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Deb_Wat
09-06-2006, 06:26 PM
Hi Gang,

In preparing for our test next week, we have noticed that there are different answers in regards to a 3:1 light ratio.

The London book states that a 3:1 ratio is a difference of 1.5 stops.

But, I have found other sources (my lighting class at CPW and other lectures attended this year) that state a 3:1 ratio is a one stop difference.

Can someone clear this up for me. Why are there two answers and which is the correct one for the sake of the PPA Certification test?

Thanks

Deb

mlphoto
09-06-2006, 06:36 PM
You are correct that there are multiple definitions of 3:1. I personally do not agree with the definition of the PPA but hey, it is a label...that's it. The key is to know the meaning behind the label.

The definition that PPA uses is that 3:1 has a 1 stop difference when dealing with strobes or hot-lights. 1 light unit hitting the entire face as fill and then twice as much light (1 stop more) hitting one side.

Now, when dealing with reflectors, PPA's definition of 3:1 includes a two stop difference. The "proper way" to measure that ratio is to meter off the reflector.

Personally, I feel this is just confusing.

David_A._Lottes
09-06-2006, 07:14 PM
3:1 ratio example - fill on subject meters at F5.6 with main flash turned off and main meters at f8 with fill flash turned off for one stop difference. Been a long time since I took the exam so I can't guarentee this answer but if it's wrong my ratios have been off for twenty years. Which could very well be. :D

Deb_Wat
09-06-2006, 07:24 PM
The definition that PPA uses is that 3:1 has a 1 stop difference when dealing with strobes or hot-lights. 1 light unit hitting the entire face as fill and then twice as much light (1 stop more) hitting one side.

Now, when dealing with reflectors, PPA's definition of 3:1 includes a two stop difference. The "proper way" to measure that ratio is to meter off the reflector.

Personally, I feel this is just confusing.

The sample question on the certification website states a 3:1 ratio is 1.5 stops difference. Should I just go with that answer for the test?

Anyone know why their are different answers.

Mark_Turner
09-06-2006, 07:43 PM
I'm not sure there. I do know that if you have a unti of 5.6 hitting both sides of the face, and one unit of (2x5.6=8) hitting one side then that is 3:1, with 3 units on one side and only 1 on the other. However when you meter that with a meter (incident) pointed at the camera from subject you get 8.3 to 8.5 total exposure. Maybe that is where the extra 1/2 stop is coming from.

Now what to answer on the test....

mlphoto
09-06-2006, 07:44 PM
If that is what the certification sample test says, then, yeah...I would go with it.

The different answers arise due to differing opinions on a variety of subjects:

1) Do you add up all the light units hitting the subject or do you only count the light units coming out of/off from the light source (light or reflector)
2) Is your fill light positioned near the camera?
3) yada, yada, yada.

Personally, I only count the light coming out of the light source. I also notate the amount as follows:

main:fill:separation:kicker:background

So, I can have a 4:1:1:0:3 ratio. It helps me remember how I set lights weeks or months later.

But, that is just how I do it.

Derek_Alvarez
09-06-2006, 08:20 PM
Ratios



here is a way to look at it


1 ..2.. 4.. 8.. 16.. 32.. 64.......


each number doubles these number are your ratios

1:1 ... 1:2 ...1:4 ...1:8 etc

so let put this to an fstop

a 1:1 is f11/f11
1:2 is f11/f8
1:4 is f11/f5.6


now lets add 1/2 stops

go back to the ratio chart
1 ..2 ..4 ok
here is how it will look

1..2..3*..4..6*..8..12*..16........

the * number is the 1/2stop ratio

so a 1:3 ratio is 1 and 1 1/2
or F11/F 6.7

does that help

David_A._Lottes
09-06-2006, 08:23 PM
The sample question on the certification website states a 3:1 ratio is 1.5 stops difference. Should I just go with that answer for the test?

Anyone know why their are different answers.

Well if I have my fill metering 5.6 and my main metering 8 then the highlight side probably will read 8.5. So the difference between the fill alone and both lights turned on is 1.5. But what a convoluded way to arrive at that conclusion :confused:

Seems like a very tricky question depending on the way it is worded. The answer of 1.5 would be missleading if it were phrased as (Where do you set your power) as opposed to (what does your meter read). Now if it is phrased as what is the difference between the meter reading of the brightest highlight and the meter reading of the shadows for a 3:1 ratio I guess 1.5 stops would make sense. This would be handy to remember in available light.

Michael_Gan
09-06-2006, 08:42 PM
Deb!

The reason it becomes 1.5 stops is because you have to take into account the amount of light from the main light that "spills" onto the fill light area. Although your mainlight is 8 and your fill is 5.6, there is about 1/2 stop added to your fill source thus reducing the light differential to 1.5.

Michael

David_A._Lottes
09-06-2006, 09:00 PM
Oh MAN am I getting mixed up. I thought it was the FILL light bumping my main up from 8 to 8.5 thus INCREASING the differential from 1 to 1.5 stops. :confused:
I'm going to leave this to smarter people. Sorry if I've confused you Deb. If you ever work for me and I ask you for a 3:1 ratio just remember to meter the lights seperately and put the main light at a 45 degree angle from and metering one stop brighter than the fill and I'll be happy. :)

Derek_Alvarez
09-06-2006, 09:26 PM
Also Remember

your Main/Key light is always expessed as 1
your Highlighs are always expressed to the left of the of the Main/Key
which gives you an expression of 4:1 3:1 2:1 etc.
and your shadows are expressed to the right of the Main/Key
1:2 1:3 etc

So if you have a Main ,Fill and Hightlight/Accents
you might see this

2:1:2 which is F16,F11,F8

David_Quisenberry
09-06-2006, 09:29 PM
Deb!

The reason it becomes 1.5 stops is because you have to take into account the amount of light from the main light that "spills" onto the fill light area. Although your mainlight is 8 and your fill is 5.6, there is about 1/2 stop added to your fill source thus reducing the light differential to 1.5.

Michael

That totally depends on your light placement. If the fill is directly behind you [d you are shooting head on to the subject] and your main is placed on one site then that would not be accurate. In that case your fill would be spilling on your main.

Michael_Gan
09-06-2006, 09:44 PM
OK, come to think of it, the fill is adding the 1/2 stop to the main. either way, your adding light to the "dark side" which lowers the f stop ratio.

Michael

David_A._Lottes
09-06-2006, 11:18 PM
Guys......We are all looking really bad here. I've been googling lighting ratios and it's time for all of us to go back to class. :o First of all the accepted value of an F stop is two units of light. So if your key light is F8 that equals 2 units of light. Now your fill at F5.6 equals 1 compared to F8. See where this is going? Main light set at 8 fill at 5.6 equals a 2:1 ratio. So for a three to one ratio add another unit or half an F stop of light to the main. That means F8 plus one half stop equals 3 in relationship to F5.6 which equals one a 3:1 ratio. Now this is what the London book says, so be it. Michael, I love you man, but 1.5 is more than 1 thus the difference between a highlight side that is 1.5 stops brighter than the shadow side is a HIGHER ratio than a highlight side that is 1 stop brighter than the shadow side. Derek, love you to man but your Key light is the number to the left of 1. The one is the fill. The Key or main is the 2, 3, 4 ,5 etc. and it represents units of light. Now I would argue that the spill Michael is talking about tweeks this formula by at least a half a stop as we all know from experience. So Even though I may not be creating a by the book 3;1 ratio with my one stop difference i bet it is more like a 2.5:1 ratio than a 2:1 as the book says and I think it looks better for general purpose portraiture than a ratio of almost 4:1 doing it by the book. Anyway Deb, and everyone else out there prepairing for the test, the answer is a 3:1 ratio is a difference of 1.5 stops between main light and fill. Sorry this took so long to get right. I'm putting my pointed hat back on now and sitting in the corner. :cool:

Michael_Gan
09-06-2006, 11:26 PM
David, I feel the love, but there is a confusion between light ratio, and exposure. 5.6 is 1/2 the amount of light that is added to the entire scene. Your exposure will be higher than f8.

Michael

Derek_Alvarez
09-06-2006, 11:46 PM
ratio than a highlight side that is 1 stop brighter than the shadow side. Derek, love you to man but your Key light is the number to the left of 1. The one is the fill. The Key or main is the 2, 3, 4 ,5 etc. and it represents units of light. Now I would argue that the spill Michael is talking about tweeks this formula by at least a half a stop as we all know from experience. So Even though I may not be creating a by the book 3;1 ratio with my one stop difference i bet it is more like a 2.5:1 ratio than a 2:1 as the book says and I think it looks better for general purpose portraiture than a ratio of almost 4:1 doing it by the book. Anyway Deb, and everyone else out there prepairing for the test, the answer is a 3:1 ratio is a difference of 1.5 stops between main light and fill. Sorry this took so long to get right. I'm putting my pointed hat back on now and sitting in the corner. :cool:

David

I would have to disagree with your Google

I just pull out my old books here and all say the same thing

the Main Light is always 1. as to which way the light Spills that I'm not sure not sure????

Derek_Alvarez
09-07-2006, 12:10 AM
David

Where did you find those results i've never heard that and would like to read it :)

Derek_Alvarez
09-07-2006, 01:55 AM
David BTW i'm willing to except I'm wrong on what the 1 is but I'd like to hear what and where it comes from :D

David_A._Lottes
09-07-2006, 02:03 AM
Here you go Derek

http://www.vividlight.com/articles/1916.htm

I read bunches of stuff but this one was the clearest to me. I think part of our problem here is we don't have the exact wording of the question. Does it ask how to set up your lights or how to meter your subject. Like Michael said there is a difference between metering of each light and the metering of the over all exposure. Anyway I hope you enjoy the link. It makes sense to me. I think?

Sorry I didn't post this sooner, I had an evening consultation with a couple for a wedding next June. BOOKED EM on the new and and increased prices! Thats a good thing, I think?

KirkDarling
09-07-2006, 02:14 AM
Here is what to understand for the test:

The camera sees the fill light as the sole illumination of the shadows. Call that 1 unit of light. The key light--which we've adjusted to be twice as bright as the fill--illuminates the part of the face it strikes with 2 units of light.

But from the viewpoint of the camera position, that 1 unit of fill light ALSO strikes the same areas visible to the camera as the key light does. So from the camera position, the highlights are getting 3 units of light while the shadows are getting 1.

Going from 1 unit of light to two units of light is a one stop increase. But adding just one more unit of light is only a half stop increase above THAT. In order to make it a 2 stop increase, we'd have to have fully doubled the light a second time (2 units) not just add 1 more unit.

1 unit + 1 unit = 1 stop increase.
1 unit + 1 unit + 1 unit = 1.5 stop increase.
1 unit + 1 unit + 2 units = 2 stop increase.

Here is the diagram, so you can clearly see where a 2x difference in the lights equals a 1:3 ratio.

You get a 1.5 stop difference if you meter incorrectly. If you turn the meter to the fill light with the dome extended, then do the same toward the main light, you will see a 1.5 stop difference. But the metering to determine the ratio should be done with the domes retracted (Sekonic) or with the flat diffuser (Minolta).

Derek_Alvarez
09-07-2006, 11:56 AM
Ratios



here is a way to look at it


1 ..2.. 4.. 8.. 16.. 32.. 64.......


each number doubles these number are your ratios

1:1 ... 1:2 ...1:4 ...1:8 etc

so let put this to an fstop

a 1:1 is f11/f11
1:2 is f11/f8
1:4 is f11/f5.6


now lets add 1/2 stops

go back to the ratio chart
1 ..2 ..4 ok
here is how it will look

1..2..3*..4..6*..8..12*..16........

the *number is the 1/2stop ratio

so a 1:3 ratio is 1 and 1 1/2
or F11/F 6.7

does that help

Deb
Ok this Portion of my post is correct :D


However the jury is still out on this next part

All of the Portrait lighting Books I have show
the Main/Key as 1 :(

which is always the way I read the Ratio :confused:



"

your Main/Key light is always expessed as 1
your Highlighs are always expressed to the left of the of the Main/Key
which gives you an expression of 4:1 3:1 2:1 etc.
and your shadows are expressed to the right of the Main/Key
1:2 1:3 etc

So if you have a Main ,Fill and Hightlight/Accents
you might see this

2:1:2 which is F16,F11,F8


There is very little Info on this part of the Ratios listed on the internet
but there is a conflict

David_A._Lottes
09-07-2006, 02:11 PM
Thanks for the diagram Kirk
If you look at Kirk's diagram it says 3:1 ratio. The 3 expresses the value of the Main light and is to the left of the 1. Now I'm not sure about the dome thing. A human face is in a dome shape so I would think that is what you would want to use to meter it. Not a flat 2 dimensional diffuser. I would think the only time to use the flat diffuser would be for a reflective reading or an incedent reading on a flat surface. But that's another can of worms.

Deb I would really love it if you could post the question as written for the test. I think it would help us all to understand the answer of 1.5 stops difference better.

Now I've decided from this thread that my interpretation of a three to one ratio being a 1 stop difference between main and fill is wrong (According to the book). But I have yet to see anyone come up with a comprehensive and clear cut explaination of a 3:1 ratio on this thread. How sad. We are CPPs Masters and Veterans of years of experience and something as basic as Ratios still draws conflicting conclusions. This is a testimony to the way we as an industry have allowed the basics to slip away from us. In a world of "If it feels good do it" photography the fundamental skills are being forgotten. :(

Thank you Deb for the wake up call and thank you everyone who contributed to this thread. You've all given me alot to chew on. :)


Where did I put that pointed hat??????

Mark_Turner
09-07-2006, 02:17 PM
Where did I put that pointed hat??????

Well don't sit down til you find it. Ouch!

Derek_Alvarez
09-07-2006, 02:42 PM
oK

The Ratio 0f 1:3 or 3:1 are the half ratios

a 6:1 ratio would then give 3 stops
again it is because the value has doubled :D

KirkDarling
09-07-2006, 02:55 PM
Thanks for the diagram Kirk
If you look at Kirk's diagram it says 3:1 ratio. The 3 expresses the value of the Main light and is to the left of the 1. Now I'm not sure about the dome thing. A human face is in a dome shape so I would think that is what you would want to use to meter it. Not a flat 2 dimensional diffuser. I would think the only time to use the flat diffuser would be for a reflective reading or an incedent reading on a flat surface. But that's another can of worms.

David, that "3:1" in the diagram is a typo on my part. It should be "1:3" as I've learned it.

As I was taught, it was a shadow illumination:highlight illumination ratio--not a fill light:main light ratio. That's why it comes out 1:3 instead of 1:2. If it were mere fill light:main light, it would merely be an expression of the bare difference in power--stops, watt seconds, watts, whatever. But because it's a ratio of the illumination that actually strikes the subject, it includes the additive effects of overlapping lights.

Derek is adding a number for the accent lights, too, and although I can't recall seeing that expressed, it's certainly valid.


Deb I would really love it if you could post the question as written for the test. I think it would help us all to understand the answer of 1.5 stops difference better.

Yep.


Now I've decided from this thread that my interpretation of a three to one ratio being a 1 stop difference between main and fill is wrong (According to the book). But I have yet to see anyone come up with a comprehensive and clear cut explaination of a 3:1 ratio on this thread.

I just gave it to you. It's the ratio of the shadow illumination compared to the highlight illumination, taking into account that the highlight illumination is a combination of both the fill light and the mainlight. It is not a ratio of the power of the lights (at least not as far as getting the question right on the PPA exam is concerned).

Karen_Linsley
09-07-2006, 03:01 PM
I beleive Deb is referring to this question, from the sample test on this web site: http://certifiedphotographer.com/files/public/SampleItems.pdf

8. In a portrait illuminated with a 3:1 lighting ratio, comparisons of the highlight side of the face with the shadow side would produce a difference of how many stops? a. 0.5; b. 1.5, c. 2.5; d. 3.5.

Correct answer according to the test sample is ...........B. 1.5 stops.

David_A._Lottes
09-07-2006, 04:17 PM
Bless you Karen
I would have gotten that wrong before Deb posted this thread. I would have assumed it meant how many F stops difference do you set between your Main and your Fill. Now because 1 is not an option I might have figured out 1.5 as being the brightest highligt compared to the shadow side. As for the 1:3 - 3:1 confusion It seems however you learn it will work if you remeber the order, but it seems easier to remember the set up if you think of 1 as the fill (less) and 3 as the main (more). If I tried to think of my main as 1 and my fill as 3, I would get mixed up and think my fill needed to be brighter than my main. So for the test my answer would have been wrong. Now in the real world when I've set my Main to F8 and my Fill to F5.6 I am creating a 3:1 ratio, and yes the meter will read 8.6 or so in the brightest highlight. So I'm doing it right I just didn't know how to explain it. What can I say Thanks again everyone! Old dogs can learn new tricks :D

David_A._Lottes
09-07-2006, 04:59 PM
It's the ratio of the shadow illumination compared to the highlight illumination, taking into account that the highlight illumination is a combination of both the fill light and the mainlight.

That's really more of a definition than an explaination ;)

Sorry Kirk.....Just being a Smart*** :D

Mark_Turner
09-07-2006, 05:11 PM
Hey David, Smart"aleck" has 5 *****. Don't want to get in trouble now.

David_A._Lottes
09-07-2006, 05:27 PM
OOOPS! Thanks Mark :D

Yes Kirk
I know Explaination is a Synonym for definition. Just being a Smart *****
;)

Deb_Wat
09-07-2006, 07:33 PM
Okay. I realllyyyyyy appreciate all the input on this. I know how I will answer the question on the test depending on how it is worded, I think :eek:

Deb

KirkDarling
09-07-2006, 09:16 PM
I beleive Deb is referring to this question, from the sample test on this web site: http://certifiedphotographer.com/files/public/SampleItems.pdf

8. In a portrait illuminated with a 3:1 lighting ratio, comparisons of the highlight side of the face with the shadow side would produce a difference of how many stops? a. 0.5; b. 1.5, c. 2.5; d. 3.5.

Correct answer according to the test sample is ...........B. 1.5 stops.

Hah! Yes! Look at my diagram and my explanation above. The shadow side is being illuminated by 100 watt seconds, the highlight side is being illuminated by 100 ws + 200 ws, which is triple the amount of illumination.

If it were 100+100 (doubled) it would be one stop more. It would need to be 100+100+100+100 (quadrupled) to be two stops more. It's only tripled, which is 1.5 stops more.

There it is.

David_A._Lottes
09-07-2006, 09:54 PM
Yes Kirk
Your Diagram was spot on.......but you also said....... "You get a 1.5 stop difference if you meter incorrectly"..........not what I would call nailing it. :rolleyes:

Let's face it none of us gave a clean answer here even though we all knew how to accomplish the task. That's a shame. :o

I blame the loosey goosey manner we've been encouraged to imulate. Even if our image making skills aren't rusty our communiction skills are.

But you sure came closer than me..........I'M NOT WORTHY, I'M NOT WORTHY, I'M NOT WORTHY (spoken while genuflecting).

Respectfully - David

Dan_Leary
09-07-2006, 11:57 PM
OK,
I was very confused about all of this until I heard David Ziser explain it. It's basically just like Kirk's diagram but he broke it down like this...

Pretend your main light is F11 and your fill is F8. F8 is assigned 1, since it is the weaker of the lights. F11 is assigned 2 cause as we all know F11 is 2X as powerful as F8. So, you have 2 "units of light" (for lack of a better term) hitting the highlight side of your subject and 1 unit filling in the shadows, BUT light is additive SO the one unit filling in the shadows is also hitting the highlight side as well. So 2+1=3 "units of light" on the highlight side and 1 unit of light on the shadow side. 3:1... at least that's how I understand it.

KirkDarling
09-08-2006, 12:40 AM
Yes Kirk
Your Diagram was spot on.......but you also said....... "You get a 1.5 stop difference if you meter incorrectly"..........not what I would call nailing it.

I was confused by the link. I had it right in my explanation above the diagram, but metering the two lights individually didn't come to 1.5 stops difference unless it was being done incorrectly.

Joseph_Froeschle
09-08-2006, 02:51 PM
Hello all, I thought I'd chime in on this topic. I think I may have a simple way to understand ratios.

The question that was asked in the original post had to do with the CPP exam. The CPP exam question gave us the difference in f/stops between the shadow and highlight (in this case it was 1.5 stops) and asked what the lighting ratio was given this information.

Each time we open up or close down one f/stop we are either allowing 2 times as much light to enter through the lens or reducing that amount by 1/2. So everything we do regarding ratios is either multiplied or divided by 2. If we know that there is a 1.5 stop difference we multiple 1.5 x 2 which equals 3, hence a 3:1 ratio. This works every time as long as we know what the f/stop difference is between the highlight and shadow.

If the question had given us a ratio, for instance 7:1 and asked how many stops difference there is between highlight and shadow, we would divide 7 by 2 which equals 3.5. So the answer is 3.5 stops.

How each photographer meters to obtain the difference between highlight and shadow can be up for discussion. I've seen it done several different ways. And each way might yield different results. But once you decide on a value between highlight and shadow, simply multiplying by two will yield the correct ratio for that value.

I hope this helps anyone struggling to understand this topic.


Joseph Froeschle

Deb_Wat
09-08-2006, 03:56 PM
Each time we open up or close down one f/stop we are either allowing 2 times as much light to enter through the lens or reducing that amount by 1/2. So everything we do regarding ratios is either multiplied or divided by 2. If we know that there is a 1.5 stop difference we multiple 1.5 x 2 which equals 3, hence a 3:1 ratio. This works every time as long as we know what the f/stop difference is between the highlight and shadow.

If the question had given us a ratio, for instance 7:1 and asked how many stops difference there is between highlight and shadow, we would divide 7 by 2 which equals 3.5. So the answer is 3.5 stops.Joseph Froeschle

Okay, yeah, woooo hooo, I get it! :) :) :) :) I just needed that last boost to put me over the edge to understanding.

Thanks Joseph!

Deb

David_A._Lottes
09-08-2006, 04:11 PM
QUESTION: In a portrait illuminated with a 3:1 lighting ratio, comparisons of the highlight side of the face with the shadow side would produce a difference of how many stops?

Answer:
1.5 x 2 equals 3, hence a 3:1 ratio.

NOW THAT WAS CLEAN!

You Rock Joseph!

David_A._Lottes
09-08-2006, 05:44 PM
Knowing those sneaky testers they will phrase the question differently on the actual exam. So let's see some different questions. We could do this like the "Name That" thread. I'll ask a ratio question and whoever gets it right can ask the next one. But wait for confirmation from the person who asked the question. Here goes.

A portrait with a 5:1 ratio, comparisons of the Main light to the Fill light can be created with which combination?

F8.5 (Main) and F5.6 (Fill)

F11 (Main) and F5.6 (Fill)

F11.5 (Main) and F4 (Fill)

KirkDarling
09-08-2006, 06:05 PM
So everything we do regarding ratios is either multiplied or divided by 2. If we know that there is a 1.5 stop difference we multiple 1.5 x 2 which equals 3, hence a 3:1 ratio. This works every time as long as we know what the f/stop difference is between the highlight and shadow.

Wow, Joseph, thanks. Great thumbrule. Everybody who is going to take the test, be sure to memorize this. I'm not kidding.

Deb_Wat
09-08-2006, 06:37 PM
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.

David_A._Lottes
09-08-2006, 06:46 PM
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. ;)

Derek_Alvarez
09-08-2006, 06:47 PM
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

David_A._Lottes
09-08-2006, 07:06 PM
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!

Michael_Gan
09-08-2006, 07:14 PM
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

David_A._Lottes
09-08-2006, 08:34 PM
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

Joseph_Froeschle
09-08-2006, 10:03 PM
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!

David_A._Lottes
09-08-2006, 10:58 PM
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

KirkDarling
09-09-2006, 02:16 AM
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.

Joseph_Froeschle
09-09-2006, 04:17 AM
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.

David_A._Lottes
09-09-2006, 10:37 AM
Joseph
Clear your mind and take a deep breath. The difference between the Highlight side and the shadow side would meter at 6.5 stops. In order to create this ratio the power of your lights woul be increased 3 stops. Keep in mind that each time it is increased a stop it doubles the units of light so (2x3=6) plus the overlap of the fill .5 = 6.5. This is why Michael keeps saying not to confuse exposure with ratio.

Ratios are not merely something we observe. We can create them. that is the whole point behind the math. To be able to predict and manipulate results.

There are a many reasons your tests may be yeilding different results. Distance of lights to subject, setting of power going to each light, bounce off walls acting like reflectors, use of any automated self adjusting devices on the lights. All of these things could be throwing your results off. Thats why they invented math. Trust the rules not your eyes. Keep running tests and making adjustments until you get the target.

Your rule of thumb was a stroke of genius and it works beautifully. Using your rule and Kirk's diagram I worked my way backward through the math. To get to know the rule forward and backwards.

As you can see we are all learning and re-learning on this thread, don't give up!

David_A._Lottes
09-18-2006, 03:05 PM
Hello all!
I can't wait to see if someone cleared this up in the down time! I have been re-working the numbers and still can't figure out where we're going wrong, or if we are! Let's work this one out! And if we do lets have another question from the sample test. This is a GREAAAT thread!

Jay_Kilgore
09-18-2006, 04:39 PM
I was always taught that 3:1 was a 1.5 stop difference. For example, Art Ketchum (www.artketchum.com), the guy who taught me lighting ratios, taught me the "perfect" setup is key @ f8 fill @f4.5, Kicker/rim light/s @f5.6 and hair either f11 or f16 adjustable by color of models hair.

Michael_Gan
09-18-2006, 05:16 PM
Jay, the 1.5 stops is an exposure difference, not a difference between the main and the fill. The main and the fill are a 1 stop difference.Michael

Don_Chick
09-20-2006, 04:19 PM
When you are working with this ratio issue you need to know that there are 2 different ratios discussed.

1 - Lighting Ratio: The ratio of light quantity falling onto the subject.

2 - Light Ratio: The ratio of light quantity from the light source.

Lighting Ratio is better thought of as contrast ratio. Thinking in these terms you think of the contrast between the high-light and shadow side of your subject's face (for a portrait).

Light Ratio is thought of in terms of how much light output each light is providing. It doesn't take the subject (or what the light is falling on) into consideration.

Be very careful when you read the question to determine exactly what they are asking as the answer will be different in each case.

A lighting ratio (contrast ratio) of 1 stop will produce a 3:1 ratio.

Fill light at f8 has a light output of 1 unit. Main light at f11 has a light output (relative to fill) of 2 units (1 stop more light). The fill provides 1 unit of light on the shadow side of the face and 1 unit of light on the highlight side of the face. The main provides 2 units of light on the highlight side and 0 units on the shadow side. 2+1=3, 1+0=1. 3:1

A light ratio of 1 stop will produce a 2:1 ratio.

The fill at f8 provides 1 unit of light. The main at f11 provides 2 units of light (relative to the fill). What the light is falling onto is irrevelent. Therefore a 2:1 ratio.