I scan my V850 at 16 bits and notice I can see what looks like grain in Tmax 400 but not Tmax 100. What does that mean relative to your point?
(If you click on the images they will zoom in to see the detail better in the skies)
4x5 Tmax 400 at 16 bits https://www.flickr.com/photos/alank...43ZPF-2k44Fak-2k43ZPq-2jcarE6-2jcbMjA-2jcbMiJ
4x5 Tmax 100 at 16 bits https://www.flickr.com/photos/alanklein2000/49843392888/in/album-72157714124881023/
The important thing is whether there is noise in the scan. The noise could come from a combination of film grain and other sources. The other sources could include (but not limited to) sensor noise or even shot noise.
Shot noise comes from the quantized nature of light in combination with the statistics of photon detection, and in theory it can show up at low signal levels. I don't know if shot noise is a factor in scanners, but I would not be surprised if it is. To give you an idea of how the statistics work, the standard deviation is equal to the square root of the number of photons. For example, if 100 photons are detected then the standard deviation is 10 photons. Suppose that the analog to digital step size at low signal levels is equivalent to ten photons. In that case shot noise would be more than enough to produce effective dithering. It would also apply at high signal levels. For example, if a high signal level were equivalent to detecting 10,000 photons the standard deviation would be 100, so if the step size were 10 photons it would be more than enough to produce effective dithering.
Sensor noise (as distinct from shot noise) is very likely at low signal levels. This can come from various sources, primarily electronic in nature. For example, there is something called thermal noise, which comes from the random thermal motion of electrons. There is also something called flicker noise. Anyway, a lot of scanner reviews talk about the existence of noise in the shadows. This probably comes mostly from some combination of sensor noise and shot noise.
None of the noise sources mentioned above go away at high signal levels. They become proportionally less important compared to the signal level if the signal level is high, but from the point of view of how it relates to the effective dithering effect the important comparison is not the comparison to the absolute signal level but rather the comparison to the step size of the analog to digital converter. This means that if there is effective dithering taking place at low signal levels it will also be there at high signal levels.
Things get a little more complicated if a non-linear transformation is applied to the signal somewhere in the signal chain. If so then it is possible that the effective dithering effect might go away due to roundoff error. I am told that when a scanner saves in 8 bit mode there may be a non-linear transformation, and I can't say to much about that possibility. However, I understand that in some signal processing systems when a high-bit word is converted to lower bits the software may add pseudo-random noise in order to make sure that dithering is present. I don't know if this applies to 8 bit scanners.
Now to the question of t-max 100: here is one way you can investigate this experimentally using your own scanner and your own photographs. I'm not talking about your regular photos but rather photos generated specifically for the purpose of testing this. Get some t-max film. Find a perfectly uniform object to photograph, like a blank wall. It will also help if you will defocus the lens to make sure there's no small-scale variability in the image. You might even consider taking the lens off altogether. Take photos of the image. Some at high exposure, some at moderate exposure and some at low exposure. Develop the film.
Scan the images in 8 bit mode. Open a file. Look at the histogram of crop of a very small portion of the image near the center of the image. Zoom in on the histogram (along the horizontal axis) and see if there is just a single spike in the histogram or if you can see several spikes. If there are several spikes then there is significant noise in the image (whether from film grain, sensor noise, or some combination), and that should satisfy the effective dithering requirement. Do this for the blank images that you photographed at various exposure levels.
You could also take a photo of an image having a smooth brightness gradient. Scan in 8 bit mode. Copy the image to form a second identical file. Open one of the copies and do some extreme image manipulation until you can see banding. Next open the second copy and convert it to 16 bits. (That conversion is very important. Leaving it in 8 bit mode will nullify the test.) Next do exactly the same extreme image manipulations that you did on the first file. Do you see banding? if not then the 8 bit scan hasn't hurt anything and you are good to go.
Always remember, if you scan your regular photos in 8 bit mode then be sure to convert them to 16 bit before you do any image manipulation. Keep it in 16 bet mode, preferably forever, but if not forever then at least as long as you are doing any image manipulations on the image.
Now for a question: Why would anyone even want to scan in 8 bit mode? The first answer is that some systems may only allow 8 bit scanning. Leaf brand scanners when used in certain configurations fall into that category. The other reason (not a very strong on these days) is that 8 bit scans save storage space.