I've read again one extremly useful article about Dynamic range, 24 or 30 or 36 and so on:
http://www.scantips.com/basics14.html
Below is citing (rather big, but interesting):
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Said again because it's the main point in a difficult subject: Because 10 bits can contain 4 times the range and number of steps of tones that 8 bits can, 10 bits makes a fuller range easier, and can also extend the range into deeper areas. In a 30 bit scanner, the hard work of handling logarithmic data has the advantage of working with 10 bits of data, instead of 8 bits. The improvement is in the extended Dynamic Range, i.e., the ability to differentiate deep black tones.
Some scanners can return more than 24 bits to the output port, to be used for histogram and curve processing in external software. However, there is little point of this if the scanner can do it first. The truth is that very little image programs can handle 30 or 36 bit images. PhotoShop 4.0 can, but can only do histogram and curves, and then the image must be converted to 24 bits for any additional processing. And that job is better done in the scanner. It is the reason the scanner does it. Scanner firmware is the hard way to do it, but it is best, so it is provided. And most image programs or file formats can't handle more than 24 bits either. Frankly, it's not important, whatever would you do with such an image anyway?
The benefit of 30 or 36 bits is only in the internal processing in the scanner, to be able to get a "better 24 bits", and we we are going to get to this in just a few more paragraphs. The main point of this extra range internally within the scanner is that it allows the 30 bit scanner to capture a higher range of image density for its internal gamma processing. It is often said that the 30 bit scanner returns the "best" 24 bits, as if it simply discards a bit or two of noise here and there. That is an extremely misleading and simplistic way to look at it. It's more correct to say it returns better 24 bit values (especially at the black end).
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So, this is what a 30 bit scanner does and how it does it. 30 bits do not affect colors. It does NOT deliver better color in the RGB sense, it cannot, it returns only 24 bits. It does deliver better detail in shadow tones, if such deep dark detail exists in the image. The 30 bits remain internal, and are not returned to the scanner driver
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Conclusion: to get good 24 bit for post-processing in PS, use 30-bit pre-processing in Twain driver.