Confused about light intensity

[Jeff Heywood]

We're doing some high-speed shooting and we're not used to working with advanced lighting, which has ended up underscoring the fact that there are some simple lighting things we don't understand.

For instance...

2 identical fixtures does not necessarily increase light intensity over one of the same fixture. True or not?

I would have assumed, if I was lighting a 3x3 foot space and I had one diva over it and I added another diva the amount of light in the space would increase therefore higher shutter speeds for me. Is this not true?

What is the theory I need to learn here? If I took two identical HMI's and tightly focused them on the same spot would their be more light on that spot than if I only focussed one there?

Suffice to say I have become confused.

[Carl Middleton]

Stating first that I'm not too great with lighting (I'm trying to learn though!) here's my take on it:

You are adding more light with multiple lights, yes.

Whoever told you that was probably referring to the fact if they're used differently, say, back and fill lights, it wouldn't NECESSARILY give you a higher shutter speed. Not necessarily. But 500w + 500w = 1kw if all else is the same (distance, focus, direction, etc) but it's kinda hard to put lights on top of one another. So it'll never actually truly double. Kinda like highway MPG ratings. :)

C

[Jack Walker]

Two lights the same on the same spot will double the amount of light to that spot and thus cut the length of the exposure in half if the f-stop doesn't change.

Imagine a lamp with two bulbs. Turn on one bulb and you get a certain amount of light. Turn on the second bulb and you get twice as much light from the lamp.

If a light is moved nearer or farther, the inverse square law applies (in general, since a focused light will not follow it strictly): if the distnance of the light from the subject is doubled, the amount of light falling on the subject will be 1/4 of the amount falling when the light is closer.

Here is a page that covers some basics on light intensity:
http://www.internetcampus.com/tvp029.htm

Links from the page will take you to some other basic lessons.

[Steve Oakley]

well diffused lights fall off much faster than point source like open face lights. that said, with the deva's if they are just a few feet away they should roughly double the light level if you go from 1 to 2. at 10ft way the light level would still increase in theory to double, but maybe not quite because they fall off so fast. light meter would tell the truth. should be close to double

if shooting high speed, I'd go for HMI's with flicker free ballasts and use silks to diffuse as needed. you can dump a crazy amount of light into a small space this way. I'd also shoot tests depending on the FPS

[Jack Smith]

Although adding a second identical light from the same distance will roughly double the light, you would only change your exposure by 1 stop.eg. using 1 light at 1/60th shutter and f 2.8, 2 identical lights at the same distance to the subject would be either 1/60th shutter and f 4.0 or 1/125th and f 2.8
Doubling light only changes exposure by 1 stop.

[Brian Drysdale]

I'd be careful about flicker if you're planning to use HMIs for high speed work. Flicker can also be an issue with the smaller tungsten lights for this type of work and using anything smaller than a 1k can have a problem. Of course, it depends on how high a frame rate you're planning to use.

[Jeff Heywood]

We're planning to use 500 but possibly up to 1000fps.

We have very delicate subjects so heat is a major problem. It's hard to figure just how much light we'll need.

[Jack Walker]

Originally Posted by Jeff Heywood

We're planning to use 500 but possibly up to 1000fps.

We have very delicate subjects so heat is a major problem. It's hard to figure just how much light we'll need.

What are you shooting? How big is it? Can you shoot it outside? Etc.

Can't you just turn on the camera, light an 18% gray card until you get the right exposure with the f-stop and speed you want to use, then measure how much light you are using with a light meter. For the test you can use regular hot lights, just adding or moving in until you get the right exposure.

Once you know how much light you need (from the meter, based on foot candles or a matrix of f-stop/shutter speed/apparent ASA) you can figure out how to get that much light of the right type on the particular subject you are shooting.

I don't know what shutter speed you will use, but let's assume the minimum speed would be the same as the fps.

For 1/500 shutter speed you need roughly 8x the light that you need for 1/60, and for 1/1000 you need at least 16x the amount of light.

Here is a little chart, for example:
say f/4, for the same exposure you need:
100 watts 1/60
200 watts 1/125
400 watts 1/250
800 watts 1/500
1600 watts 1/1000

High shutter speeds require a lot of light or a high ASA speed, and for practical purpose with high ASA translating to high gain on the electronic camera.

[Bill Davis]

Jeff,

You simply cannot think of light without thinking about the distance from source to object.

Perhaps it will help to think of it like this.

One light, 1ft away from something and you get light at 1X (call that distance A)
Add a second light at a distance of 1ft you get = 2X

That's simple and intuitive.

However...

Move the first light to a distance of 5 feet you get 1/25th of the light you got from A (the inverse of 5 squared)

Add a second light from the same 5 feet to "double" things and you get 2/25ths of A - hardly any increase!

Yes the light "doubles" but that doubling has MUCH less effect than the fall off over distance and the inverse square principal.

Want the effect of actually increasing the light on something 5 feet away in the same way you can increase it on something 1 foot away? Rent a BIG grip truck, you're gonna need 49 more of those lights so you can honestly "double" the light from 1/25th to 50/25ths!

Nobody does that, of course, because a well constructed light like a Deva has enough output to light a subject at a reasonable distance (like 5 feet)

But you still can't ignore the physics.

And yes, if you're working at a distance which puts you at the outer limits of a quick fall off light like a Kino - you can add another light and hardly notice the difference.

In lighting, like in sound, the distance from a light to an object has WAY MORE impact than simply adding more lights.

Does that help?

[Jeff Heywood]

Thanks for the info all. Bill that last description really sunk home. Sure we'd be doubling light, but doubling a very small amount. Makes much sense.

[Jeff Heywood]

Thanks for all the help on this. We shot the footage and it looks phenomenal in our galleries.

For fun we cut a little youtube video of the footage.

http://www.youtube.com/watch?v=eKcztOR6UYM

[Carl Middleton]

I love it!

Congratulations - you took all the info you were asking for and made it into a wonderful piece - I understand the sensitivity and heat requirements, and shutter speed requirements now. :)

You got your point across in a beautiful way - and told the story. I had no idea the situation was that grave.... quite a sad story.

Carl

[Jeff Heywood]

Thanks for the compliment and thanks for the help understanding light.

This video was just for fun, and to get the word out, but we have a spin browser in the gallery that has the full clips of 4 different types of frogs eating.

People turn the wheel fast or slow and can see all the movements of the frog at whatever speed they like. It works amazingly well.

[Carl Middleton]

I WANT ONE!

Is it Christmas yet? :D

[A. J. deLange]

Think of it this way. Double the watts --> 1 stop less exposure (square root of 2 f number change i.e. f/2 --> f/2.8) or halved shutter (1/30 --> 1/60). Double the distance from the lights to the subject --> 2 stops exposure change (required f/ number halves e.g. f/4 --> f/2) or required shutter speed quadruples (1/60 --> 1/15). If one light is appreciably closer than the others it is the distance and wattage of this light that determines the exposure.

Loved the frogs and loved the Aquarium (got some great shots of the Belugas) last summer.