June 12, 2017 at 3:31AM

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How does the distance between the light and diffusion material affect the quality of diffusion?

Assuming the diffusion is fixed at 3 feet from the subject and the light source is at 5 feet from the diffuser, what will happen to the specular highlights and the shadows when we move the light source to 10 feet (diffuser still at 3 feet from subject). We adjust the intensity of light of course but want to understand how does the quality of diffusion change. Also, will light softness remain same? (since the size of the diffuser wrt the subject remains the same)

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If the diffusor is 100% effective (not reflective, but effective), then things will be the same (other than possibly light output). But if the diffusor is 50% effective (and thus 50% specular), then there will be a hot spot. The size of the hot spot will be a function of how divergent or parallel are the rays hitting the diffusor. The hot spot from a light source 5' away will be larger (not more diffuse, but larger) than the hot spot from a light source 10' away.

June 12, 2017 at 7:50AM

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I kinda disagree with Michael here, Softness in light is dependent on the size of the source from the perspective of the subject. So if you are the subject looking at the source, then from your perspective, the size of the source dictates the softness. So a 2x2 up close can be softer than a 20x20 from in the distance, the sun 430,000 miles wide. Thats the biggest source possible, but its 92 million miles away, so when you look up, from your perspective, its a small hard source. It just depends on how much of your field of view does the source take up.

The first thing to note is that once there is a diffusion frame in between the subject and the light, the light is no longer the source the frame is, You have to look at how much of the frame is evenly illuminated to determine how much of the frame your new source is. If the diffusion is heavier, than the light will fill more of the frame and then the source will be the full size of the frame. If the diffusion is lighter, than only the center of the the frame will be evenly illuminated, and the size will be only slightly larger than the source you used, in this case lets say a 8 inch fresnel lens. Then with a light diffusion like 251 you might only get a 10x10 circle that is will be your source. It doesn't matter whether you have a 2x3 frame or a 4x4 frame, the source will still only be 10x10. That will be soft very very close to the subject and hard at a normal distance. If you move your light further back from the frame the hot spot will get smaller in the frame, so your effective source will be smaller, so it will be harder.

Just stand where your subject is being lit and look at the source to determine how big or small your source really is, if you put your light very close to 216 you'll see a smaller source, if you use lighter diffusion but don't shoot straight through it, come at it at an angle where the subject cannot see the source through looking through the diffusion frame, and it will fill the frame completely and be as soft if not softer than 216. The effectiveness of the diffusion is determined by where you put the light behind the diffusion and how much of the frame are you filling so just stand where the subject is and take a gander at the source!

Side note: source size and distance is proportional a 4x4 from 5 feet will be identical in softness to a 8x8 from 10 feet, because from the perspective of the subject they will be the same size. Also you have to factor in inverse square law as well, source will get softer the closer they get to the subject, but you have to find a balance with distance because the closer you get then the more abrubt the fall off. If you do have your source 3 feet away from the subject and the subject moves 1 foot the exposure will change by a stop. So you have to balance that with the benefits of closer sources for softness and find a happy medium where you are getting the quality you want.

June 15, 2017 at 3:41PM

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Indie Guy
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