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The size of a mirror used for redirecting light

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#1 Tom Yanowitz

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Posted 27 November 2016 - 11:03 AM

Hello,

I haven't had a chance to use mirrors on sets yet.

My question is : if your goal is to reflect a light source onto the subject, to extend the virtual source to subject distance for fall-off reasons, or for any other reasons,

how does the area of the mirror influence the lighting ? especially for Fresnels, or any relatively small and harsh sources ?

 

Unrelated PS : David Mullen, please come and show The Love Witch in France, I want to see that film !!


Edited by Tom Yanowitz, 27 November 2016 - 11:03 AM.

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#2 David Mullen ASC

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Posted 27 November 2016 - 12:10 PM

The mirror itself will crop/cut the light so it has to be big enough so that you don't see its borders on the pattern it throws, unless that is OK for the shot.  In other words a 4'x4' mirror board, for example, will throw a square-ish light pattern from reflecting the hard source, but the pattern is usually large enough for most purposes.  The closer the mirror is to the light, the larger the pattern will be but the less you are extending its throw compared to shining the light directly.  In other words, you could have a smaller mirror right next to the light that would throw a larger pattern but would hardly add to the distance that the light is traveling.  The spread of the pattern is determined by the size of the mirror, which is cropping the original source.


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#3 Tyler Purcell

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Posted 27 November 2016 - 12:57 PM

The mirror trick is one that I pull out when I've got a big room to light with only one or two sources. The way I use it, enables the room to seem well lit from multiple sources, without taking the time to do so. It does require some sort of high power light, a 2k tungsten would work for locations with no windows. If there are windows, I'll use bigger HMI's and simply color balance for outdoor light.

Since I'm always working with little to no money, I go to a local 99 cent store and buy the cheapest flexible tall mirrors (they aren't made of glass). We then attach them to C stands or combo light stands by drilling holes in the top and bottom and using a ratcheting clamp bracket with a pin you buy from home depot. This way you can bend the mirror, lock it in place and get light refraction instead of simply reflection. We'll then use gel covered flags to tint the refracted light and it looks amazing.

The great thing with the smallish mirrors is that all the light that passes by one mirror, can be reflected by another. So we'll literally have a bunch of these mirrors in one place in a given room, all reflecting to a different location, with gels, gobo's and sometimes other smaller mirror devices helping to spread out the beam.

When you're dealing with actors key light, you've gotta bend the mirror so the middle is concave. This makes a stronger beam of light right in the center of the mirror. Then all you've gotta do is put a silk inline with the actors to diffuse the harshness of the light. This is the tricky part because you want the silk as close to the actors as you can get it and I've found throwing a tiny bit of very light orange gel (I buy the stuff in full rolls) over the silk, warms it up just enough to get good face tones with this setup.

The only downside to mirrors is the time it takes to get it right. You really need a crew who has worked with them before to understand light refraction vs reflection. I've had gaffers scratch their heads and wasted time on set setting this rig up for them. In the long run, the look is fantastic, but it's VERY time consuming.
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#4 John E Clark

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Posted 28 November 2016 - 01:01 PM

The only downside to mirrors is the time it takes to get it right. You really need a crew who has worked with them before to understand light refraction vs reflection. I've had gaffers scratch their heads and wasted time on set setting this rig up for them. In the long run, the look is fantastic, but it's VERY time consuming.

 

What do you mean by 'refraction vs  reflection'?

 

Refraction -- the change in direction when light passes from one medium to another, most commonly seen in for example a stick half in water, half out, and it appears to be 'broken' at the water's surface. Refraction also figures in lens designs as different types of glass or other materials have different refraction indices.

 

There is a small amount of 'refraction' if the mirror has a thickness of glass and the reflective surface is on the opposite side of the glass from the incident light. But if the surface of the mirror is the reflective surface, there would be no 'refraction'.

 

Of course creating a concave reflective surface will 'focus' the incident light.


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#5 David Mullen ASC

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Posted 28 November 2016 - 01:24 PM

The original question was primarily about using a mirror to increase the distance between the hard source and the subject in order to decrease the rate of fall-off and to increase sharpness, not so much about using mirrors to make one source into multiple sources or for passing the light through diffusion once it reaches the subject.

 

One problem with mirrors is that they can jiggle and shake, which cause the light pattern to also move around.

 

I've used them on the underside of the eaves of a house outside of a window or up against a ceiling in order have a beam of light come back down at a steeper angle than would be possible.


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#6 Michael LaVoie

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Posted 28 November 2016 - 01:50 PM

A fun mirror trick is to stripe the face of it with clear packaging tape and then gently and randomly crack it from behind.  Shining a light into it will then produce a broken up pattern of light similar to using a cookie.  


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#7 Stuart Brereton

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Posted Yesterday, 12:07 AM

The mirror trick is one that I pull out when I've got a big room to light with only one or two sources. The way I use it, enables the room to seem well lit from multiple sources, without taking the time to do so. It does require some sort of high power light, a 2k tungsten would work for locations with no windows. If there are windows, I'll use bigger HMI's and simply color balance for outdoor light.

Since I'm always working with little to no money, I go to a local 99 cent store and buy the cheapest flexible tall mirrors (they aren't made of glass). We then attach them to C stands or combo light stands by drilling holes in the top and bottom and using a ratcheting clamp bracket with a pin you buy from home depot. This way you can bend the mirror, lock it in place and get light refraction instead of simply reflection. We'll then use gel covered flags to tint the refracted light and it looks amazing.

The great thing with the smallish mirrors is that all the light that passes by one mirror, can be reflected by another. So we'll literally have a bunch of these mirrors in one place in a given room, all reflecting to a different location, with gels, gobo's and sometimes other smaller mirror devices helping to spread out the beam.

When you're dealing with actors key light, you've gotta bend the mirror so the middle is concave. This makes a stronger beam of light right in the center of the mirror. Then all you've gotta do is put a silk inline with the actors to diffuse the harshness of the light. This is the tricky part because you want the silk as close to the actors as you can get it and I've found throwing a tiny bit of very light orange gel (I buy the stuff in full rolls) over the silk, warms it up just enough to get good face tones with this setup.

The only downside to mirrors is the time it takes to get it right. You really need a crew who has worked with them before to understand light refraction vs reflection. I've had gaffers scratch their heads and wasted time on set setting this rig up for them. In the long run, the look is fantastic, but it's VERY time consuming.

As John Clark pointed out, refraction is when light changes direction as it passes from one medium to another. A cheap plastic front surface mirror is not refracting light at all, but rather focusing it as it is bent, rather like the reflector in a lamp. A 'gel covered flag' wouldn't transmit any light at all, as a flag is designed to cut light entirely.

 

Having all your mirrors 'in one place in a given room' is unlikely to yield a useful result, anymore than lighting a room from one position would with movie lamps.

 

If you have examples of rooms lit with this technique, Tyler, perhaps you can share them.


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#8 Tyler Purcell

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Posted Yesterday, 01:23 AM

Sorry for the confusion...

Gel covered flag FRAMES. I missed the word "frame", sorry that confused you.

I always thought bent light was "refracted" light. I gather that's not completely accurate.

When you put mirrors together, in front of a beam of light, you are able to reflect that light into different parts of a given room. Thus, allowing for quicker setup's and one light generally covering more of a given space, with a distinctly creative look.

I will post some examples of this in a bit.
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Pro 8mm

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Aerial Filmworks

Paralinx LLC

Visual Products

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Technodolly