Question about light fall off?

[Safaran Qureshi]

Can anybody help out with some confusion I seem to be having about light?

Lets say;

I have one light in a scene (Keylight). 

Object A is 3 feet away from the key light.

Object B is 6 feet away from the key light.

I shoot two takes. One where my key light is at 100% brightness and one where the key light is at 50% brightness.

Does the exposure of both A and B change by the same amount in both takes?

 

Please answer with 'Yes' or 'No' and then why.

 

Why am I asking?;

When doing split lighting for example, I want to know if I expose one side of someones face at t2.8, will I get the same amount of bounce return (fill) as if I expose at t5.6?

 

Thanks.

[Brian Drysdale]

Assuming that you're using the stop reading to set lighting levels, rather than a meter that directly reads out the light levels in foot candles or lux, f2.8 is 2 stops from f5.6. That's a 25% difference compared to the other, since a stop is a halving or doubling, depending on which direction you're going.

It's common to keep the same f stop on the camera in order to have a similar look from the lens, so you have to adjust the lighting levels to maintain the contrast and depth of field within the scene. This is often done by putting metal scrims or ND gels on the light. Moving the light closer or further away can also be used, although that introduces other variables regarding shadows etc.

You will get a 4 to 1 lighting contrast ratio if you key with a light at f5.6 and the fill at f2.8 on your meter.

[imran qureshi]

1 hour ago, Brian Drysdale said:

Assuming that you're using the stop reading to set lighting levels, rather than a meter that directly reads out the light levels in foot candles or lux, f2.8 is 2 stops from f5.6. That's a 25% difference compared to the other, since a stop is a halving or doubling, depending on which direction you're going.

It's common to keep the same f stop on the camera in order to have a similar look from the lens, so you have to adjust the lighting levels to maintain the contrast and depth of field within the scene. This is often done by putting metal scrims or ND gels on the light. Moving the light closer or further away can also be used, although that introduces other variables regarding shadows etc.

You will get a 4 to 1 lighting contrast ratio if you key with a light at f5.6 and the fill at f2.8 on your meter.

Hi, thanks for posting! It may be my wording but this isn’t what i’m trying to get at with my question.

I'm not asking what the difference between object A and B is.

I'm asking;

With the inverse square law in mind,

Let's say I meter object A at f2 and Object B at f4 (take one)

then I decide to do a second take where I increase the output from my keylight and meter object A at f4, does that now mean that object B is f8 as you would expect?

my question is about how light output affects fall off.

Hope this makes sense.

 

 

[Brian Drysdale]

The inverse square law still applies, the lighting contrast ratio will remain as long as you maintain the same relationship.