resolution

[Robert Feinman]

I've been scanning 6x7 color negatives on an Epson 4870 using
Vuescan. Using half resolution of 2400 gives a nominal print
size of 17x22 which is already bigger than my final print.

The other day I scanned an image at the full resolution of 4800
dpi since I was planning to crop it. The image appeared
"sharper" than the one at 2400 even when both were scaled to
final print size. So, my question:
Does the scan software use just every other sensor to get to
half resolution or does it use all of them and then interpolate
to downsize in software? If the latter, does it use the same
algorithms as in Photoshop or something else?

What I'm aiming at is it better to scan at full resolution and
then resample in the editor software or not? Scanning a large
negative like this at 16 bit depth makes for a really large
file and slow editing, so reducing the size in the scanning step
has some advantages from a workflow point of view.

 

[Scott W]

I've been scanning 6x7 color negatives on an Epson 4870 using
Vuescan. Using half resolution of 2400 gives a nominal print
size of 17x22 which is already bigger than my final print.

The other day I scanned an image at the full resolution of 4800
dpi since I was planning to crop it. The image appeared
"sharper" than the one at 2400 even when both were scaled to
final print size. So, my question:
Does the scan software use just every other sensor to get to
half resolution or does it use all of them and then interpolate
to downsize in software? If the latter, does it use the same
algorithms as in Photoshop or something else?

I did a quick test with my Epson 1650, scanning at 50ppi and 300ppi, it
is clear they
are scanning at 50ppi and not a higher ppi and down sampling as the
50ppi scan has a lot of aliasing that the 300ppi scan down sample to 50
does not have.

My guess is that they do the same for 2400 vs. 4800, just picking every
other pixel and scanning only half as many lines, but this is just a
guess. The worry then at scanning at 2400 ppi would be grain aliasing
might make the grain seem worse then it really is, which would among
other things limit how much USM you could use. Of course the optics on
the 4870 are likely to be poor enough that grain aliasing is not an
issue when scanning at 2400.

There you go, a whole lot of speculation with no clear answers.

Scott

 

[David J. Littleboy]

If you are printing to 13x19, though, it's not all that much larger. I
usually downsample to 2200 ppi (from 4000 ppi) for 6x7. With the Nikon 8000,
that results in a very sharp image.

I did a quick test with my Epson 1650, scanning at 50ppi and 300ppi, it
is clear they
are scanning at 50ppi and not a higher ppi and down sampling as the
50ppi scan has a lot of aliasing that the 300ppi scan down sample to 50
does not have.

My guess is that they do the same for 2400 vs. 4800, just picking every
other pixel and scanning only half as many lines, but this is just a
guess. The worry then at scanning at 2400 ppi would be grain aliasing
might make the grain seem worse then it really is, which would among
other things limit how much USM you could use. Of course the optics on
the 4870 are likely to be poor enough that grain aliasing is not an
issue when scanning at 2400.

There you go, a whole lot of speculation with no clear answers.

My guess would be that scanning at 4800 ppi and hitting the scan with Neat
Image or Noise Ninja at 4800 ppi would be the way to get the cleanest and
sharpest downsampled images.

There might be a detail advantage to sharpening at 4800 ppi after noise
reduction and prior to downsampling, but you'd have to be careful to make
sure that you didn't introduce or aggravate any aliasing that occurs during
the downsampling. This would require some testing.

David J. Littleboy
Tokyo, Japan