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#1 Dave Clark

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Posted 30 March 2009 - 09:38 AM

I'm doing a lens flare study with the aim of reproducing them in my 2D post production package, and I'm having a hard time working out how colour changes when reflected in the lens

my reference is footage of a white flashlight being shone at the camera in a dark room, filmed using a todd a.o. 38mm anamorphic lens. I can reproduce that flare very nicely as long as the light stays white

for example I see white light turned into blue with those horizontal anamorphic flares, but in some movies when that light is sunlight I see those flares become orange - so it's not as simple as tinting all the reflections blue, and it's also not as simple as performing a hue shift because then other colours wouldn't match, so I'm guessing I need to map certain colours into other colours in a very specific way, and also maybe take brightness into account

I've been reading this page on lens coatings, there's even a chart there that shows what colour is reflected with a typical coating, but it doesn't help me enough unfortunately, I understand the principles of lens coatings but clearly not well enough to work this out by myself, I'm hopeing someone here with experience of them can help me
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#2 Ralph Keyser

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Posted 31 March 2009 - 06:18 PM

I'm not sure I understand where you are trying to go with this. It certainly sounds like a very hard nut to crack. Every lens has its own combination of lens elements and every element has its own shape and coating (or lack thereof). The color of the reflection changes with the color of the incident light (as noted in the referenced web page), and, of course, since each manufacturer's coating is different and unique, the color shifts are unique. The elements of a lens flare are made up of all the internal reflections, so both the front and back side of some or all of the elements in the lens contribute to the flare. The result is that every lens type has its own characteristic flare pattern and colors. I'm not one of them, but I'm sure there are people that can look at a lens flare and tell you the make and model of lens.

Without detail manufactuer's information on glass and coatings to feed to some high-end optics design software, it seems like an emperical approach might be the best. For each lens you are interested in, you'll need to look at the flare generated by various temperatures (colors) of light. I imagine that this is why most post production lens flare is done with a handful of knobs that let you play with flare shape, number of elements, and color shifts so you can pick the flare that goes best with the shot if it's not a natural flare.
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