Using Convex Mirrors for Sunlight bounce

[Trent Watts]

I was wondering if anybody has experience bouncing sunlight with convex mirrors to light up a large area with sunlight. Any tips? Would this actually work? I'm looking at this 36" dome mirror from U-Line for this.

 

https://www.uline.com/Product/Detail/H-1885/Safety-Mirrors/Full-Dome-Acrylic-Mirror

[JD Hartman]

It would certainly disperse the light, then you'd spend a lot of effort flagging the light off areas where you don't want it?

[Ed Conley]

I don't see how that would even work.

It wont direct light anywhere as the the light hitting the sides will just fall off.


It would be better to just use the standard 4x4 Mirror board in to a 12x12 Ultra bounce/Muslin or whatever or just use a 12x12 rag to bounce light into the area you need.

[Trent Watts]

Light dispersion (not direction) is what I'm trying to achieve. But I realized that the dispersion will reduce the sun's intensity as well, so it won't work. Thanks for the feedback though.

[Frank Wylie]

Flat mirrors into large sliks in frames, but tracking the sun is harder than it seems and if you have wind, you have large sails to contend with.

[Jarin Blaschke]

We actually had convex mirrors made for “The Northman.” I put an 18k into it to emulate sunlight or moonlight in the studio or outside at night, so I can’t speak about bouncing the real sun. It would be much more forgiving when tracking the sun, and it would put out a pattern more wide and desirable compared to the standard 4-foot flat mirror. However there is substantial light loss with the convex.

For we light snobs, the convex mirror was a very useful tool because it shrinks the source to a proper point, and the “sun” or “moon” starts looking like the real thing. Crisp, believable shadows. To shrink an 18k to the relative size of the sun, we calculated that we’d have to place it over 250 feet away. The convex mirror allowed us to place the lamp 15 feet away from the mirror, and the mirror 30-40 feet from set. You just had to be sneaky about hiding the diverging angle of the rays.

Our mirrors were 1.2 meters wide and 150 or 300mm deep depending on what we needed to do. Light loss was about 3.5 stops with the 300, - you’ve been warned.

-Jarin

Frank: Large silks defeat the mirror of course, and unless your silk is only 4’ wide, you’re only using a fraction of it. Unless you mean multiple mirrors?

[Stephen Sanchez]

@Jarin Blaschke Brilliant solution! Truly creative. I love reading about stuff like this.

This made my day. It's so obvious when you think about it.

Trophy for you my friend!

[Nicolas POISSON]

I am a bit confused: I would think a "convex" mirror like the one in the first post would broaden the source, making a distant source (the sun...) to appear like a nearby point source. To have a near source appear like a very distant one, I would think one would use a concave mirror, like a parabolic one with the actual point source at its "focal length".

Is there something I did not get, or is just we interpret convex/concave a different way ?

Edited April 1, 2022 by Nicolas POISSON

[Stephen Sanchez]

On 4/1/2022 at 4:30 AM, Nicolas POISSON said:

I am a bit confused: I would think a "convex" mirror like the one in the first post would broaden the source, making a distant source (the sun...) to appear like a nearby point source. To have a near source appear like a very distant one, I would think one would use a concave mirror, like a parabolic one with the actual point source at its "focal length".

Yes. That's correct.

Jarin's objective as it appears was to light a large space with a single sharp source. The drawback is that source is objectively closer, which he mentioned about having to hide the "divergent rays."

A parabolic fixture does make the source objectively more distant, creating parallel rays. I don't believe it appears as a smaller source. And it's inherently spotted, requiring more fixtures for a wider coverage area.

[Jarin Blaschke]

Ideally, for the most realistic results you’d have the source 50 or more meters away by using flat mirrors the size of the set windows, but convex is a shortcut with the aforementioned drawbacks.

We calculated that 70 meters’ distance would match the relative size of the sun or the moon.

With a convex mirror, we have moonlit scenes in Nosferatu that are actually much harder than real moonlight. The convex mirror took away too much light to be used for sunny interiors where I want +4 exposure for sun patches. Convex was fine for studio “exteriors” where sun only needs to be +1