Scanning Film with a Digital Camera
At one time the measure of a good negative was the ease or difficulty it provided in making a paper print with an enlarger. Sadly the technology for creating analog prints from color reversal appear to have been lost forever. I still value a B&Wnegative that prints easily, but in most cases ease of scanning is even more important because the digital representation is the first and maybe only way that other people will see the photograph.
Scanning with a Digital Camera
For 35mm and medium format I prefer a digital camera over a consumer-grade scanner for several reasons: speed and control over exposure.
Always use the 2-second self-timer to get to get a sharp image. You probably need to use manual focus, but contrast-based systems are also good enough. I usually take macro shots at f/8 or f/11.
Black and white negatives capture a relatively wide tonal range which does not map the linearly to the digital camera's dynamic range. This means that in post you will almost certainly want to adjust the shadows and highlights using an S curve which vaguely mimics the restrainer in the highlights and depletion of halides of shadows on photo paper.
There are means of capturing a scan that are analogous to split-grade printing. This is possible by bracketing the exposure by +/-1 stop and combining the three layers. Combining three exposures also provides far less loss because of curve adjustment. In Gimp I will often merge two layers at 50% opacity using the Grain Merge mode. In this example I am also applying a luminosity mask to one layer
Another option is to apply some simple curves to each layer and then adjust the opacity of each. Blending the tree exposures in this way you will be able to prevent banding.
Slide film is a real joy to inspect by eye, but requires creativity to scan because it has very rich details embedded in the shadows. Because of this I always auto-bracket by 1 stop. If you're in a hurry you can simply pick the best of the three for each frame. This isn't usually what happens. More commonly two of these exposures will be selected for a selective merge using a luminosity mask. Using luminosity masks we can create a kind of high-dynamic range image that hopefully comes close to what we see with our eyes.
I have no formula for this step, but in often I start with darker image and add an overexposed image to bring back the shadow detail. I may use any combination of masks ranging covering very light to very dark, but I try to limit the composition to two or three layers or less for mental simplicity.
Color negative film is easy to develop, but very difficult to scan because it has no inherent color balance. For this reason I avoid color negative film if possible.
Final color correction is hard. In Gimp you might get close by using Mid-point eyedropper in Levels. More likely you will need to adjust curves. I have found the following envalope to work reasonably well with Fuji Pro 400H
This is a preset I save called my Color Envalope gives you six control points, two for each channel that work by giving you separate control over compression of hilights and shadows as well as relative saturation.
Using a digital camera
Drum scan resolutions