Do scanners adjust exposure ?

When the scanning software is set to deliver a brighter or darker image, does that setting affect (a) the scanning process itself, or (b) only post-scanning interpretation of the data recieved from the scanner ? In other words, does it affect the exposure ?

If the answer is (b), that means the scanner does the same scan every time, and what you get is wat you get. You can't ask it to read extra deeply into the shadows, or darken the highlights, etc. You only do that after scanning, for better or worse. The same would apply to color balance correction, etc.

Is there a difference here, between cheaper versus expensive scanners ?

Ken, I don't know about other scanners, but my Epson 3200 scans slower with a dense negative, so it appears to have a longer exposure time in this way, compensating for the denser negative. On another webside I red that a Nikon scanner can ajust the briteness of the lamp. (The Epson lamp burns at only one level.)

Most desktop scanners, as far as I know, are single-pass systems, meaning the intensity of the imaging light remains constant. That is probably desireable for several reasons, including consistency of color temp of the light tube. So, the answer for the typical scanner would be "b" - the scanner software setup is using overall brightness and curves to make the adjustments to the raw scan data..

Some high-end scanners, like the Imacon, I believe, have multi-scan capabilities, or perhaps even multiple light tubes, which would enable better shadow detail. But, not being a real scanner expert, I could easily be wrong.

Even using post-processing adjustments within the scanner, however, multiple scans of the same neg or transparency can be useful. Occasionally, better results can be obtained by doing one scan where the highlights are favored, and a second scan to brighten the shadows. Then the two scans can be selectively merged using layers in Photoshop or another image editing package that has layers capabilities. The "selectivity" can be a function of adjusting the individual layer's opacity, or by carefully erasing parts of one layer to allow the lower layer to show through.

Every scanner I've ever used has gone more slowly as the density of the negative being scanned increases - effectively increasing the 'exposure time' for each line of the scan. I would say the answer is 'a'.

As I understand it, multi-pass or multi-sample scanning is not an exposure tool but instead a way to get multiple samples for each pixel. Why get multiple samples? Because noise will fall (as a rough approximation) as the square root of the sample size. That is, if you sample four times, you've halved the noise; if you sample 16 times, you've cut it to a quarter of the original.

I may have an answer in very general terms-- "C, all of the above"

Your scanner first has to set its lower limit based on the barest signal that is coming through the sensor even when its not scanning - this is the black point. The black point changes with temperature which changes with how long the machine has been turned on and how much scanning its been doing. Just above that is its threshold of detecting signal from your film. Slower scans (longer exposures) are often used to read more info from the denser parts of the film. At the other end are the bright areas which must be controlled or they're just gone off the scale of possible recording (where the sensor is overloaded with signal). Somewhere in between is the range that the scanner can see. It may not be able to "see" all this with one setting. Times may vary and the scanner may "figure" how to do this or not on its "auto" setting. You may click a box that tells it to do "auto" or manually adjust settings - black and white points, tone curves, etc. to change this. The scanner may change apertures to adjust exposure, or you may do so manually on some old machines. I'm not sure anyone can answer how each and every scanner works (I'd love to hear it) but in general the answer is that the scanner does not "do it the same every time", hardware or software.