GH2 asa rating of the GH2 sensor

[misterbarryman]

I did some tests in cinema mode at different asa ratings and looked at images blown to the pixel level to determine where there there were chromatic and resolution changes. Although my tests were anything but exhaustive I perceived that the the images were identical up till about 640 asa and then started to deteriorate at 800.

Assuming that the sensor has a specific sensitivity and that information from it is processed so that too much information/light will have the levels clamped, and too little will have gain applied, my thought is that changing the "ASA ratings" will implement this kind of processing electronically. I wonder whether shooting at the lowest asa (as with the RED) risks having less detail in the highlights because of the clamping.

My next series of tests will be to shoot some high contrast material where I shoot at 640 asa with two stops of ND, and shoot clean at 160 ASA. Has anybody done anything similar or have any comments on my assumptions.

 

[l.breyers]

I did alot of iso tests before, and for blowouts and blackouts it was more of an issue of exposure. ISO really only affects fixed pattern noise, which only becomes visible in underexposed areas.

 

[misterbarryman]

I have now completed more tests and have discovered the following:
1. The ASA rating bears little relation to readings using a spot meter and a grey card (18%). However, if the camera is pointed at a grey card (filling the screen) and the manual exposure is adjusted to 0, the grey card is a pretty reliable way to calculate a good exposure for the sensor.
2. With identical (tungsten) lighting and areas in the frame of at least two stops under and over exposure, I have determined that at 640 ISO there are some color shifts and blockiness in the blacks, whereas at 160 ISO, there appear to be some blooming in the highlights. This does no appear at 320 ISO, which is starting to look like the ideal ISO rating for the camera in tungsten light.