light on the subject, it must be stronger than
the fill light. In color production the fill should be about one-half the intensity
of the key.
This key-to-fill brightness
difference is expressed in terms of a lighting ratio.
If the key light is twice
as bright as the fill, the ratio will be 2:1 (which is the standard for color
TV). Using the 2:1 ratio, if the key light is 2000 lux, the fill will be 1000
lux; if the key light is 90 foot-candles (FC) the fill light would be 45 FC.
Although many lights may
be used in a scene, the lighting ratio refers to the ratio between just two
lights: key and the fill.
The key-to-fill ratio affects
how the form, dimension and surface texture of subject matter will be seen.
To achieve dramatic effects, and occasionally to meet the needs of special subject
matter, ratios other than 2:1 can be used. More on that later.
If a lux or foot-candle
meter isn't available to establish the proper lighting ratios, a standard photographic
light meter can be used. The f-stop difference between the intensity of lights
can be translated into a lighting ratio.
To achieve a standard 1:2
ratio, make the key light one f-stop brighter than the fill. The key light in
this example could (when measured by itself) call for an exposure of f:16 and
the fill light (by itself) an exposure of f:11.
Lighting Ratios
With Differences (In
F-stops) Required Between Key and Fill Light Intensities.
1:1
- no difference (flat lighting)
2:1
- One f-stop (for general color photography, videography)
3:1
- One and two-thirds f-stops (for general black and white photography, videography)
4:1
- Two f-stops (for low-key dramatic effect)
8:1
- Three f-stops (for a very low-key dramatic effect) For ratios beyond this
the dark areas will probably just be rendered as black, without discernible
detail.
Back
to top
Background
Light Intensity
Because the background is
of secondary importance to the center of interest, it should receive a lower
level of illumination. Generally, the intensity of background lights should
be about 2/3 the intensity of key lights. This will insure that the central
subject matter stands out slightly from the background.
In case you have forgotten
Math 101, you can get two-thirds of any number by multiplying it by two and
dividing the result by three. Therefore, if the key is 2000 lux, the light falling
on the background should measure about 1300 lux.
If you are
using a photographic meter to set light intensities, the background light should
read 1/2 to 2/3 of a stop less on the exposure meter than the key light.
 Since backgrounds
are typically one-dimensional (flat) and of secondary importance to the main
subject matter, the placement of the lights and their angles is not critical.
But, the light across the background should be even--especially if you are using
chroma key. By walking along the background with a light meter, any dark or
bright areas can be quickly found.
Back
to top
Subject-to-Background
Distance
Shadows on backgrounds from
mic booms, moving talent, etc., can be distracting and annoying. Background
lights will lighten, but normally not eliminate, shadows. However, by moving
subjects 3 meters (9 or more feet) away a background, you will find (if the
key is at an elevation of 45 degrees) that shadows will end up on the floor
(out of sight) instead of on the back wall behind the subject.
Sometimes, however, it's
necessary for talent to move in close to a background. An example would be someone
shadows from the front lights almost invisible--if you don't mind the soft,
diffused look it will create in the video. Otherwise, you will just need to
use a key angle that doesn't create distracting shadows.
Unduly dark
backgrounds can be brightened up by using explaining a chart on a wall.
Back
to top
|
