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filmscanners: RE: filmscanners: RE: filmscanners: Pixels per inch vs DPI

Austin wrote:
>Why would you want to output at a fixed 300 PPI?

Because that's the requirement of the offset printer which many of my recent
photos are going to.  Aside from that, 300 dpi is as a general rule of thumb
the "best" resolution *most* printers (pc and otherwise) work with.  Some
are more, some are less, and the manufacturers muddy the water by talking
about the size of individual ink dots not the size at which a pixel is 
as a "dot" on the print.

> If you are outputting to an inkjet printer, you are best to
> just choose your image output size and let the PPI output
> to the printer fall where it may.

So what do you set the dpi to in the file?  If you create a TIFF file, there
will be a figure for the dpi embedded in it.  I use 300dpi.  When I actually
print from PSP the "real" dpi is hardly ever precisely 300dpi - it depends
on the page layout and how the picture is cropped.  So yes, I'm effectively
doing what you suggest above when prniting on my own printer.  But I have
to set the file's dpi to something, and it makes more sense to set it to
300dpi than 2700dpi or 100dpi for a full frame 2700spi ( :) scan.

> If you do any resizing of the PPI to make some fixed number,
> then you are resampling, which degrades the image.

I was talking about the setting in the file.  You can set the output dpi
of Vuescan (or Nikonscan I think) to anything you want.  It makes no difference
to the number of pixels.  Setting it to 300 dpi means that you'll get a
meaningfully sized print (roughly a page) out of a 2700 spi scan on most
printers.  Leave it at 2700dpi and you'll have a print the size of the neg
frame.  Set it to 100dpi and the size will be silly for printing.

>> Epsons seem to work quite well at 240dpi because of the
>> integer relationship with the 1440dpi native dot size.
> That?s pretty much been proven to by a myth.  It is true,
> to some degree, for lineart, but not at all for halftoned images.

In my own personal experience it's true that prints having an integer 
between the output dpi and 1440 on my Epson 1160 will be sharper and have
less visible dithering than at other scales.  It was also true on my previous
Photo 700.  I expect that newer printers, particularly those with more than
4 colours, will give better results.

I don't know for sure about other printers - for instance the 12x0 series
probably have fine enough patterns from 6 colours at 1440 or 2880 dpi that
variations in the source dpi make much less difference. I don't know because
I don't have one.

What I *do* know is that Epson had on their own web site an equation for
calculating the ideal source resolution which was based on an integer 
with the printer's native resolution.  Epson themselves said it was the
best thing to do.  The story may have changed since they wrote that FAQ.


Rob Geraghty harper@wordweb.com


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