I just bought a little 870 to see what I could do in 4K. It is nice that the unit has a manual iris, but is it really an iris? The settings seem to change the exposure by increasing the gain (in dB, that's gain, not exposure). Is there a way to adjust the exposure while keeping the gain at a sensible 0 dB?

In page 67, the user manual says : "When the iris value is adjusted to brighter than OPEN, it changes to the gain value"
It sounds like Gain is at 0db until you go beyond full open Iris value...

When changing the iris value, the displayed iris/gain value is changed only for every other step up or down, while the actual brightness of the picture changes for every step up or down.
So, I guess there are intermediate iris and/or gain values that are used but not displayed.
And I have no clue which one of each step in a "pair" actually matches the displayed value.
The question is: when do I actually have iris fully OPEN and NO gain ? When does gain actually kick in ?
If I increase brightness from some larger iris value step by step until OPEN/0dB is displayed, I can go one step further (brighter) and still see OPEN/0dB...is the latter now the real 0dB or did I just start adding 1.5 dB gain ???? Or was the "first" OPEN/0dB more "something between f 1.8 and OPEN=F2.0 / 0 dB" and the second one really OPEN/0dB ?