The Idea is that you set the exposure (gain) as high as you can without clipping any data.
And then lock that exposure for the entire roll, so each image gets scanned with exactly the same gain. This way they all can be inverted the same way.
Vuescan (without locking exposure ) will change the exposure setting everytime you do a preview scan and crop. So each image will have different exposure / gain settings. Sometimes you want that, often you don’t . So we need to find an exposure setting which works for the entire roll, and lock it.
This is from memory so I hope I’m correct in this :
Set lock exposure off , and set the exposure / gain setting to it’s default value (1.0 probably ).
Do a preview scan which includes a part of the unexposed film strip. A piece of unexposed film strip is the brightest your scanner will encounter. Think about it . There is no darker then absolute dark, but since it’s a negative it will be the brightest point .
Now, select thr area which contains a piece of filmstrip with the cropping tools . If you now do a preview scan again , it should set the exposure value because or the auto exposure. So the exposure / gain settings should change now . And the preview scan that appears should have no clipping , but still be kinda bright.
Now tick ‘lock exposure’. This turns the auto exposure off, so any more preview scans and normal scans will not alter the exposure setting, but blindly use what was set before.