Buy a book on the Zone System and read it carefully - it's way too involved to go into here and not nearly as simple an approach as the posts (so far) allude to.
Originally Posted by arigram
You can evalute a scene using your Gossen meter by reading shadow, midtone, and highlight areas that are near to where your camera is setup. You're trying to determine the luminance range of the subject you're photographing from the shadow area with texture that you want, to the highlight area with texture that you want.
A spot meter is certainly easier - but, you can do it with any hand held meter.
The Zone System is predicated on the idea of normal development, over development, and under development. This is often noted as "N" (normal development), N+1 (normal development plus 1 Zone), N-1 (normal development minus 1 Zone).
Over development increases contrast, under development decreases contrast. That's how you "fit" the luminance range to the film so that it prints without excessive dodging and burning to get detail in shadows and highlights.
With sheet film, because you handle every exposure individually, you can adjust 1/2 Zones, or even 2 or 3 Zones. This really isn't practical using roll film, but N, N+1, N-1 (about 95% of adjustments that really need to be made) can be done as follows.
As for using 35mm film:
Get 3 rolls of 35mm film, and label them "N," "N+1," and "N-1."
Evaluate the scene, decide which development best fits the luminance range and your visualization of the final print.
Load that roll into the camera. Take the photographs. Go to your next subject, meter the scene and determine the best development - if you need to change film cassettes for a different development factor then you do the following.
NOTE THE FRAME NUMBER OF THE FILM IN THE CAMERA AND WRITE IT IN A NOTEBOOK.
Rewind the film - but NOT so it goes into the cassette. Some power winders will allow an adjustment that leaves the film leader out of the cassette. My Nikon N-90 is setup to do this. If you have a camera with a power winder you will have to check with a reputable repair shop to see if it can be adjusted to leave the film leader out of the cassette. If not - you're out of luck or will have to get a manual re-wind camera. (There's your excuse to buy that Leica you always wanted.)
With manual rewind cameras you can "feel" the film let go of the winding sprocket.
What you are doing is swapping film cassettes, mid-roll to put the correct development cassette into the camera to match the luminance value you've measured.
When you load the cassette you swapped mid-roll back into the camera - you advance the film (with the lens cap in place) to the frame number plus 2-3 frames (so you don't over lap images). When you develop the film, you will have all of the images with "N" development on one roll, images requiring "N+1" on another roll, and images requiring "N-1" on another roll.
120 film can be done by changing backs if you have a camera with interchangeable backs by designating a back for "N," one for "N+1" and one for "N-1."
There is another way to do this - but I think it's way too hard although I have a friend who used to do this.
He developed a roll of film without exposing it so he had a blank roll of film with just frame numbers. He used this to "measure" his exposed film - more on this in a minute.
In the field, he would keep track of exposures on the roll by frame number and the development time needed. He would give 1-2 blank frames between exposures if he had a change in development time.
When he got into the darkroom, he would take the blank roll of film with only frame numbers and make a "map" of the roll that he shot on a counter top using masking tape that matched where the film needed to be cut. In the dark, he would stretch out the exposed roll over the "map" and feel along the edge of the film for the tape. When he found the tape he'd cut the film and put it into a box labeled for the correct development time.
When he was finished, he had separated the different images into small pieces of film for development.
My problem with this approach is you now have snippets of film of different lengths that cannot be loaded (easily) onto a film development reel. He didn't seem to mind and developed them in a tray. Worked for him - way too laborious for me.