Hi folks - just by way of introduction, I'm a 30-year industry vet, mostly an editor but I occasionally shoot on the Sony NX5U.
If I need to shoot a backlit scene (i.e. window in the background) and want to intentionally overexpose the shot, the camera doesn't seem to want to let me do that. Iris is open all the way in manual mode, but the camera still wants to expose for the window. Subject becomes understandably dark.
"Gain" is off. Not just zero but off.
Back Light function gets me little improvement.
I believe your issue is that the camera has variable manual settings.
First step is to make sure you have your data display turned on. If not, you can do it in the menus.
If it is turned on, you will see three numbers on the bottom of the viewfinder.
iris/aperature Gain speed/time/exposure
These will each have a legitimite number on them, like 1.8 for aperature, 0db for gain, and 60 meaning 1/60 of a second.
Now, if you have the slide lever on the left side of the camera set to auto, then all three of these numbers will be prefixed with the letter A which means automatic.
If you switch the slide switch to the manual setting, then at first the three numbers will still be in automatic evidenced by the A.
If you tap the iris button, it will toggle to manual iris, removing the A, and enabling the iris ring. The gain and exposure will still say auto and will try and get the "best picture"given the manual iris.
If you tap the shutter speed button, the A will dissappear, and it will enable the roller wheel near the menu button. You can dial in the shutter speed you want. The camera will still try and get the best picture using the only option it has, which is gain.
If you tap the gain control button, the A is removed and the three positin gain swith is enabled for you to set the gain manually.
So, you can manually control 1, 2, or all three. But only by controlling all three can you keep it from trying to get the best picture by itself.