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[filmscanners] Re: PS sharpening



Brian writes:

> If I scan a 35 mm slide or negative at 4000
> dpi in a Nikon Coolscan 4000 and I want to make
> a print in Photoshop, I alter the long dimension
> to 11 inches (the short dimension ends up at
> whatever to retain the proper dimensions).
> Since this usually ends up in a file size that is
> smaller than what it was originally, does this
> mean the image will be downsampled?

No.  By default, when you enter a dimension in the Image Size dialog box,
Photoshop will resample the image to match the dimensions you've given.  In
the case of pixels, PS simply resamples up or down to match the new pixel
dimensions.  In the case of a physical dimension like 11 inches (entered in
the "Print Size" portion of the dialog), however, PS resamples up or down to
match the new physical dimension _after_ calculating the number of pixels
required by multiplying the physical dimension by the number of pixels per
inch.  When you open a scan from the Coolscan, the ppi is set to 4000 (the
scanner's resolution); and the number of pixels in the image corresponds to
the number of pixels in a 35mm frame scanned at 4000 ppi, or about 5669x3779
pixels.  If you now enter just 11 inches as the new dimension in the
resizing dialog, Photoshop will compute 11 inches x 4000 ppi = 44000 pixels,
and will upsample the image to this size.  In general, this is not what you
want.

You should _first_ uncheck the Resample box in the dialog, then enter the
new ppi you want for your print size, then recheck the box and enter the
print size you want.  For example, you could first change the ppi to 300 (if
that's what you want on the final print), then enter the desired print size.
With a ppi of 300 and a print size of 11 inches, PS will _downsample_ from
the size of the 4000 ppi scan (because fewer pixels are required).

> If the answer is yes then how do I downsample
> in powers of 2?

Change "pixels" to "percent" in the upper portion of the Image Size dialog
box and enter 50 (percent).

> ... do I go 4000 to 2000 to 1000 to 500 to 360,
> sharpening at each step as you suggest?

That's what I do (except I'd skip it on the last downsample, because the
step from 500 to 360 is too small and sharpening at that point might look
too messy--sometimes I try it both ways on the last step and pick what looks
best).

In theory you can also downsample in one step and unsharp mask once, but
then you must calculate the proper radius based on the number of pixels lost
and unsharp mask up front.  For example, if you downsample in one step of
500%, you'd use a radius of 4.9 pixels or so.  I don't do it this way so I'm
not sure how it turns out (it's easier to unsharp mask in steps afterwards,
and look at the partial results after each step), but you can always try it.



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