Lets say you photograph a gray card. Would you expect the exposure to
have a true black and a true white?

 And, the quality of light is very important. If you have low contrast
lighting, don't expect a contrasty image out of that.

  Some cameras have "gamma" adjustments that let you control the
contrast and the number of stops that is covered. Many photographers
shoot in RAW  mode and adjust the exposure when they convert out.

 You have an image editor, as long as the highlights and shadows are
not clipped, you can alway set a true black and a true white in your
editor. Certain caveats apply, of course.

With that said, not every image needs a true black or a true white. The
image subject will dictate that.

Keep in mind the guidelines, but don't force them on every image.

  Jeff

The problem is in your second paragraph: "below average lighting
conditions". You could however select a film with an extended range, or you
could adjust the developement to shift the image on the film's charcteristic
curve.

One word to study: Histogram.

Well, and also expose for the highlights if you are going to do post
processing. If no editing, it's possible to change the contrast in the
menu but seldom practical in the field. More often my complaint is too
much contrast because you can always increase contrast later with
editing. Anyways, study the histogram & most of the rest will become
obvious.

If the subjects brightness range is less than the dynamic range of the
sensor/film you will be able to get a black/black or a white/white but
not both without adjusting the levels in post processing.  Draw a line
6 inches long on a sheet of paper and let that be the dynamic range of
your sensor.  Label the left end B and the right end W.  Draw a
shorter line, say 4 inches long, on another sheet of paper.  If you
line up the left end of shorter line with the left end of the long
line, you can see that it falls short of the W, you won't get the
white, just a shade of light gray.  Slide it the other way and you get
your white, but you now can see it falls short of black, giving you
another, dark gray.  What you are effectively doing with the levels
control is making the long line fit the short one (shrinking the
dynamic range) so that you get the range from black to white.  It
weren't no different with film, it was just handled differently in the
darkroom.

Vance