Basic ImageJ questions: getting started

[Megan M. Mahoney]

HEllo
I am new to ImageJ, I would like to use it to analyze xray films of  in
situ. Currently we 1) take pics, and calibrate darkness and brightness at
the same time using MCID then 2) analyze using NIH Scion Image. I have
some really basic questions about image aquisition and the basic steps of
setting up. I could not find an answer in all the online info.

Specifically:
1) can I take my pics of my xray film (with a lightbox) without worrying
about the calibration of light/dark (turning light up and down)? AS long
as I take pics that are all under the same conditions.

2) Can I just use the image of the white-black optical density step tablet
(in examples) to calibrate OD? Do I take a picture of it as well under the
same conditions? Or do I need to use the film to calibrate OD?

3) Can I just use the command to subtract background and it will be true
for all my images?

Thanks in advance for your help with such basic questions.
-Megan