[Leica] Re: Shadow noise in scans

[jonathan at openhealth.org (Jonathan Borden)]

On Aug 15, 2004, at 10:10 AM, Eric wrote:

> Sometimes I need more caffeine before posting.  I'm talking about 
> grain and
> you're talking about noise.  Nevermind.  :)
>
Grain and noise are similar in that grain for film is analogous to a 
digital sensor's noise + pixelation.

Factors which affect grain (not in order of importance)

1) exposure -- when a film emulsion has varying sized crystals, the 
larger ones will tend to be exposed by lower light levels and 
consequently grain is enhanced in the shadows. This is dependent on the 
formulation of the film emulsion.

2) aliasing artifact between scanner resolution and size of grain. Try 
either increasing or decreasing the scanner resolution and this will 
vary the apparent grain (actually aliased grain).

3) digital noise -- multisampling tends to decrease this -- noise 
should be random and smooth out with more samples.

4) scanner light source -- a collimated light source will tend to 
enhance apparent grain, some scanners have a 'grain reduction' setting 
which uses an opalescent source, you can also purchase 3rd party 
filters which do the same -- this is the same as with condenser vs. 
cold light heads on an enlarger.

+ all the known issues involved with film development.

There are various Photoshop techniques which can do a great job at 
reducing grain in smooth areas of the image -- for example create a 
high pass filtered layer from your original and use this as a mask for 
a gaussian smoothed (or degrained) layer.

Jonathan