Lighting Ratios Confusion

Discussion in 'Lighting Equipment' started by hugebob, May 13, 2010.

  1. Hi All,

    I'm trying to understand the concept of lighting ratios. I'm reading a book entitled, "The Master Lighting Guide" by Christopher Grey. In it, the author explains key-to-shadow, highlight-to-key and highlight-to-key-to-shadow ratios. But, I'm confused by the examples. Take this one:
    "... if your key light reads f11 and you want a 1-stop difference between the key light and the fill light, you will set you set your fill light to read f8. The ratio between these two units would then be expressed as 1:2."
    Now, I understand the 1-stop difference and why it's 1:2. What I don't understand is why the fill light should read brighter than the key light. I would have though that the fill light should read f16: i.e. 1-stop darker than the key light. Similarly, in the author's explanation of highlight to key ratio, it's reversed as to what I would have thought. The highlight should be brighter than the key light. But, the f-stops in his examples are darker: if the key light is f11 and a 1-stop difference is desired, set the highlight source to f16. I would have thought f8. I can't believe this is a typo on the author's part.
     
  2. Two lights are metered - A and B.
    A meters at F11, which means F11 is required to get the proper exposure.
    B meters at F8, which means F8 is required to get the proper exposure.
    F8 allows in more light than F11, which means that A is a brighter light than B.
     
  3. But he's not saying it's brighter, he's saying it's weaker. f8<f11<f16
    If my light is giving me a metered reading of f8 and I want to shoot at f11 I will have to turn the power on my lights up, or move my lights closer to the subject the necessary distance to increase the power.
    When I am trying to set up a specific lighting scheme/ratio, I will first meter the fill, either ambient or strobed, because the fill hits the entire subject. From there I can figure out what I need my key/main light to be at: Ie if the fill is 5.6 and I want a 1:2 ratio, I will need my key to be 5.6 as well, because combined I will have f8. The key light is only hitting one side of the face/subject there fore one side of the face is f8, the other is 5.6 and your exposure is f8.
    If I want to add a hairlight/kicker, depending on the color of the hair/subject I'll want my light to be around f11-11.5.
    The same goes for blowing out the background.
    Hope this helps/makes sense.
     
  4. Here's my thinking. I have two lights set up: one key, one fill. I turn off the fill light and get a reading from my key light: f11. Now, I want a 1:2 ratio. I turn off the key light and power up the fill. I adjust the power on the fill light so that I measure f-what? To my mind, such as it is, I'd think that I'd want to measure f16 on the fill; one stop darker. So, when I shoot the picture at f11, the fill areas are somewhat darker.
     
  5. rnt

    rnt

    All right... The proper aperture for your key light is F11. That's where you'll be setting your camera. The proper aperture for your fill light -would be- F8, but you're shooting at F11 so the fill light is 1 stop underexposed at the proper exposure for your key...
     
  6. Ahhhhh!! Thanks Bob! That clears the cob webs. Thanks to the rest of you too.
     
  7. As Bob says! Your key light is f/11. No fill. Your camera is set to f/11. So far so good. Now we need to add fill. You fill light need to be less powerful than your key light. Remember, the aperture controls the amount of light being allowed though the lens. It has nothing to do with the light source itself. So, if I want my fill one stop darker, then I need to meter it @ f/8. Look at it this way, set your key to f/11 (your aperture is now f/11). If you set you fill to f/16 but keep your camera at f/11, you have now blown out the f/16 side: your aperture is f/11, wider than f/16 yet you have a light firing @ f/16, which is brighter than f/11.
     
  8. John, if your key light is f11 without any fill, how are you going to get a 1:2 ratio by adding fill at f8? If you set your camera to f11, you will overexpose by half a stop won't you? My understanding is that if I want to shoot at f11, both my fill and my main have to be f8 individually, which combine on the main side to give f11...
     
  9. Bob, that brings up a question for me then. Wouldn't you have to stop the fill to f16, two stops lower? Because if you light it one stop above the main at f8, and then set the camera at one stop below the fill at f11, the fill light ends up lighting the same as f11, right?
     
  10. Okay, ignore my previous question! My brain just engaged :) This is what happens when you're in your 40's...the brain slows down a bit!
     
  11. rnt

    rnt

    Maybe we should all be working in EV's or something... My brain hurts!
     
  12. The part that seems to trip people up is that they want to connect the f-numbers to the lights themselves.
    When you meter a light, the meter's telling you what you'd need to set on the camera for a normal exposure. A dimmer light is going to need a wider aperture, so the meter's going to tell you to open up (e.g., from f/11 to f/8). A brighter light's going to need a narrower aperture, so the meter's going to tell you to stop down.
    In other words, the scale works kind of vaguely backward when you're metering lighting.
     
  13. Hi Marc... it can work that way. The fundemental questions are 1) what am I taking a picture of and 2) how do I want it to look. In your decription, you are treating fill light as a baseline light then adding a key. And if that works for you, then great! Nothing wrong with that. In most learning scenarios, we are going to start with one light. The eye/brain is hardwired to "see" one light, the sun. This one light is our key. It establishes are primary light source and hence lighting pattern. Of course there are a lot of variable here! Our primary light source might be 3 strobes firing through a huge sheet of translucent material to take a shot of a tractor or something. Now a key light creates specular highlights, diffused value (or true tonailty) and shadow. Our job is to control all of this along with contrast to create an image. So, once we have our key light, we now compare what it looks like to what we want it to look like. And hey, if we are really lucky, we don't need any more lights. Done! But more often than not, we need to modify our light. How we want to shape the light is all part of our vision for the image. I might simply need to add a reflector to throw some light on the shadow side. I don't even need to meter it! I personally would have a problem starting with fill light. How do I know what needs fill when I don't have a key? I haven't created my specular highlight, my true tonality or my shadow. I haven't given my subject its depth, color and texture. How do I know what I need to "fill" without that? I think it's a matter of how you approach building an image. I can honestly say that after having watched an ASP member photographer (American Society of Photographers, what happens AFTER you get your Masters from PPA in photography!), "build" an image, I couldn't start with fill because controlling contrast is how I make an image. I can't have contrast until I have my key! However, as I said, if it works for you there is nothing wrong with it.
     
  14. We start with a two-lamp set-up
    Consider a portrait 2:1 (Flat Lighting)
    We place the main high and off to the side causing it to shine down on the subject. Let’s say the main delivers 1000 units (watts if you like) on the subject. The fill is placed at lens height, near an imaginary line drawn between camera and subject and adjusted to deliver light to arrive at the subject plane equal in light energy to the main. Thus, the main and the fill each contribute 1000 units of light. Now the frontal part of the face receives light from both. Thus, the total on the frontal areas of the face will be 2000 units. Now some areas of the face are in shadow. These are locations where the main could not reach; we are talking about dimples and valleys and nose shadow etc. Consider the circumstances; 2000 units on the frontal areas and 1000 units in shadow areas. Mathematically this can be stated as a ratio. The ration is written as 2000:1000. We handle this like a fraction that can be reduced by dividing both sides by same number; in this case 1000. The reduced ratio is written as 2:1. This 2:1 lighting ratio is flat lighting -- nearly featureless.
    Now consider a portrait set-up 3:1 ratio (bread and butter lighting). This is the one that wins contests and sells best. To achieve we reduce the fill energy at the subject plane to 1/2 power as compared to the main. We might do this by setting a knob on the fill lamp or we can just move the fill fixture further away from the subject.
    We can calculate the fill-to-subject distance (assumes both main and fill are identical). We measure main-to-subject distance and multiply this value by 1.4. The answer is a revised fill to subject distance. This added distance reduces the light energy playing on the subject by 50% (1 f/stop). The 1.4 factor is derived from a law in physics known as the inverse square law. The idea is to cause the main to deliver 1000 units and the fill 500 units. Now consider the frontal area of the face get light from both fixtures. The values are 1000 main + 500 fill. Thus, the frontal areas receive 1000 + 500 or 1500 units. Shadows receive only the fill’s 500. Ratio is 150:50 reduces to 3:1. This is achieved if the fill is subordinate to the main by 1 f/stop. You can also place main and fill using a meter. The trick is again 1 f/stop difference, fill subordinate.
    Consider 5:1 somewhat more zippy lighting.
    We reduce the fill to 1/4 power by knob on the unit if available or by setting the fill even further back. If the fill is positioned to deliver a 2:1 ratio, the main and fill are equal distance from the subject. To establish 5:1 we multiply the main-to-subject distant by 2. This calculates a revised fill-to-subject distance for the 5:1. This is a two stop difference with the fill subordinate to the main. How is this 5:1? This placement causes the fill to be 2 f/stops subordinate or 25% of the main’s energy. Now the frontal area receives 1000 from the main and 250 from the fill for a total of 1250 frontal and 250 in the shadows. The ratio is 1250:250 or 5:1. This is contrasty lighting.
    Consider 9:1 somewhat theatrical, very zippy lighting.
    If we reduce the fill to 1/8 power by knob or measurement 3 f/stops subordinate to the main. From the 2:1 position the multiplier is 2.8 from the 3:1 position the multiplier 2, from the 5:1 position the multiplier is 1.4. Main continues to deliver 1000 units, the fill 125 units. Thus, the frontal areas receive 1125 the shadows 125. The ratio is 1125:125 = 9:1. This is a 9:1 exceedingly contrastry lighting considered theatrical. Note 9:1 is the maximum ratio. Any more reduction and the shadows will be void of detail.
    To review:
    Main at 4 feet fill at 4 feet ratio is 2:1
    Main at 4 feet fill at 5.6 feet ratio is 3:1
    Main at 4 feet fill at 8 feet ratio is 5:1
    Main at 4 feet fill at 11 feet ratio is 9:1
    It is no accident that the fill-to-subject distance follows the f/number set which is 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22.
    Note each value going right is its neighbor times 1.4.
    Each value going left is its neighbor divided by 1.4
    I call this kind of math gobbledygook
     
  15. Yikes! Alan, the last line of your post is the only one that I understood. I realize that's my limitation.
    Here's a shot, using lights. I hope my upload works.
    00WSVG-244007584.jpg
     
  16. Cig is a marvelous example of a 9:1 theatrical ratio.
    The math helps us achieve a specific ratio not by hit or miss but as delibate act i.e. we can repeat the set-up again and again.
     
  17. Robert: Whether the ratio is shown as 2:1 or 1:2 the reading is always the same; The main light is 2 times brighter than the fill light. The larger number is always the main light because the main light is always equal to or brighter than the fill light.
    Important note: Portrait photographers see lighting ratios differently in that they compare the intensity of highlight areas and shadow areas on the subject itself. Two light units both having the same intensity have a 1:1 ratio with each other where they stand, but they have a 2:1 portrait lighting ratio on the subject because the highlight gets light from both units and the shadow gets its light from only one. The portrait photographer uses his meter to read the light falling on the subject. This suggests that you might set and measure the fill light first to establish a shadow that is bright enough for the camera to see, and then add the main light to it by metering the highlight created with both lights on. A gray card will make this a lot simpler. The procedure would be to adjust the main light until you have the ratio you want. You would not want to adjust the fill light any more because it lights both highlight and shadow and changing it throws the main light off too.
    The discussion that follows focuses on the light unit ratio rather than the portrait lighting ratio. BTW, you convert portrait lighting ratio to light unit ratio by subtracting 1 from the main light number. That is, in the portrait ratio the fill light always contributes only one more measure of light to the highlight areas. Alan's list of light differences above shows portrait lighting ratios.
    The confusion comes in because the author of the book you read intends for you to measure each light separately using your light meter. Actually, I prefer to measure only the main light and calculate the brightness of the other light. This is a very simple step using our modern everyday calculators. The adjustment factor from the main light to the fill light is merely the square root of the main light number in the lighting ratio itself. This adjustment factor is a number photographers will recognize as the familiar f-stop number. Whether you use it as a multiplier to calculate distance or a direct adjustment for a variable light unit, it works the same way for all ratios.
    Incidentally I would suggest that you make a separate reading of your whole set once you have it ready to get the exposure you need for the camera. All of the preliminary readings of the lights alone are to get them balanced with each other only.
    The explanation for the fact that some of the descriptions above are confusing is that a change in light intensity is exponential not linear in nature. That is, twice the light is NOT twice the distance. What is more, the relationship between light intensity and f-stop numbering is not clear when you have to adjust units that are not already marked in f-stops.
    The light unit ratio 3:1 is instructive because the adjustment factor is not an ordinary f-stop number. The square root of 3 is 1.73. Measuring your lights with a light meter will find the intensity of one light, but, as difference to the other is not a typical f-stop number, you may find yourself resorting to the simple solution of using a familiar f-stop anyway. That is, you can easily set a 4:1 ratio as a substitute for a 3:1 ratio because a two f-stop difference is easy to understand. This is not technically correct, but as many would argue, Who can tell the difference?
     
  18. Brilliant explanation Alan, that explanation was a Eureka moment for me and summed up lighting ratios perfectly Thank you!
     
  19. I knew I could rely on Photo.net readers. I read this section of my book over and over and just didn't get it. One post at photo.net and I'm straight. Thanks to all of you!!!
     
  20. The objective is command and control of the intensity of the light playing on the subject. The lighting ratio is the chief adjustment over the contrast of the finished photograph. A grasp of lighting ratio and exposure are the keys to kingdom. Otherwise your working in a trial-and-error mode and gone is the ability to repeat a set-up with accuracy.
    Radiant energy such as light, magnetism, gravity, radio waves etc. propagate according to the law of the inverse square. Everyone knows that light falls off with distance. Now the light from a candle can be seen to radiate out in all directions. Use your imagination, invasion a candle burning inside a hollow sphere.
    The candle light bathes the inside surface of the sphere. Geometry tells us that the area of the surface area of a sphere is proportional to the square of the radius. If we double the radius, the candle is moved back from the walls of the sphere. Math reveals that if we back off the candle by a factor of 2x, the now enlarged sphere has 4 times more surface area. What I am saying is, if you double the distance candle-to-target, the light must play on 4 times more surface. The light energy remains the same but is now spread out over 4 times more area its brightness is reduced to 1/4 (25%) of the original.
    Now consider, If the sphere's radius is increased 1.4 times (the square root of 2) the surface area of the sphere doubles. Now the light must play on 2x more area and the light intensity on the sphere's surface is 1/2 (50%) of the original. This is the origin of the 1.4 multiplying factor.
    Incidentally, on the same theme, if we want the camera lens to allow 2x more light to enter, we must double the surface area of the aperture opening. To accomplish we multiply the aperture diameter by 1.4. The result is this number set: 1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32. Note each number going right is its neighbor on the left multiplied by 1.4. Look familiar? This is the number set you known an love as the f/numbers.
    Controlling lights is best done by measurement with a meter. We can make approximate adjustments by distance changes or power adjustment but these methods have pitfalls. As to distance changes; technically, only a point source such as bare bulb follows the law of the inverse square. When umbrella or soft box or diffuser or bounce is utilized, the law of the inverse square goes out the window. This is why umbrellas and soft boxes work, they diffuse and in doing so, they diminish the extent of falloff with distance. These devices change the point source to a broad (expansive) source and this act mitigates intensity changes with distant.
    Stick with ratios induced by 1 f/stop (full f/stop) changes. These are the sequence 2:1 - 3:1 - 5:1 - 9:1. Likely, will never hit these values on the button however, they are the targets to aim at.
     

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