Correct Answer for 3:1 Lighting Ratio - Page 5
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  1. #41
    Join Date
    Sep 2005
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    Highlands Ranch CO
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    Default

    Okay. I got that it's 2.5 stops.

    But, I don't see any of your answers as 2.5 stop difference. Correct me if I'm wrong, please!

    My rookie logic...

    5.6 to 8.5 would be 1.5 if I could find 8.5, I find 9.5 at 1½ stops.

    5.6 to 11 is only 2 full stops

    4 to 11.5 would be 3½ but again, I find 13.5 instead of 11.5

    So I think the answer would be:

    5.6 to 13.5 for 2.5 stops, 5:1 ratio

    I'm using the aperature chart in the London book as a reference to answer this question as this is one of the recommended books for the test.
    Last edited by Deb_Wat; 09-08-2006 at 06:40 PM.

  2. #42

    Default

    Deb
    Your on the right track. The difference in F stops between the highlight side and the shadow side is going to be 2.5. But that is for the overall exposure not each light metered seperately. Now go to Kirk's diagram and insert some of my F stop combinations into his formula. One of them will give you a circle that has 500 for the main and 100 for the fill.
    Last edited by David_A._Lottes; 09-08-2006 at 06:57 PM.

  3. #43
    Join Date
    Aug 2005
    Location
    Starke Fl
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    4,106

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    Deb

    Most auto SLR (35) /Digital cameras adopted the 1/4 point and 1/2point fstops on the LCD's it made it easier to read 6.7 rather than 5.6.5 or 5.6 1/2
    so you see on the camera 1/2 or 1/4 stops like 4.5, 6.7 etc.

    and on some camera and meters you can choose a setting of 1/2 or 1/4 points
    to show on the display
    “The price of anything is the amount of life you exchange for it.” Henry David Thoreau

  4. #44

    Default

    Deb
    Assume that whatever F stop value you use is 100 watts in Kirk's diagram and remember to double it everytime it goes up a stop. For example if 5.6 is 100 and that's the fill than F8 would be 200, F11 would be 400 and F16 would be 800 watts. Then add the original 100 watts from the fill. I'm going to lunch, Good Luck!

  5. #45
    Join Date
    Aug 2005
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    Petaluma, CA
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    5,479

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    Deb

    Remember not to confuse light ratio with exposre. Given Kirk's wonderful draw out drama (old Mad Magazine term), if you are using 8 for your main and 5.6 for your fill, your main lit side will read f9.6, not f8 - you need to add that half stop of light to the intensity.

    Michael
    Michael Gan,M.Photog.Cr. CPP,
    Meritage House of Photography

    If your business depends on you, you don't own a business-you have a job. And it's the worst job in the world because you're working for a lunatic... You can't close it when you want to, because if it's closed you don't get paid. You can't leave it when you want to, because if you leave there's nobody there to do the work. You can't sell it when you want to, because who wants to buy a job?
    —Michael Gerber
    http://www.meritageonmain.blogspot.com

  6. #46

    Default Gotta Go!

    Sorry Gang
    I gotta go. The middle answer is what I was looking for. A 5:1 Ratio is 2.5 stops difference between the highlight side and the shadow side. If 5.6 is the fill and has a watt value of say 50 watts, you need 5 times that to get the 5:1 ratio 5 x 50 = 250. Now if you open up one stop from 5.6 you get F8 and need 100 watts for that, open up another stop to 11 and you need 200 watts. Add the 50 from the fill and you get 250 watts. So your Main meters at 11, your fill meters at 5.6 and combined the exposure is 11.6 or about 2.5 stops difference from the shadow side to the highlight side a 5:1 ratio. Have a great weekend! - David

  7. #47

    Default

    I hate to throw another wrench into this but it has always been my understanding that the ratio is a comparison of the main to the key metered separately. Given that I have to agree with Deb there is no right answer. And unless I’m doing something wrong with the circle diagram it is not consistent in my tests. I’ll provide some examples that I just did in the studio. FYI: I used a Sekonic L-358 for these tests.

    First example:
    High Contrast
    Fill = f2.8
    Key = f/8.5 ½
    Since this is 3.5 stops difference I would state this as a 7:1 ratio
    Using the circle diagram the fill = 100, key = 700, resulting in an 8:1 ratio.

    I arrived at the key amount by adding 100 units for each ½ f/stop increase over f/2.8 given that the fill is 100 units:
    F2.8 ½ 100 units
    F4.0 100 units
    f/4.0 ½ 100 units
    f/5.6 100 units
    f/5.6 ½ 100 units
    f/8 100 units
    f8 ½ 100 units
    total 700 for the key light

    Metering the lights separately I set the fill at f/2.8 and the key at f/8.5 ½. With both lights on I took a meter reading with the meter dome pointed to the camera position. That measurement resulted in f/8.5 ½. The fill added no additional exposure. Result is a 7:1 ratio.

    Example 2
    Mid contrast
    Fill = f/2.8
    Key = f/4.0 ½
    This is 1.5 stops difference so I would call this a 3:1 ratio (my favorite)
    Using the circle diagram fill = 100 key = 300, which = 4:1 ratio.

    Again I set both lights and metered toward the camera. The result was f/4.0 7/10, an increase of 2/10 of a stop. Not quite the ½ stop increase the circle test would indicate.

    Test 3
    Low contrast
    Fill = f/2.8
    Key = f/40

    On stop difference, I would say this is a 2:1 ratio.
    Using the circle fill = 100, key = 200, which is a 3:1 ratio.

    Both lights on resulted in f/4.0 ½; the fill finally added a full ½ stop of additional light.

    I may not understand the circle diagram correctly, if not please enlighten me. I'm sure it would be helpful for everyone to know.

    For my purpose I always meter each light separate and set the ratio that I believe is appropriate for the given situation. And then I usually change that further with the addition of a reflector to help fill the shadow side. Go figure HAHA!
    Last edited by Joseph_Froeschle; 09-08-2006 at 10:09 PM.

  8. #48

    Default I'm Back

    Hi Joseph
    Your first example of the 7:1 ratio throws us into a Zone system kind of curve. The difference between the shadow side and the highlight side is so extreme the Key light is over whelming the fill. With a ratio this high you may as well have no fill as have a fill of only 2.8. I say zone system because as it goes there are only seven (I think) zones of texture beyond that your either black or white . You've gone to the point of no return. Your next test is about right. 2/10th not much but something. Then finally you get to the the 3:1 ratio and it works. I suggest you try your tests again shooting for a 2:1, 3:1, 4:1, 5:1, and 6:1 using Kirk's circle. I think you'll find it works. Good luck - David

  9. #49
    Join Date
    Aug 2005
    Location
    Normal, Illinois
    Posts
    7,783

    Default Check the Ratios Again

    I hate to throw another wrench into this but it has always been my understanding that the ratio is a comparison of the main to the key metered separately. Given that I have to agree with Deb there is no right answer. And unless I’m doing something wrong with the circle diagram it is not consistent in my tests. I’ll provide some examples that I just did in the studio. FYI: I used a Sekonic L-358 for these tests.

    First example:
    High Contrast
    Fill = f2.8
    Key = f/8.5 ½
    Since this is 3.5 stops difference I would state this as a 7:1 ratio
    Using the circle diagram the fill = 100, key = 700, resulting in an 8:1 ratio.



    Joseph, check your ratio-computations again. Remember that you must double the light level for each stop of increase.

    Look at the attached table.

    Going from 100ws to 1200ws (the same thing as going from f2.8 to f~9.6, as per your test) is a ratio of 100:1200 which to 1:12, not 1:7.

    But be careful here, because remember that the ratio we're talking about here is not the simple power ratio of fill light:main light. We're talking about the ratio of subject illumination, that is shadow illumination:highlight illumination. If the main light overlaps the fill light, then the area of overlap is additive.

    So if the shadow is illuminated by a 100ws light and the highlight area receives that 100ws plus the 1200ws, then your test has actually produced an illumination ratio of 100:1300, which 1:13...not 1:7.

    As David has said, you are already well into a dynamic range that's going to push one end or the other into non-detail, so you're not going to see any difference when you push the dynamic range even farther.
    Attached Images Attached Images
    Last edited by KirkDarling; 09-09-2006 at 02:29 AM.
    --Elephants can swim...
    ...and very gracefully.
    Knowing that,
    I do believe
    Anything is possible for me.

    Kirk Darling, CPP

  10. #50

    Default

    Kirk,

    So what your saying is:

    If I set my key for f/9.6 metering from the highlight side of my subject, then metering from the shadow side with the dome retracted achieve a reading f/2.8 that would be a 13:1 subject brightness ratio?

    If this is in fact accurate then my suggestion of multiplying or dividing by 2 would be completely inaccurate as it relates to subject brightness ratio. Because a 13:1 ratio would yield a difference between the highlight and shadow of 7.5 stops (13/2=7.5). Looking at your table it's only 3.5 stops.

    I'm always open to new ways of looking at things but I'm having a hard time digesting this.

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