Sony FX 6 Quick Start Manual page 70

Exposure: AUTO ND instead of AUTO Iris
Another good option is to automate the ND filter instead of automating the iris.
ND filters are used to cut light to the sensor. They are mostly used when you cannot
lower the exposure in any other way. Make sure that you never have any GAIN
applied before using ND filters.
Using ND filters does not affect image quality at all.
Using a ND filter can also allow you to select the manual iris setting you want for
the desired depth of field, not an iris setting that is dictated by the exposure. This
method works well because this camera is able to apply incremental amounts of
ND.
There are three ways to use the ND filters: preset, variable and auto.