With the release of Firmware version 1.13 for the PMW-F5 and F55, Sony enabled the use of EI gain when shooting using the Cine EI mode. It’s a little unusual in that the EI gain only goes below the default/native gain and not above.
So what is EI gain and how do you use it?
EI or Exposure Index gain is a way of changing the gain of the monitoring outputs, but not the gain of the primary recordings. Currently with the F5 and F55 the signal seen on the HDSDI, HDMI output and viewfinder outputs are the same signals as recorded to the internal SxS cards, so EI gain is also added to the internal recordings. However EI gain is not added to the raw recordings recorded by the R5, these are always done at the native ISO of the camera, 1250ISO for the F55 and 2000ISO for the F5. The EI Gain is selected in the camera menu, it cannot be selected from the hot keys around the info LCD.
The F55 supports Exposure Index values (400EI/640EI/800EI/1250EI) in Cine-EI mode.
The F5 supports Exposure Index value (640EI/800EI/1250EI/2000EI) in Cine-EI mode.
So as we can see from above we can reduce the EI gain, not increase it (the F3 and most other cameras allow an increase in gain). What happens when we do this? Lets say we decrease the EI gain on an F5 to 800EI or 640EI on an F55, just a little over a one stop reduction. The image seen on the monitor or viewfinder screen gets darker. If your using a light meter to expose you would set your light meter to the new EI ISO.
If exposing according to that light meter, it would tell you to open the iris by a little over 1 stop. This would then return the brightness on the monitor or viewfinder back to where it was before you selected the new EI ISO. Now remember that the actual raw recording ISO does not change. So as you open up the iris (as that’s what the light meter or monitor image tells you to do) the raw recordings become brighter. With most conventional cameras this could be a problem as it might result in an overexposed image. But the F5 and F55 have so much dynamic range that for most typical scenes the exposure will still be OK. My experience with the F5 and F55 is that when shooting at the native ISO you normally have plenty of headroom. Raising the recording levels like this will allow more shadow and dark detail to be seen. So what you are doing is in effect shifting the cameras latitude down by just over one stop. Assuming your correctly exposed at the cameras native ISO you get +7 stops and -7stops, but now with the EI ISO set 1.25 stops lower, the dynamic range becomes +5.75 stops -8.25 stops. In addition as you will be bringing levels down in post production (compared to footage shot at the native ISO) you will end up this less noise in the final results.
So the current EI ISO settings on the F5 and F55 allows you to shift your exposure range down to produce a less noisy image with greater shadow latitude, but decreased highlight latitude. Interestingly this is the opposite to the way EI ISO on the F3 works, the difference being due to the F5 and F55’s much higher dynamic ranges but more importantly the linear raw recording allowing for better use of the top end of the cameras dynamic range. A camera like the F3 that uses log recording cannot be significantly overexposed as the log compression of the highlights would cause problems with the brighter parts of the scene becoming excessively compressed. I would imagine that Sony will include EI ISO settings that are higher than the native ISO in future firmware releases, but for the moment we can only go down.