"ecm" <> writes:
>I use Irfanview - it's a very good basic image editing program (only
>lacks layers and levels, and a good crop tool, really) available as
>freeware at:
>http://www.irfanview.com/
>Load your negative scan, go to the "image" menu, and select "negative"
>- voila!
This will give you a low-contrast positive, not one that matches the
original scene. The reason is that a negative has reduced contrast
compared to the original scene. Technically, its gamma is approximately
in the 0.5-0.6 range. When you convert a negative back into a positive
image, you need to invert this by using a function with a gamma of
about 1.67 (really an exponent of -1.67).
The simple "negative" operation is linear, with a gamma of 1.
If you want to be more accurate, you need to deal with the fact that the
film transfer function isn't a simple straight line in log-log space;
it's an S-shape. And even the straight-line center section has a
different slope for each primary colour.
Dave