missunderstanding, lightmeter

[Michael Nash]

Here's a question for you though (I had a friendly disagreement over this one on a shoot once):

 

If you're using reflectors bouncing sunlight onto your scene, then you move the reflector 1/2 the distance for shot 2, is the distance you're computing the inverse square law:

 

1. Only the distance from the reflector's 1st position & 2nd position?

OR

2. Do you add the distance from the sun to the reflector into the equation?

 

MP

 

It depends on the reflectance of the reflector. A theoretically perfect mirror would return 100% of the light rays with no added diffusion of light -- therefore you would calculate the inverse square from the source (the sun). But in the real world mirrors aren't perfect, and silver reflectors scatter the light even more. The more the light gets scattered, the more quickly it falls off.

 

I think people try to over-think the inverse square law thing too much. I know we've had disagreements here in the past about the falloff of hard vs. soft, etc. After years of doing this the best way I've found to deal with the concept is to just get out there an USE the sources and you'll begin to know intuitively how quickly things will falloff. Save the inverse square stuff for lighting plots when you're using a lamp's photometrics.

 

I'm just a pragmatist. ;)

[Patrick Neary]

I think you would figure in the distance to the sun. So if you move the shiny board 10 feet further away from the sun, you need to calculate that against the 93 million miles (according to the third Google hit for "distance to the sun") because now your shiny is 93 million miles and 10 feet away from the sun.

 

I'd open up half a stop. :)

[Michael Collier]

I think you would figure in the distance to the sun. So if you move the shiny board 10 feet further away from the sun, you need to calculate that against the 93 million miles (according to the third Google hit for "distance to the sun") because now your shiny is 93 million miles and 10 feet away from the sun.

 

I'd open up half a stop.

 

hahaha. nice one. half at least. If you want to overthink the inverse, then keep on thinking on it. If the sun is that far, I think it has fallen off as much as it can.

 

As for the reflector, it doesnt really apply. The law only applys to the theoretical zero-space point of light projecting equally in all directions.

 

Since the reflector is inconsitent in both surface shape and light spread (no reflector would ever bounce light in a perfect 180 degree spread) As was mentioned earlier, think of a mirror. It would return light still paralell to eachother, and so the light would not have any fall off. the more the reflector diffuses as it reflects, the quicker the light will fall off.

 

 

The inverse is a theoretical model of how light works. Its not a hard-and-fast rule that should be used on set. Its best to only think so far on that as to understand that light has a specific fall-off behavior. Similar to that of the DoF relation to Focal Lenght, or DoF/appeture relationship. They are concepts that help you think logically about what seems abstract to most.

[Matt Pacini]

I asked, because I had a friendly disagreement with a DP who was shooting for me. He's great, so I didn't argue, but I wondered ever since...

 

We were shooting outside using reflectors, & he said we needed to 1/2 the t-stop because we moved the reflector twice the distance from the subject, & I didn't think we needed to do it, for reasons stated above.

 

However, I think the statements about the size of the source (the reflector is smaller at a distance, therefore not as bright) are certainly relevant, and come to think of it, probably work out to the same results as if you were using the inverse square law for your calculations, so I guess in theory I was right, but in practice, I was wrong!

 

MP

[Laurent Andrieux]

As it's been mentionned, the "inverse square law" only strictly applies if the source is a point.

 

It is a good thing to consider a bounce board as a source itself, not just a mirror. It is a "secondary source". A mirror is not a source by itself. It can cut off light, but it will cut the same amount of light wherever it's placed on the light's way. It doesn't change the nature of light it is reflecting and its position beetween source and subject only acts as a flag frame (changing the lit surface), but it behaves as a secondary source only if not reflecting only but diffusing the light as well.

 

If a diffuse source was of an infinity dimension, the "inverse square law" would not apply at all, since the light catched by the object would always remain the same, whatever distance beetween source and lit object.

 

Then the real situation of a bounce board of a finite dimension is beetween the 2 theorical situations. The light decreases but not as fast as if it was a point source.

 

It depends only on the surface size of the bounce board and the surface that is lit (it also depends on the ability to diffuse the light : the more it diffuses light, the wider this lit surface will be).

 

Basically one can consider that the smaller the bounce board is the faster the bounced light will fall off.

 

It is possible to calculate the light but you need to know these surface' sizes.

 

I've been making funny experiences with a readhead bounced on a poly. The question being : if I need more light, is it worth having the source closer from the poly board (you then have to open up the barndoors) or worth moving only the poly board and not the source etc. Some results are surprising when you meter it.

 

Anyway, the mentionned calculation is not that easy. The best way to get the answer is the light meter.

[Greg Gross]

I suggest that you review in American Cinematographer Manual- reflected light readings,incident light

readings to help refresh your mind on metering. You can stand at the subject and point the meter at

the camera or stand at the camera and point the meter at the subject-IF THE CAMERA IS IN THE SAME

LIGHT AS THE SUBJECT(as this will help to expedite shooting under time restraints). Also in order to ex-

pedite shooting-YOU CAN METER YOURSELF IF YOU ARE IN THE SAME LIGHT AS THE SUBJECT IS. I am

not suggesting that you do this routinely. As your skin is much more different than Angelina Jolie's is.

One really learns metering when they start to practice all of its possibilities. You will also become more

comfortable with metering. Now your next problem will be when you are unable to meter. Lost your meter,

your meter just broke,batteries dead(no spares),your out in the desert,in the shade in a city park,early

evening scene,its snowing in Buffalo N.Y.. ????????????

 

Greg Gross