Lighting ratios

[Don Davis]

Hello all,

 

My apologies if this question has come up before--I couldn't find it in the archives. I'm curious about whether lighting ratios refer to f-stop measurements or magnitudes of light intensity. In other words, is a scene with a 5.6 key and a 2.8 fill a 2:1 ratio (two stops difference) or a 4:1 ratio (four times as much light)?

 

Also, I've heard the ratio explained both as the relationship between key and fill, as well as between "key plus fill" and "fill". Which is it?

 

Many thanks for humoring a beginner!

 

Don Davis

Student DP

Los Angeles

[Laurent Andrieux]

Usually , when you put ratios in terms of 2:1, 4:1, etc, they refer to illumination ratios, therefore a ratio of 2 stops (2.8 to 5.6) is a 4:1 ratio.

 

Ther's been many talks about the key/fill or key+fill/fill ratios. it all depends on how you meter the lights. If you meter illuminations, you're supposed to use a flat disk on the meter. The illumination of a source is measured from the location in the direction of the source. If you have several ones lighting a point, you then are supposed to add them. Imagine you'd have one source for the fill and one for the key, you'd then calculate k+fill/fill.

 

If you meter an iris aperture, you use a spherical bulb that integers the incoming lights and therefore, you don't have to add them. The key light you're metering includes the enventual influence of the fill, if ever and then, you consider this as your keylight... you would then consider the ratio as key/fill...

 

I think this is a lot of blah-blah, my self....

[Don Davis]

If you meter illuminations, you're supposed to use a flat disk on the meter.

 

Do a flat disc and a dome give the same reading when pointed at a key light providing no other light is falling on the dome?

 

Don Davis

Student DP

Los Angeles, CA

[Michael Nash]

I think this is a lot of blah-blah, my self....

<{POST_SNAPBACK}>

 

I agree, it gets more complicated than it really needs to be. But we need that accuracy so that we all know we're talking about the same thing.

 

I use the tried and true practice of cupping my hand over the meter's dome and angling it toward the light source to measure sources independently. And yes, the purpose of the dome shape is to measure the composite of all the sources (respective to the lens) and give a measurement of that.

 

But I have never, ever, in 15 years of shooting used a "contrast ratio" as described in texbooks. I think of key and film in terms of stops of over- or under-exposure. Does the fill measure one stop down? Two stops down? Does that look how I want? Very immediate, accurate, and to the point.

 

It's always seemed kind of silly to me, although I understand it's a principle that's been around much longer than I have. Would a painter fill his pallette with colors according to a ratio, or by the colors that looked right and gave him what he wanted? But in the end, it's all just different ways to describe the same thing.