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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 26 Mar 2014 (Wednesday) 07:07
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Lighting math question

 
Vitoflo
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Mar 26, 2014 07:07 |  #1
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Hi,

If I have the main light set at f/8.6 placed to one side of the subject' face while the fill light is at f/5.6 placed above the lens axis (both of them gridded...what aperture should I set my camera and why?

What's the sum of f/8.6 + f/5.6?

Thanks




  
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medicdude
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Mar 26, 2014 07:21 |  #2

If I understand right, set it at f8, this just tells you that your fill will be 1 stop under the key. I would see if you could get it to f8 for accuracy and ease, since there's no f8.6


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drvnbysound
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Mar 26, 2014 07:41 |  #3

To know that your lights are at 8.6 and 5.6 respectively, I assume you are using a meter. Once you have those dialed in, take a meter reading with both lights on. Shoot with whatever that reading is.

Let's assume that it turns out to be f/9, but you want to shoot at f/8. Adjust both lights down, by the same amount (e.g. reduce key by -1/3, reduce fill by -1/3), meter again. Reduce again as necessary. This will keep the ratio between the lights the same, while lowering the overall exposure amount.


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Vitoflo
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Mar 26, 2014 08:07 as a reply to  @ drvnbysound's post |  #4
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I was watching this video of Frank Dispensa where he says that f/8.6 plus f/5.6 equals f/11 so he set the aperture accordingly...how did he make thah math?

He took a meter reading of each light but not both, he just made the math!




  
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SkipD
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Mar 26, 2014 08:14 |  #5

Vitoflo wrote in post #16787356 (external link)
I was watching this video of Frank Dispensa where he says that f/8.6 plus f/5.6 equals f/11 so he set the aperture accordingly...how did he make thah math?

He took a meter reading of each light but not both, he just made the math!

The math may work for the lights being in one set of positions relative to the subject, but as you move the lights around the reading made by a meter would prove the math to be hokey. Using a meter or guessing and a lot of chimping would be the only ways to deal with multiple-light exposure calculations.


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Vitoflo
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Mar 26, 2014 08:20 |  #6
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SkipD wrote in post #16787367 (external link)
The math may work for the lights being in one set of positions relative to the subject

so how did he go about getting f/11 ..by trial and error?




  
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apersson850
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Mar 26, 2014 09:27 as a reply to  @ Vitoflo's post |  #7

My math makes it closer to 10 than 11. Or 10.3 to be more precise.


Anders

  
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BTNorris
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Mar 26, 2014 13:45 as a reply to  @ apersson850's post |  #8

Consider the side-lighting scenario:

f/8 light ONLY on the left side of the face
f/5.6 light ONLY on the right side

Light doesn't overlap, so no calculation needed.

Other scenarios may be 100% overlapping or somewhere between, so there is no set math.


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Alveric
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Mar 26, 2014 13:53 |  #9
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Ahh, the old source vs. additive methods controversy.

Light ratios: source vs additive (external link).

One final thought about lighting ratios. What camera setting should you use when exposing the previously described ratios? The answer is whatever your meter indicates. You should be taking a meter reading with both the key and the fill lights on and use that reading.


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drvnbysound
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Mar 26, 2014 16:31 as a reply to  @ Alveric's post |  #10

The answer is whatever your meter indicates. You should be taking a meter reading with both the key and the fill lights on and use that reading.

+1!


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dmward
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Mar 26, 2014 16:42 |  #11

When metering each light the dome is pointed toward the light. F8.6 (I am taking this to mean F8 plus .6 or F10) is the main light falling on the subject. F 5.6 is the fill light falling on the subject, including the area also lit by the main. F5.5 is 1 and 2/3 EV less than F8 and 2/3. Or, not quite 1/8th the light from the main. That means plus 1/3 EV (F11) is a good starting point for camera setting.

As mentioned if one knows the reading from the two lights to a tenth of an EV they should be able to also use the meter to get the shooting exposure by pointing the dome, at subject position, toward the lens and firing both lights, same as making a picture.

In practice, 1/3 EV or less is going to be hard to see and certainly well within the ability of processing software to fine tune.


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drvnbysound
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Mar 26, 2014 17:06 |  #12

dmward wrote in post #16788633 (external link)
When metering each light the dome is pointed toward the light. F8.6 (I am taking this to mean F8 plus .6 or F10) is the main light falling on the subject. F 5.6 is the fill light falling on the subject, including the area also lit by the main. F5.5 is 1 and 2/3 EV less than F8 and 2/3. Or, not quite 1/8th the light from the main. That means plus 1/3 EV (F11) is a good starting point for camera setting.

As mentioned if one knows the reading from the two lights to a tenth of an EV they should be able to also use the meter to get the shooting exposure by pointing the dome, at subject position, toward the lens and firing both lights, same as making a picture.

In practice, 1/3 EV or less is going to be hard to see and certainly well within the ability of processing software to fine tune.

I was assuming, and always have, that the fill light reading was a separate reading of it's own and did not include any light from the key light.

... maybe a bad assumption!

I know that I've always metered them completely separate of each other, and taken a final reading as you stated above, with both (or all) lights firing for the shooting aperture.


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J ­ Michael
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Mar 26, 2014 18:55 |  #13

Light is additive. If there is a 2:1 ratio between the two sources then the exposure where the overlap occurs is the sum of the two sources, or a half stop over the stronger source. If it was 1:1 then you would be doubling the light at the overlap. But things aren't usually that simple - the fill might be grazing incidence so more on the order of intensity sin the angle of incidence, hence the suggestion to use a meter.




  
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dmward
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Mar 26, 2014 22:19 |  #14

Some examples: Main is 45* off lens axis, fill is on axis and thus also illuminating the highlight side of the face.
F8 main : F8 fill = 1:1 Main plus fill on highlight side means 1 EV more = F11

F8 main : F5.6 fill = 1:2 Main plus fill on highlight side means 1/2 EV more = f 8.5 or F9+

F8 main : f4 fill = 1:3 Main plus fill on highlight side means 1/4 EV more or F8

It should take about 10 minutes to experiment and validate this with a couple of speedlites or monolights.


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apersson850
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Mar 27, 2014 03:46 as a reply to  @ dmward's post |  #15

Yes, the problem is of course the different blending ratios between light A and light B. But that's not so much math as understanding how the different light sources illuminate the subject.


Anders

  
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Lighting math question
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