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Re: Measuring light stops of gels

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Declan Smith
Re: Measuring light stops of gels
on Oct 11, 2015 at 10:36:28 pm

I set the lamp on a stand with the centre at 53 inches from the ground, and secured the 758 to another stand (via the 1/4" socket) with the Lumisphere retracted and also at 53 Inches from the ground, then put the meter 1M from the lamp. The lamp and the meter therefore are at a consistent position between tests. I have done the tests fully spotted and fully wide. When checking the setup, I didn't use a target, just the measurements and a rough view where the centre of the spot was landing.

I did a similar test with my open faced lamp of the same power and the relative difference again shows different light loss depending on whether the lamp is wide or spot.

Test 1: No Filter Wide: f/5, Spot f/6.3
Test 2: 1/2 White Diffusion Wide: f/4.5 EV-0.4, Spot f/5 EV-0.6
Test 3: Thick Diffusion (unsure of grade) Wide: f/4 EV-0.6, Spot: f/4.5 EV-1.0

So I am guessing this must be a property of diffusion that is more pronounced using fresnels?

The reason I am trying to understand the light loss is that on reviewing the rushes following a technical rehearsal, where I measured the light setups with the 758, I want to soften the shadows a bit with some diffusion. If I know the expected light loss I can calculate the changes I need to make to the setups prior to the filming so as not to take up any more time on set (that's my theory at least!). This approach appears to work with standard colour gels (like CTB), but I guess with diffusion, it acts differently and depends on the brightness of the source being shone through it?

Declan Smith
After Effects CS6/ FCS3 / Canon XLH1 / Canon 7D / Reason / Cubase

"it's either binary or it's not"

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