For negative conversion
Below is a brief introduction to why there is software out there for making positives from negatives.
Converting vs. Inverting
Making a positive out of negative film sounds simple - a negative is simply a the opposite of a positive, so you just invert the negative. Right?... Right?!
Mostly, no - this is what you get if you take a scan from a negative and make a straight inversion
That doesn't look great, right? Well what if we just white balance it against the border- it's way too blue!
Now the rebate around the film is nice and black but the image still looks terrible! What if we white balance against some known grey point in the picture?
This is starting to look a lot better! And yet - it is far from the beautiful image that should be on the film.
It is way dull, with low contrast and it is still weirdly red. There is also no separation between the yellow moss in the foreground and the green plants.
Now it's starting to look like a picture! We have adjusted the contrast both with a slight S-curved contrast curve and squished the extremes to get truer black and whites.
And yet - the foreground still has no separation between the moss and the grass.
This conversion was made using Negative Lab Pro, with standard settings and only a couple of button clicks.
Here we have overall pleasing colour and contrast, nice separation in the foreground.
This example is not to tell anyone that converting a negative manually is impossible or that you need to go out and buy Negative Lab Pro. However, we can learn some lessons:
Inverting negatives is not the same as converting them: For converting, the curves of individual colour channels must be adjusted.
Making a positive from a negative is an interpretive process, meaning some decisions have to be made to produce the final image.
It follows that negative to positive conversion is subjective and that there is hardly a thing such as a 'true' characteristic of a film stock.
Automatic or semi-automatic negative conversion can be a very good starting point for processing your negatives digitally - though be warned that the software is imperfect and will not let you close your eyes and lean back