reflection image when shotting flames

[Yaron Harel]

hi all.

one of our costumer try to shut a frame that the lower half of the frame light with flames and the upper half is dark the flames are feed with diesel fuel and there is diesel fume above the flame.

when they look at the monitors they see an image reflection of the flames on the dark half of the frame and they see it also in the camera PB (they haven't try PB on RED cine or RED alert).

did anyone ever bump in to such phenomenon?

thank, Yaron.

[Keith Walters]

hi all.

one of our costumer try to shut a frame that the lower half of the frame light with flames and the upper half is dark the flames are feed with diesel fuel and there is diesel fume above the flame.

when they look at the monitors they see an image reflection of the flames on the dark half of the frame and they see it also in the camera PB (they haven't try PB on RED cine or RED alert).

did anyone ever bump in to such phenomenon?

thank, Yaron.

Sounds like the old infrared problem.

Invisible infrared light from the flames might be reflecting off the smoke and fumes.

Your eyes don't see it but the camera does.

Are they using NDs?

If so, the problem is that most NDs were only designed to reduce visible light, since until single-chip cameras came along, most cameras did not pick up infrared.

If this is the problem all they can do is either get better filters or a hot mirror, or use the lens iris to reduce the exposure and live with the large depth of field.

[Yaron Harel]

Sounds like the old infrared problem.

Invisible infrared light from the flames might be reflecting off the smoke and fumes.

Your eyes don't see it but the camera does.

Are they using NDs?

If so, the problem is that most NDs were only designed to reduce visible light, since until single-chip cameras came along, most cameras did not pick up infrared.

If this is the problem all they can do is either get better filters or a hot mirror, or use the lens iris to reduce the exposure and live with the large depth of field.

Thanks Keith.

first, i meant shot of course not shut.

secondly, yesterday night i figure out that it might be the old problem of refracting infrared light I'll suggest the crew to try use ND's filter and see if it can help.

thanks for the advice, Yaron.

[Keith Walters]

secondly, yesterday night i figure out that it might be the old problem of refracting infrared light I'll suggest the crew to try use ND's filter and see if it can help.

 

I meant that NDs may be the cause of the problem.

 

Many older type NDs only block visible light, so even though the ND looks dark gray to you, under infrared they look transparent.

 

So if you have a "1/8" ND, even though it reduces the visible brightness by seven-eighths, it may not reduce the invisible infrared anywhere near as much, or at all. You might be getting 12% of the visible light through, and 95% of the infrared!

 

So you either need to get infrared blocking NDs designed for single-chip cameras like the RED, or not use NDs at all and use the lens iris to control the exposure.

 

Closing the lens iris down will at least tell you if the NDs are the problem anyway.

[Petros Nousias]

Maybe its the OLPF sitting between the lens and the sensor reflecting the lower half, especially if the camera is not looking straight at the flames but is tilted up/down. Ive seen similar reflections(especially with the sun/lights being in the frame) and were always because of this.

[John Sprung]

Maybe its the OLPF sitting between the lens and the sensor ....

 

This seems unlikely for a couple reasons.

 

If reflections off the OLPF were a serious issue, they would be encountered by a great many users.

 

The OLPF of the Red is parallel to the sensor image plane, to me it looks like perhaps 8 - 10 mm away. To get a reflection from it, light bouncing back from the sensor would have to go back to the OLPF and reflect to the sensor again. This would put the reflected image farther from the center, rather than taking it from one half of the frame to the other. Reflections across the center of the image can be created in the lens, because of the many curved surfaces. That seems more likely to be the cause. Another possibility is a kick from the front of the OLPF to the lens and back. That could give you a reflection across the center.

 

Shooting the same setup with a variety of lens designs will reveal whether it's the lens or something else.

 

 

 

 

-- J.S.