As others have reminded me, some of my lab work at the Germaine School
of Photography in NYC involved masking in 4X5. Now I see the
connection. Thanks.
John
Kenneth Sloan wrote:
>
> > Someone once said (maybe on this group), that when you scan an image,
> > you should do it so you only have to scan it once, then use this "raw"
> > or "master" image for all future images, whether it be downsizing or
> > creating a thumbnail.
> > And YES, there is a huge improvement in resized image quality when I
> > use the "unsharp" filter. It really sharpens it (why do they call it
> > "unsharp"?)
>
> For shame! When I teach this to Computer Science nerds I always tell
> them that photography folk have been doing this for years - and have
> lots of darkroom techniques that seem "unusual" in the digital world.
>
> The technique is called "unsharp masking". Start with a slightly
> out-of-focus negative. Make an even MORE out-of-focus POSITIVE
> transparency. Now...print the sandwich and what do you get? A sharper
> print. Why? Because the print is a blend of the original image (which
> contains high and low frequency energy) and the unsharp mask (which
> contains very little high frequency energy, but lots of low frequency
> energy). Reversing the sense of the two image means that you have:
>
> Image - UnsharpMask = "sharper" image
>
> In the digital world, you can either do this directly:
>
> Blur <Image >UnsharpMask
> Blend Image -UnsharpMask > SharpImage
>
> Or...you can combine both operations by convolving with a kernel which
> is a combination of "blur" and "subtract", viz:
>
> -1 -1 -1 0 0 0 1 1 1
> -1 9 -1 = 0 1 0 - 1 1 1
> -1 -1 -1 0 0 0 1 1 1
>
> (the above is just an example...please don't pick at the details, or
> I'll write more...you have been warned!)
>
> I like to create the unsharp mask and then play with the amount of
> Blend. I use a function:
>
> Blend Image1 Image2 t >SharpImage
>
> where 't' is the weight of Image1 and (1-t) is the weight of Image2.
> This allows everything from image mixing to image SUBTRACTION (which is
> what we want when Image2 is an UnsharpMask).
>
> And...yes - I have found that when my son scans prints on his cheapo
> scanner, it helps to run the sampled image through an "UnsharpMask"
> step. Use with moderation, though! You can take out the low frequency
> information, but nothing can put back in the high frequency information
> that you DIDN'T GET when you scanned originally. Better to make sure
> that you get the best possible original scan!
>
> --
> Kenneth Sloan sloan@xxxxxxx
> Computer and Information Sciences (205) 934-2213
> University of Alabama at Birmingham FAX (205) 934-5473
> Birmingham, AL 35294-1170 http://www.cis.uab.edu/info/faculty/sloan/
>
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|