Digital BW, The Print

On 05/07/2010 outdoornm wrote:
> You are not reading what Michael has said. He said that a higher 
> resolution scan created a file that showed more grain and was softer. 
> We all know the mantra to scan at high res for possible future use, 
> but possibly higher res scans magnify grain. Has this issue been 
> covered?

No, higher res does not "magnify grain", but you can get grain aliasing 
artifacts when scanning at any resolution below the Nyquist limit for the 
emulsion.

Grain aliasing is similar to moire as a random interference between 
spatial frequency of sampling and grain distribution, and produces false 
grain as an artifact.

Because grain size and clumping is highly variable, depending on emulsion, 
exposure and development, the Nyquist limit for the grain structures in 
any emulsion is is a range of frequencies. Aliasing effects can therefore 
be localised and limited to certain areas of the image, or across the 
frame, or absent.

I first described this c.1999 on the filmscanners list when Polaroid were 
puzzled by exaggerated grain in customer scans. You'll find more thorough 
subsequent discussion about the effect by Pete Andrews at 
http://www.photoscientia.co.uk/Grain.htm

Some 2700ppi scanners like the earlier Nikon Coolscans, with their 
semi-collimated LED lightsource, could produce horrendous false grain in 
areas such as blue sky on ISO100 slide materials. Many users thought they 
were seeing real ISO100 grain, and that meant that 2700ppi was more than 
adequate. In fact they were seeing aliasing products that looked rather 
like grain, from material that appears essentially grainless to the eye.

Higher res scans mitigate the effect.  IME upwards of 4000ppi largely 
avoids the effect on most materials. However I've seen it with overexposed 
Fuji200 colour neg and also with some of the ISO3200 B&W neg films. 
TMax3200 is essentially unscannable for me, using a Polaroid SS4000, the 
aliasing turns out an image made of gravel.

-- 
Regards

Tony Sleep
http://tonysleep.co.uk