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Deleted
02-24-2006, 09:10 PM
On the sample CPP test questions, there is a question that asked the following:

In a portrait illuminated with a 3:1 lighting ratio, comparision with the highlight side of the face with the shadow side of the face would produce a difference of how many stops? The answer is 1.5.

Can someone explain that? There is 2 stops difference between the two lights, right? So why would the difference between the highlights and shadows be 1.5 stops?

KirkDarling
02-25-2006, 01:11 AM
On the sample CPP test questions, there is a question that asked the following:

In a portrait illuminated with a 3:1 lighting ratio, comparision with the highlight side of the face with the shadow side of the face would produce a difference of how many stops? The answer is 1.5.

Can someone explain that? There is 2 stops difference between the two lights, right? So why would the difference between the highlights and shadows be 1.5 stops?


John, the camera sees the fill light as the sole illumination of the shadows. Call that 1 unit of light. The key light--which let's say 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.

Another way in real terms:
100 watt seconds gives us, say f11.
200 watt seconds gives us f8
300 watt seconds only gives us f6.3 (or thereabouts)
To get to f5.6--2 stops above f11--we need 400 watt seconds.

You knew that.

PhotogCraig
02-25-2006, 03:49 AM
Another way in real terms:
100 watt seconds gives us, say f11.
200 watt seconds gives us f8
300 watt seconds only gives us f6.3 (or thereabouts)
To get to f5.6--2 stops above f11--we need 400 watt seconds.

You knew that.


You were doing pretty good in the first half of your post, but what you placed here at the end is the reverse of the way it is in real terms.

Deleted
02-25-2006, 04:27 AM
You were doing pretty good in the first half of your post, but what you placed here at the end is the reverse of the way it is in real terms.

Ok, so if I understand all of this correctly, it should go like this:

100watts - f5.6
200watts - f8
300watts - f9.5 (or there abouts)
400watts - f11

then would it continue like this:

600watts - f13
800watts = f16

?????????

KirkDarling
02-25-2006, 04:34 AM
You were doing pretty good in the first half of your post, but what you placed here at the end is the reverse of the way it is in real terms.

It's late here. Sheesh.

PhotogCraig
02-25-2006, 04:47 AM
I guess you are allowed one.

KirkDarling
02-25-2006, 04:56 AM
I guess you are allowed one.

Not being blonde, I have to chalk it up to a "senior moment."

Dave_Swager
03-13-2006, 04:29 PM
Someone correct me if I'm mis-remembering what I learned in basic photography about f-stops. Progressing one f-stop in either direction either halves or doubles the amount of light. The progression in light intensity one f-stop at a time would therefore be 1,2,4,8,16...etc., or 1,1/2,1/4,1/8,1/16...etc. A lighting ratio of 3:1 would therefore be a difference of 1.5 stops.

Dave Swager

Dave_Swager
03-17-2006, 02:35 PM
If anyone's still looking at this thread I have an additional question about lighting ratios. Why the dichotomy with regards to speaking in terms of lighting ratios vs. f-stops? I suppose it may have something to do with the mechanics of setting up lighting equipment in a studio environment, but having spent no time in a studio myself I don't really know. The equipment I use (lights and light meter) is referenced in f-stops, and it is therefore much easier to think in terms of f-stops when discussing lighting. Are there other methods or tools that make referencing lighting ratios preferable?

I think it is also worth noting for the benefit of those relatively new to the subject that the effect of lighting ratios varies on the basis of the exposure latitude of the medium being used for capture. Media with a narrow exposure latitude shows greater contrast, all other things being equal.

Dave Swager

Michael_Gan
03-17-2006, 04:57 PM
Hi Dave,


Hope this helps.

Yes, the difference between one stop is either double, or 1/2.

Now, let's say your fill light is f/5.6, and your main is f/8. so the "dark" side of the face is 5.6. But the "light" side of the face is f/8 PLUS f/5.6 (so you're adding 1/2 stop to that side of the face!). So, you should get a reading on that side of the face of f/9.6 (or sumthing like that). So, your ratio of f/5.6 to 9.6 is 21/2 times more light which is (tada) 3:1.

The 3:1 is not 3 stops to 1, 3:1 is expressed in the amount of light diferences.

Michael

Dave_Swager
03-17-2006, 05:23 PM
Reply to Michael Gan.

I understand the phenomenon, my question is why do we use two methods to refer to the same thing? In your example what's the benefit of saying the lighting ratio is 3:1 vs. saying the difference is 1.5 stops. It's the same information so far as I can see without the added step of converting f-stops to lighting ratios.

Dave

KirkDarling
03-17-2006, 10:59 PM
Dave,

What if you don't have a light meter, but know the output of the flash units?

This is getting down to understanding the basics of what's going on at the level that light strikes subject. I could figure out a thumbrule of f-stops and never think anything as concrete as "1:3 light ratio." But if you never really understand the underlying concept of what's happening, you can't break away from the thumbrule or even understand when and how you might need to break it.

Example of a goatrope: For the last few years there have been perennial disputes in photo forums about whether the "1/focal length shutter speed for handholding" thumbrule works for APS-C cameras. People who understand the concept underlying the rule don't have to think more than five seconds to realize the correct answer...and it has nothing to do with pixel diameters or length of rotational axes or any of that other stuff. It has to do with the acuity threshold of the human eye and keeping the physical size of image blur below that threshold.

That's the level we're getting down to here. The light ratios start from stating the result we want in quantifiable terms and then figuring how to apply our tools to reach it.

This would make it easy: If we were to take a spotmeter, set it to show Exposure Values (EV) and take spot measurements of each area of the subject's face. We would see, then, that there was a ratio calculatedly between the highlight side and the shadow (fill) side.

The shadow side may give us an EV of, say, 3 while the highlight side gives us an EV of, say, 9. Guess what--1:3 highlight to shadow ratio. Notice that the actual tone of the subject is nearly irrelevant in this context (within some broad limits)--the values could have been 7 and 21 for a lighter-toned subject or 20 and 60 for a really bright subject, but the ratio would be the same.

But the question is how do you make that ratio (or any ratio) that happen intentionally?... because making things in the picture happen intentionally is the craft of the matter, and if one is going to be a certified professional in any endevour, one should know the underlying concepts.

Dave_Swager
03-18-2006, 12:08 AM
I think I may be detecting the difference in language. If one thinks of f-stops only as a camera setting and not as an indicator of lighting differentials then I understand the different usage. I thought perhaps there was a mechanical/methodological reason for the different language.

Dave

BTW - If I know the output of a flash unit but don't have a meter I still know something about f-stops.

KirkDarling
03-18-2006, 05:22 AM
I think I may be detecting the difference in language. If one thinks of f-stops only as a camera setting and not as an indicator of lighting differentials then I understand the different usage. I thought perhaps there was a mechanical/methodological reason for the different language.

Dave

BTW - If I know the output of a flash unit but don't have a meter I still know something about f-stops.


That's better than me, then. I don't know what f-stop 2000 lumens is at five feet.

Dave_Swager
03-18-2006, 02:15 PM
That's better than me, then. I don't know what f-stop 2000 lumens is at five feet.

All other things being equal I know that the difference between 2000 lumens and 1000 lumens is 1 f-stop which could also be stated as a ratio of 2:1.

Dave

KirkDarling
03-18-2006, 02:56 PM
All other things being equal I know that the difference between 2000 lumens and 1000 lumens is 1 f-stop which could also be stated as a ratio of 2:1.

Dave

Only if the areas of the subject lighted do not overlap at all, but do border each other precisely. If the overlap, you have 3:1, if they have a gap you will have something higher.