Sensitivity & exposure problem...

[huascar]

Hi everybody. I made some tests with my RED ONE in the last days, but I have problem to understand a thing. This is the setup used:

RED ONE with Nikon mount
Nikkor 50mm f2.0
3 HMI lights 1200W
my girlfriend (as model)

I use false color to exposure well the image. Everything is ok at 320 ISO.

BUT, I decided to check 100 ISO, because I had so much light (320 ISO, 180 degr. shutter, f5.6), and wanted to decrease noise. So I set ISO at 100 and open iris to f2.0.

HERE IS THE PROBELM:


False color and zebra become useless!!! I mean, they start working bad! If I have false color on (when I was at 320 ISO) I see perfect exposure; then I go down with ISO, and I see changing exposure with false color (also violet zones); I open iris, without reaching a good exposure zone... BUT looking in the LCD I HAVE MANY OVEREXPOSED ZONES!!!! It seems I have the exposure at 320 ISO with f2.0 (many overexposed zones) and going down to 100 ISO, only go down the brightness of the image... but the overexposed zones remain!!!!

So, what is the usage of 100 ISO???

I hope to have explained well.... SORRY FOR MY ENGLISH!!!!

Please help me!??!?!


huascar

[Andrew clemson]

Im a bit confused. What view mode did you have the camera in? which build?

[Anders Holck]

Huascar,
the deal is that basically the RED ONE is a fixed ISO 320 camera.
If you set the sensitivity below ISO 320, to for example 160, the sensor basically takes the exposure at 320, then the camera DSP lowers the exposure by half.

This of course means that the parts of the image that is clipped at 100% will be dimmed to 50% (linear)

One reason for using ISO 100, is if you shoot a dark flat lighted scene without specular highlights, you can force your exposure higher (Use more light to light the scene) while you still see the dim image on screen. This helps raise the noise floor.

Compared to a DSLR (where the ISO is not fixed) where you do get better noise with thesame dynamic range by lowering the ISO to 100. Not so on the RED ONE.

You need to use ND filters to cut the exposure so you lower it before it hits the sensor.

[huascar]

Thanks Anders! For the other reply I'm using 20.1.3 version, and the tests were made in REC709 view mode.

So, It's always better to shoot at 320, and if I have a lot of lights use ND filters (or going up with shutter speed...)?!?

And last question about this problem:

If I shoot 320 ISO, then in CS4 plugin set the ISO at 50 and raise up the exposure level, I have the same noise quantity?! It's better or worste?

Thank you very much!!!

huascar