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Doubling the amount of light


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#1 grantsmith

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Posted 08 September 2006 - 03:58 PM

Hi,

As an example, if I am lighting a white card in a dark room and using a 1k light and it is exposed correctly at 2.8 (just an example exposure).

If I swapped the light for a 2K (assuming the lens and reflector etc are the same) what kind of stop would I be looking at to expose the card correctly.

My guess is 4.

Is that right? Or would it be a lot more closed like 5.6 or 8?

Thanks,

grant
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#2 Nathan Milford

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Posted 08 September 2006 - 04:13 PM

Well, I wouldn't go by 1K or 2K as the photometrics of the unit may not be the same depending on the type of lamp, if it's in spot or flood or if it has any optics.

But if you get f/2.8 with 100 foot candles, you get f/4 with 200 foot candles, f/5.6 with 400 foot candles etc..
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#3 grantsmith

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Posted 08 September 2006 - 04:49 PM

thanks nathan
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#4 Michael Morlan

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Posted 24 September 2006 - 04:26 PM

More specifically, if you are trying to match lights to their use on your set and desired f-stop, read the manufacturer's photometric tables. They provide footcandle measurements at various common distances. (Or, you could meter a light instrument yourself and build your own table.)

Read my article "Choosing the Right Light for a look at the process of matching lights to desired exposures.

Have fun out there.
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#5 Kristy Tully

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Posted 25 September 2006 - 02:26 PM

Hey grant-

You are right. doouble the light, get one more stop. Take away half the light, loose one stop. You can so this with distance, (moving the unit) , or adding/ suntracting units . Of course there may be some fluctuation, but in general, I wanted you to know you were right, simple as that.

A great resource is Harry Box's Book, set electrician's handbook. It has tables in the back of most lights and their footcandles at varying distances, at flood and spot. Very handy.

kristy




Hi,

As an example, if I am lighting a white card in a dark room and using a 1k light and it is exposed correctly at 2.8 (just an example exposure).

If I swapped the light for a 2K (assuming the lens and reflector etc are the same) what kind of stop would I be looking at to expose the card correctly.

My guess is 4.

Is that right? Or would it be a lot more closed like 5.6 or 8?

Thanks,

grant


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#6 Michael Morlan

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Posted 26 September 2006 - 08:37 PM

Yep, just switching to a 2K fresnel from a 1K fresnel does not necessarily double the light. There are different efficiencies of globe, reflector, lens, etc. between instruments. As an example, examine the photometric tables for these three lights (Click the links):

1kW Mole 407 Baby
2kW Mole 4131 Baby Junior
2kW Mole 412 Junior

While the 4131 is roughly twice the illumination of the 407, the 412 is quite a bit brighter. So, there's no automatic doubling of wattage resulting in doubling of light (except for, perhaps, adding a second, matching instrument at the same settings as the first. That would be pretty close.)

Harry Box's book is great but you can also get photometric tables on most manufacturer's sites as well.

I actually created a webpage with photometric charts for all the lights I rent and use here. It's a great reference for me and my clients. Building your own tables/diagrams is a good exercise in learning the tools.
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