Lighting Ratios and Incident Metering Demystified
Light ratios are confusing because there is more than one way to describe the difference in intensity between lights and because incident readings used to infer reflected highlight-to-shadow ratios. So first let's start by defining terms:INCIDENT RATIO: An incident ratio expresses the difference in intensity between two or more lights based on a meter reading taken at the point the light hits a reflective surface.
REFLECTED RATIO: A reflected ratio expresses the difference in intensity between the light reflecting from the highlights vs that reflecting from the shadows of the same three dimensional object of similar tone like a white ball or the front of the face. The reflected ratio is also referred to the highlight:shadow or H:S ratio. A 2:1 H:S ratio indicates that the highlights appear twice as bright as the shadows.
The conventional for ratio notation in still photography is reflected highlight:shadow values. Thus a 2:1 ratio means the highlights on the front of face appear twice as bright at the shadows on the front of the face. Other fields such as cinematography and stage lighting which deal with a many lights typically use incident ratios. Thus a cinematographer asking for 2:1 ratio between to lights will get what a still photographer would call a 3:1 From this point on all ratios are reflected H:S, unless written like this: 1:1 (incident)
Design of an incident meter
The Sekonic L-358 which I use has a retractable white dome which is designed to mimic the way light hits a three-dimensional object. The meter is used two different way for two different purposes: 1) setting ratios with the dome down pointing at each light separately, and; 2) determining the exposure with the dome up, pointing at the camera lens while all lights are fired. Both applications the meter requires memorization of f/stop values which represent a doubling in the light intensity:
| f/22 | f/16 | f/11 | f/8 | f/5.6 | f/4 | f/2.8 | f/2 |
| 128x | 64x | 32x | 16x | 8x | 4x | 2x | 1x |
| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
Each increase in the numerical f/stop value represents doubling in light intensity. For example a reading of f/16 indicates the light is twice as bright as an f/11 reading. The shutter speed is also set on the meter, but it does not affect the exposure in a studio environment.
The Overlap Factor
Lighting ratios are set by measuring the incident strength of the key and fill lights separately and then inferring, based on an understanding of how they overlap on the subject, what the reflected intensity is.
The fill light is the "anchor" of the lighting ratio and is always denoted as 1 on the right side of the N:1 ratio. The underlying assumption is that the fill is even over the entire surface measured. The need for even fill for consistent ratios and lighting patterns is so important I've written a separate tutorial to the role of fill.
When the fill light is placed near the camera axis any other light placed between the fill and the front of the subject's face will overlap with it. A light placed behind the subject such as an accent hair light may or may not overlap the fill. To be able to visualize that it is necessary to know what the fill is actually illuminating in a lighting scenario. The best way to do that is to shoot a test shot with only the fill. This is also an excellent way to spot any gaps in the fill which will occur anywhere it casts a shadow.
From this point "neutral fill" is defined as a light source placed near the camera axis where it will illuminate the entire scene the camera photographs as evenly as possible creating the least amount of shadows when view separately. "Key" is defined as a light source equal or stronger in incident strength relative to the fill which is used to create the highlight and shadow pattern on the face or object photographed. "Background" light is defined as a light illuminating the background behind the subject but not hitting the subject. "Accent" lights refer to any light other than the key light which hits the subject and creates a highlight (e.g. hairlight).
The key light will always overlap neutral fill with the resulting reflected value equalling the sum of the individual key and fill reflectances. In otherwords 1+1 = 2 in the highlights when key and fill are identical incident strengths. Since there is only 1 unit of light hitting the shadows the resulting ratio is 1+1:1 or 2:1. If the key light incident strength is greater than fill ratios greater that 2:1 will occur. For each f/stop difference in the key vs fill indicated by the incident reading there is a doubling of the key light intensity, but not a doubling of the ratio:
| Key Light Difference | Math | H:S Ratio | Suggested usage |
| +0 f/stops | 1K+1F:1F | 2:1 | Women, kids, anyone on white background |
| +1 f/stop | 2K+1F:1F | 3:1 | Men, older boys, still life, etc. |
| +1-1/2 f/stops | 3K+1F:1F | 4:1 | Character studies where aged look or less shadow detail is desired |
| +2 f/stops | 4K+1F:1F | 5:1 | Profile views where less shadow detail is desired |
Metering Technique - Sekonic L-358
Adjusting the meter to match actual results
A meter, out of the box, will be accurate per ISO standards but usually your camera will not be. For example, at ISO 100 the true speed of Canon cameras is about 120. If you find the reading you take with the meter using the correct procedure of pointing the dome at the camera and firing all lights does not create a perfectly exposed file then perform the following test / adjustment:
All that is really needed for the test is a white terry towel, but the other targets are useful references.This is test shot from a recent calibration of my L-358. The numbers are the RGB eyedropper readings in this file which I determined was best exposed per the texture of the towel. Paper white should be about RGB=240. The only 255 values should be in specular highlights. I don't try to keep exposure on the "bleeding" edge of overexposure. That leaves no margin for error. It is trival to adjust a slightly underexposed file when editing, but its much more difficult to clone in blow highlights.
Set up one light. Perform custom WB on a gray card to ensure the RGB channels are all exposing the file equally. Take a meter reading, then shoot a bracketed set of files from 2-stops under to 2-stops over the reading by adjusting camera aperture. Open the files in an editing program using normal workflow. Look at the texture in the towel.Pick the file which reproduces it most accurately. If it is not the file taken at the meter reading, then compute the difference in f/stops. For example if the meter reading was f/5.6 but the f/6.3 file produced the best exposure the difference is 1/3 f-stop.
Turn on the meter Press ISO 1 and ISO 2 buttons at the same time. It should read "Adj 0.0". Turn the wheel to enter the exposure compensation. The meter uses decimals .3 and .6 for the 1/3 stop increments. For example, if the meter reading was f/5.6 but the best exposure was f/6.3 the wheel would be turned until "Adj 0.3" appears.
Go back and take another reading of the lights and the meter will now read the same as the best exposed fike and match the results the camera is producing. A square black icon containing [+/-] will appear on the meter readout. All subsequent readings will now be accurate.
Sekonic L-358 Meter Preflight Checklist:
- With meter off set DIP switches in battery compartment to UP-DOWN-UP-UP
- Turn on Meter. Press Mode and turn wheel to select the desired metering mode -
e.g., manual (lightning bolt), via PC cord (lightning bolt + C) - Press ISO 1 button and enter ISO 100
- Turn wheel until it reads 125 (1/125th sec). The letter "T" should be visible above it.
- Press ISO 1 and ISO 2 buttons at the same time to check compensation factor. It should be the value determined in the test above.
- Set Camera to M (Manual), ISO 100, and adjust shutter to 1/125th sec. Check other camera settings like white balance.
Step 1 - Adjust the Fill Light Intensity
Turn on only the fill light. Lower the meter dome and hold the meter where the fill light hits the center of the face aiming it directly at the light. Fire the light and note the reading displayed as an f/stop value. Adjust the intensity of the light until the meter reads f/8. Take several readings to insure the flash power has stablized. Turn off the fill light.
Step 2 - Adjust the Key Light Intensity
Turn on only the key light. Position about 45 degrees to the right of the centerline of the subjects nose and about a foot higher. Hold the meter, dome still lowered, where the light hits center of the face and aim it directly at the key light. Fire the light and note the reading, displayed as an f/stop value. We want a 3:1 H:S ratio so asjust intensity of the light until the meter reads f/11, one stop brighter than the f/8 fill. Take several readings to insure the flash power has stablized.
Step 3 - Make the Exposure Reading
Turn the fill light on so key and fill are overlapping and creating a highlight:shadow pattern on the subjects face. Make a mental note of what a 3:1 ratio looks like by eye.
A different metering technique is used for the exposure reading. All the lights are turned on and fired. The meter dome is raised. The meter dome is pointed directly at the lens of the camera. The goal here is to get the light to hit the dome the same way the camera sees it hit the face. The camera axis fill will illuminate the entire dome, the key light only about half of it. The two are averaged by the meter to determine a "middle-gray" exposure value that should correctly expose the subject. It is important not to shade the meter with your body or refect light from your clothing into it when taking readings. Standing behind the subject on the side opposite the key light will eliminate this problem.
With a bit of experince you should be able to just look at the light pattern on the dome and compute the exposure mentally using the fill reading as baseline. Here it will be the sum of all the fill, plus about half the key light intensity because the key light is only hitting about half the dome. Fill is at f/8, Key is 1-stop brighter
As many variables as possible should be eliminated when preforming baseline tests. In this exercise only key and fill were used. Later If background and accent lights are used they should be turn on also before making the exposure reading to take any reflected spill into account. When I'm not sure what the lights are doing or what they might be reflecting off I'll put a wide angle lens on the camera after I've done the ratio set-up and taken the exposure reading, then shoot a series of photos at the meter indicated exposure, one light at a time. Its very illuminating!
Light falls off at the inverse-square if the distance. In plain English it means if the distance is doubled the intensity is two-stops lower. An easy way to remember light fall off is to use the f/stop numbers. As a light is moved from out 2.8, 4, 5.6, 8, 11 feet, meters, or cubits the light will fall off by one stop at each increment. Thus if the subject is 8ft. from the fill light and the background is at 11ft. the background will get hit by one stop less light than the subject. If the background is white it look gray. If gray it will look darker gray. A separate background light may not even be needed.
Reading the Background: When neutral fill is used it will illuminate the background. Any other light or reflective surface will also contribute to its illumination to some extent via bounce. Because of this I make the background light reading with the key and fill lights on with the meter dome placed dome up at the spot I see behind the subject in the viewfinder, setting it to match the Key+Fill exposure reading.
Reading the accent lights: Accent lights should be used judiciously especially on dark backgrounds where bright equals distraction. They should be positioned on the subject in such a way that their brightness pull the veiwer's eye over the face to reach them. Readings for accent lights should be taken where the light hits the subject with the dome down. For a hairlight that means the top of the head, not down in front of the face. Since accent lights don't typically overlap the fill light their meter reading will match their appearance. Since I don't want them to be a distraction I will usually start by setting them to match the fill light reading and adjust visually by viewing form camera position.
Evaluating exposure
An incident meter is an averaging device. It sees all the light in the room including what is bounced off walls, the ceiling and furniture. Because of this the default exposure reading of the meter will not produce an ideally exposed digital file, which for digital means one which retains detail in all but the brightest specular (mirror-like) refections in the photo.
The camera histgram is wonderful tool for evaluating exposure, but only when used with the same consistent target from session to session. I use a test target which combines a Kodak 18 percent gray card, a Kodak Color Separation Guide and Grayscale, an a tri-color QPCard target.
Above is a test shot from a series I used to determine the correct exposure for various shooting distances with my DIY reflection/diffuser. I use the same target while setting ratios and exposure at the beginning of every studio session.
The white, gray and black targets create visible spikes on the histogram. Finding the correct exposure is just a matter of adjusting the aperture until the spike from the white patch kisses the edge of the histogram window on the right. The photo above show the Photoshop levels histogram for a slightly underexposed file. The camera histogram looks similar.
Throw in the towel: Since highlight detail is critical all that is really needed for the exposure test is a white object with texture, like a white towel with texture. Its histogram spike will be a rough barometer for exposure. Enlarging the file in the camera viewfinder or editing program will reveal if the texture has been retained.
Contact: Chuck Gardner
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