To measure the contrast ratio you will need a light meter. The process starts with you measuring the main source of light, or the key light.
Get a reading from the brightest area on the face of your subject. Then, measure the area lit by the secondary light, or fill light. To make sense of what you have just measured you have to understand that the information you have just gathered is in F-stops, a measure of light. With each additional F-stop, for example going one stop from f/1.4 to f/2.0, you create a doubling of light. The reverse is also true; moving one stop from f/8.0 to f/5.6 results in a halving of the light.
Let’s say you grabbed a measurement from your key light of f/8.0. Then, when you measured your fill light area, you get a reading of f/4.0. This will lead you to a contrast ratio of 4:1 because there are two stops between f/4.0 and f/8.0 and each stop doubles the amount of light. In other words, two stops x twice the light per stop = four times as much light at f/8.0 than at f/4.0.
theslantedlens.com/2017/lighting-ratios-photo-video/
Examples in the post
What Can’t Ratios Tell You?
Ratios are great but they don’t solve all the challenges of a lighting set up for a DP. Ratios help with the mood but there are a number of other factors that need to be considered as well.
1. Type of Lighting:
Soft light vs. Hard Light. Ratios can’t help you determine whether to use soft light or hard light. You have to know what the differences are and when to employ them.
2. Direction of Lighting
Ratios can’t help you determine where the Key light should be coming from.
3. Color
Ratios are no good for color. You have to develop your project and understand how the color temperature of lights impacts mood and feeling.
To get the most out of the images below you have to know what you are looking at. The FSI False Color is user adjustable (one of the reasons it is so powerful) and they way I have set it up reads like this.
RED = 3 Stops Over
Yellow = 2 Stops Over
Light Green = 1 Stop Over
Grey = Key
Teal = 1 Stop Under
Light Blue = 2 Stops Under
Dark Blue = 3 Stops Under
Black = 4 Stops Under
Replicating the fake color process through Foundry Nuke:
set cut_paste_input [stack 0]
version 13.2 v7
push $cut_paste_input
Expression {
expr0 "(r > 2) || (g > 2) || (b > 2) ? 3:0"
expr1 "((r > 1) && (r < 2)) || ((g > 1) && (g < 2)) || ((b > 1) && (b < 2))\n ? 2:0"
expr2 "((r > 0) && (r < 1)) || ((g > 0) && (g < 1)) || ((b > 0) && (b < 1))\n ? 1:0"
name Expression4
selected true
xpos 631
ypos 54
}
Blur {
size 100
filter quadratic
name Blur1
selected true
xpos 631
ypos 120
}
Blurring an image using the same scheme:
set cut_paste_input [stack 0]
version 13.2 v8
push $cut_paste_input
Dot {
name Dot27
selected true
xpos 446
ypos 2259
}
set N329d7800 [stack 0]
Expression {
expr0 "(r > 2) || (g > 2) || (b > 2) ? 3:0"
expr1 "((r > 1) && (r < 2)) || ((g > 1) && (g < 2)) || ((b > 1) && (b < 2))\n ? 2:0"
expr2 "((r > 0) && (r < 1)) || ((g > 0) && (g < 1)) || ((b > 0) && (b < 1))\n ? 1:0"
name Expression1
selected true
xpos 521
ypos 2167
}
set N66560500 [stack 0]
Shuffle2 {
fromInput1 {{0} B}
fromInput2 {{0} B}
mappings "4 black -1 -1 rgba.red 0 0 rgba.green 0 1 rgba.alpha 0 3 black -1 -1 rgba.blue 0 2 rgba.green 0 1 rgba.green 0 1"
name Shuffle7
selected true
xpos 621
ypos 2241
}
push $N329d7800
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy1
selected true
xpos 621
ypos 2315
}
Blur {
size 50
maskChannelInput rgba.alpha
name Blur3
selected true
xpos 621
ypos 2384
}
push $N66560500
Shuffle2 {
fromInput1 {{0} B}
fromInput2 {{0} B}
mappings "4 black -1 -1 rgba.green 0 1 black -1 -1 rgba.red 0 0 rgba.blue 0 2 rgba.blue 0 2 rgba.blue 0 2 rgba.alpha 0 3"
name Shuffle1
selected true
xpos 730
ypos 2242
}
push $N329d7800
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy2
selected true
xpos 793
ypos 2318
}
Blur {
size 100
maskChannelInput rgba.alpha
name Blur4
selected true
xpos 793
ypos 2380
}
Merge2 {
inputs 2
operation matte
name Merge3
selected true
xpos 621
ypos 2459
}
push $N66560500
Shuffle2 {
fromInput1 {{0} B}
fromInput2 {{0} B}
mappings "4 rgba.red 0 0 rgba.red 0 0 rgba.red 0 0 rgba.alpha 0 3 black -1 -1 rgba.blue 0 2 black -1 -1 rgba.green 0 1"
name Shuffle8
selected true
xpos 521
ypos 2241
}
push $N329d7800
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy3
selected true
xpos 521
ypos 2315
}
Merge2 {
inputs 2
operation matte
name Merge5
selected true
xpos 521
ypos 2506
}
The two options together
set cut_paste_input [stack 0]
version 13.2 v8
push $cut_paste_input
Group {
name FalseColorCompositionTest
selected true
xpos 389
ypos 192
}
BackdropNode {
inputs 0
name VIEWER_1
xpos 458
ypos -174
}
BackdropNode {
inputs 0
name VIEWER_2
xpos 722
ypos -108
bdwidth 161
bdheight 68
}
BackdropNode {
inputs 0
name VIEWER_3
tile_color 0x8e8e8eff
note_font_size 42
xpos 738
ypos 301
bdheight 110
}
BackdropNode {
inputs 0
name VIEWER_4
xpos 394
ypos -43
}
Input {
inputs 0
name Input1
xpos 467
ypos -326
}
Dot {
name DotIN
xpos 501
ypos -258
}
Dot {
name Dot2
label "\n"
xpos 501
ypos -144
}
set N28599ad0 [stack 0]
Dot {
name Dot27
xpos 611
ypos -144
}
set N2859e960 [stack 0]
Expression {
expr0 "(r > 2) || (g > 2) || (b > 2) ? 3:0"
expr1 "((r > 1) && (r < 2)) || ((g > 1) && (g < 2)) || ((b > 1) && (b < 2))\n ? 2:0" expr2 "((r > 0) && (r < 1)) || ((g > 0) && (g < 1)) || ((b > 0) && (b < 1))\n ? 1:0" name Expression4 xpos 828 ypos -148 } push $N2859e960 Expression { expr0 "(r > .6) || (g > .6) || (b > .6) ? 3:0"
expr1 "((r > .22) && (r < .6)) || ((g > .22) && (g < .6)) || ((b > .22) && (b < .6))\n ? 2:0" expr2 "((r > 0) && (r < .22)) || ((g > 0) && (g < .22)) || ((b > 0) && (b < .22))\n ? 1:0"
name Expression1
xpos 710
ypos -186
}
Switch {
inputs 2
name Switch1
xpos 760
ypos -78
}
set N28650240 [stack 0]
Dot {
name Dot1
xpos 878
ypos 5
}
set N286582c0 [stack 0]
Shuffle2 {
fromInput1 {{0} B}
fromInput2 {{0} B}
mappings "4 black -1 -1 rgba.red 0 0 rgba.green 0 1 rgba.alpha 0 3 black -1 -1 rgba.blue 0 2 rgba.green 0 1 rgba.green 0 1"
name Shuffle7
xpos 844
ypos 41
}
push $N2859e960
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy1
xpos 844
ypos 115
}
Blur {
size 100
maskChannelInput rgba.alpha
name Blur3
xpos 844
ypos 184
}
push $N286582c0
Shuffle2 {
fromInput1 {{0} B}
fromInput2 {{0} B}
mappings "4 black -1 -1 rgba.green 0 1 black -1 -1 rgba.red 0 0 rgba.blue 0 2 rgba.blue 0 2 rgba.blue 0 2 rgba.alpha 0 3"
name Shuffle1
xpos 953
ypos 42
}
push $N2859e960
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy2
xpos 953
ypos 111
}
Blur {
size 200
maskChannelInput rgba.alpha
name Blur4
xpos 953
ypos 173
}
Merge2 {
inputs 2
operation matte
name Merge3
xpos 844
ypos 259
}
push $N286582c0
Shuffle2 {
fromInput1 {{0} B}
fromInput2 {{0} B}
mappings "4 rgba.red 0 0 rgba.red 0 0 rgba.red 0 0 rgba.alpha 0 3 black -1 -1 rgba.blue 0 2 black -1 -1 rgba.green 0 1"
name Shuffle8
xpos 744
ypos 41
}
push $N2859e960
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy3
xpos 744
ypos 115
}
Merge2 {
inputs 2
operation matte
name Merge5
xpos 744
ypos 354
}
push $N28650240
push $N28599ad0
Saturation {
saturation 0
name Saturation1
xpos 398
ypos -15
}
push $N28599ad0
ContactSheet {
inputs 4
width 6100
height 2501
rows 1
center true
name ContactSheet1
xpos 482
ypos 437
}
Dot {
name DotOUT
xpos 512
ypos 553
}
Output {
name Output1
xpos 478
ypos 630
}
end_group