How can I scan at predetermined exposure settings?

[dikaiosune01]

This is related to as question I asked in another thread, but the nature of the question has diverged from gear to digital processing.

Situtation: I'm trying to scan three negatives that are shot at three different exposure settings to test the dynamic range of film.

Problem: When the scanner scans it, the software automatically adjusts it for optimal scanning, which over compensates and makes three exposures look relatively equal.
How can I scan the three negatives at the same exposure settings? I've tried adjusting contrast and brightness, but I believe that only gets me so far. Is there something I can do that is more percise?

Technology used:
Scanning HP5 film, in Epson v700 using Epson scan software.

[Marek Warunkiewicz]

Turn off all the automatic setting and do a raw scan. The Epson software has an automatic and advanced setting. Go into the advanced setting and turn off all automatic adjustments.

[D. Bryant]

Turn off all the automatic setting and do a raw scan. The Epson software has an automatic and advanced setting. Go into the advanced setting and turn off all automatic adjustments.

What Marek said.

[Ken Allen]

what Mark said with a note. Understand that most flatbed and film scanners do not have a variable aperture, or exposure times. So when you do make any adjustments in the scanner software you are only processing the scan in software not increasing or decreasing the actual exposure. So if you are capable in processing the file in Photoshop post scan you gain nothing by using the scanner software to adjust brightness, contrast, etc.
Things that are useful from software are: creating a scanner profile and embedding it into a modern version of the popular file formats so that Photoshop actually recognizes that there is an embedded profile. And/or decent color engine to convert from the scanner color space to a standard color space like Adobe1998 RGB. And that's about it.

Some scanners may adjust gain a little bit. Which is paramount to changing the ISO on your digital camera. Except that the chips in most linear array based scanners don't see the any benefit just more noise. Unlike a digital camera which seems to do well with increasing gain/ISO. I wish I new why digital camera chips are getting so much better and linear arrays have not seemed to have improved in over ten years?