Light ratio question on sample test?

[Deleted]

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]

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]

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]

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]

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]

I guess you are allowed one.

[KirkDarling]

I guess you are allowed one.

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

[Dave_Swager]

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]

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]

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