C100: Native ISO Question
- Thread starter John Y
- Start date
greenheron
Well-known member
ISO 850 is base for the chip/camera.
cowpunk52
Veteran
Yes, and no. As explained by the C300 Sensitometric Characteristics white paper:
"For the EOS C300 camera the nominal rating of its exposure index at 0 dB master gain is specified to be 640 ISO for a picture capture rate of 23.98 Hz using an equivalent 180-degree shutter.
Overall subjective image quality is bound up in the three key image sensor parameters of sensitivity, signal to noise, and dynamic range. Their combination is often termed the Sensitometric characteristic of the camera system. Dynamic range of the image sensor is a variable with ISO speed — this, because of noise in the dark areas of a scene and clipping in the highlight areas (due to sensor well-saturation). For the reference 640 ISO setting of the EOS C300 camera, and using the Canon-Log nonlinear transfer function, the gain calibration of the camera system is such that at 0 dB Master Gain setting, correct exposure of the 18% gray card will afford 4.8 stops of exposure latitude above that exposure and 7.2 stops of latitude down into the dark region.
If the Master gain is raised by 2.5 dB then the noise floor is amplified, but the lens setting can be readjusted for an ISO 850 setting to restore the correct 18% gray card exposure — under which condition the overall effective dynamic range is increased from 600% to 800 %. This raises the exposure latitude to 5.3 stops above the 18% exposure level affording a higher degree of protection of the capture of scene’s brightest highlights. This is sometimes termed the 'Push Process' in cinematography."
So, therefore, the nominal rating of the sensor is 640, and Canon Log base sensitivity is 850. You will get maximum highlight headroom by using 850 ISO and above with any gamma, but 0db gain on the sensor is equivalent to 640 ISO.
"For the EOS C300 camera the nominal rating of its exposure index at 0 dB master gain is specified to be 640 ISO for a picture capture rate of 23.98 Hz using an equivalent 180-degree shutter.
Overall subjective image quality is bound up in the three key image sensor parameters of sensitivity, signal to noise, and dynamic range. Their combination is often termed the Sensitometric characteristic of the camera system. Dynamic range of the image sensor is a variable with ISO speed — this, because of noise in the dark areas of a scene and clipping in the highlight areas (due to sensor well-saturation). For the reference 640 ISO setting of the EOS C300 camera, and using the Canon-Log nonlinear transfer function, the gain calibration of the camera system is such that at 0 dB Master Gain setting, correct exposure of the 18% gray card will afford 4.8 stops of exposure latitude above that exposure and 7.2 stops of latitude down into the dark region.
If the Master gain is raised by 2.5 dB then the noise floor is amplified, but the lens setting can be readjusted for an ISO 850 setting to restore the correct 18% gray card exposure — under which condition the overall effective dynamic range is increased from 600% to 800 %. This raises the exposure latitude to 5.3 stops above the 18% exposure level affording a higher degree of protection of the capture of scene’s brightest highlights. This is sometimes termed the 'Push Process' in cinematography."
So, therefore, the nominal rating of the sensor is 640, and Canon Log base sensitivity is 850. You will get maximum highlight headroom by using 850 ISO and above with any gamma, but 0db gain on the sensor is equivalent to 640 ISO.
groveChuck
U-matic Member
This shows it pretty well-
Not only ISO / Gain equivalents and S/N ratios, but Log (850) and non-Log (640) native ISOs and stops of DR above and below. And as Brian said, the most highlight headroom is 850 and above :
Not only ISO / Gain equivalents and S/N ratios, but Log (850) and non-Log (640) native ISOs and stops of DR above and below. And as Brian said, the most highlight headroom is 850 and above :