[EMAIL PROTECTED] wrote:
Interestingly, the better the resolution/focus of the scanner,Aliasing theory tells us this should be the case.
the more grainy/sandy/aliased the image.
You might experiment with defocusing the scannerI've done quite a bit of experimenting with the focus settings now, and I can tell you that defocusing definitely helps in controlling grain artefacts, but getting rid of them altogether means losing an unacceptable amount of sharpness. (IMHO)
to see if this reduces the appearance of grain.
To defocus with VueScan, turn off the "Options|Auto focus"
option and manually change the focus value.
Another very interesting thing that came to light is that the direction of defocus is very important. For reducing grain without losing too much general sharpness, the defocus must be in a positive direction. That is, you must add to the number that appears in Vuescan's focus dialogue box. If the number that appears there is negative, then you must make it less negative. If it's already positive, then make it more positive. I presume the positive and negative sense hold for all scanners, Ed?
A shift of +0.05 made the scans from my nightmare film almost acceptable, and there was a slight further improvement up to +0.08 shift. (This nemesis film is Jessops own brand 200 ISO colour negative, a definite thumbs down for this one.)
I've been reading up why needing a positive focus shift should be the case, and for those of you that are interested a short explanation follows: ( For those that aren't interested: Just think of it as the grain gremlins and the focus fairies having a fight, and a positive shift helps the focus fairies win.)
The real reason is that, at higher spatial frequencies, sagittal MTF responds differently from tangential MTF to defocusing. In the case of 35mm scanners, the lens's sagittal light rays run across the short dimension of the frame, tangential rays run lengthwise.
The high frequency sagittal contrast drops off rapidly with positive defocus. However, the MTF contrast can actually rise with negative defocus, giving us the opposite effect to what we want. In short; positive defocus good, negative defocus bad.
The results I got were quite encouraging. Those scans with a positive focus shift actually had a better overall brightness, and improved sky and cloud colour. A small negative focus shift reduced the overall brightness slightly and gave a grey, grainy pall to the scan, compared to the standard autofocus setting. Even a large negative shift wasn't terribly effective at improving the image.
BTW. Another vote for Konica's negative film from me. It's excellent stuff!
Regards, Photoscientia.
====================================================================
The filmscanners mailing list is hosted by http://www.halftone.co.uk
To resign,
