Minimum d.p.i. for drum scanning

[Tony W.]

What's the minimum number of d.p.i. that I should get my three- by
nine-inch transparencies scanned at on a drum scanner for them to prints
sharply and with maximum detail on an Epson 7600 printer?

The prints will be professional quality art prints.

Each print will be about 7.5 by 23 inches in size.

 

[Mac McDougald]

What's the minimum number of d.p.i. that I should get my three- by
nine-inch transparencies scanned at on a drum scanner for them to prints
sharply and with maximum detail on an Epson 7600 printer?

The prints will be professional quality art prints.

Each print will be about 7.5 by 23 inches in size.

Unless this printer breaks the norm, 300ppi at actual size is the
ballpart ppi folks shoot at for printing.
Which would mean your 3x9" would need about 760ppi. Apprx 44MB files in 8
bit, double that for 16 bit.
But then, if you're gonna pay for drum scans, why not have them scanned
even higher, just in case a higher output ppi would add quality (you can
only ultimately know by experimentation with the output device).
The service bureau charge by scan rez differences ?

Mac

 

[Kennedy McEwen]

What's the minimum number of d.p.i. that I should get my three- by
nine-inch transparencies scanned at on a drum scanner for them to prints
sharply and with maximum detail on an Epson 7600 printer?

The prints will be professional quality art prints.

Each print will be about 7.5 by 23 inches in size.

The Epson 7600 printer has a native resolution of 360ppi,
and printing at anything different from that will bring one artefact or
another. So, for maximum detail, stick with the native output
resolution.

Assuming a 3" original scaled up to a 7.5" output means a scale of 2.5x
and thus a scan resolution of 900ppi is the norm. This is the minimum
that I would recommend for output on that printer.

However, you might well want to print larger than that in the future and
integer multiples of this baseline are what you are looking for - say
2700ppi (3x). There isn't much point in going beyond that, unless you
change printers, since the 7600 has a maximum feed width of 24".

Big files though - 900ppi is 131Mb and 2700ppi is 1.2Gb, assuming 16bpc
depth. There is no point in scanning at less than this depth because
you may need to do some manipulation of the final images and the more
headroom you have the better.

I would imagine that a 22.5" x 67.5" print from such a transparency
would look absolutely stunning!

 

[Tony W.]

Unless this printer breaks the norm, 300ppi at actual size is the
ballpart ppi folks shoot at for printing.
Which would mean your 3x9" would need about 760ppi. Apprx 44MB files in 8
bit, double that for 16 bit.
But then, if you're gonna pay for drum scans, why not have them scanned
even higher, just in case a higher output ppi would add quality (you can
only ultimately know by experimentation with the output device).
The service bureau charge by scan rez differences ?

Mac

Thanks for your detailed and careful answers, Mac and Kennedy.

Yes, I think the service bureau charges more for higher resolution scans.

Tony

 

[Tony W.]

Why did you and Mac write "ppi" instead of "dpi". The manual for the
Epson 7600 uses the term "dpi".

Also, why did you write that 360 ppi is the "native resolution"? The
manual says,

• Normal - 360 dpi
For faster printing and everyday needs

• Fine -720 dpi
For high quality photos and prints

• Superfine -1440 dpi
Better than photo lab quality and

• Superphoto - 2880 dpi
For the highest quality photos and prints; provides an astonishing level
of photographic quality.

Tony

 

[Kennedy McEwen]

Why did you and Mac write "ppi" instead of "dpi". The manual for the
Epson 7600 uses the term "dpi".

Because ppi (pixels per inch) is what I mean, not dpi (dots per inch) -
the two are NOT the same!

Also, why did you write that 360 ppi is the "native resolution"?

Because that is the native resolution of the printer driver - 360ppi.
Every image that you send to the printer is first resampled to this
resolution BEFORE it is rendered by the dot halftoning algorithm. Since
a dot of ink can either be present on the page or not, and there are
only a few colours of ink dots and 3 dot sizes, the number of tones that
can be reproduced by individual ink dots is very limited. So the
printer implements a halftoning process to dither the placement of the
dots. This means that the actual colour produced on the page requires
many ink dots to be averaged - which your eye is very good at doing
automatically if the dots are small enough. If you only have one dot of
ink per pixel then you need to average a lot of pixels to get the
precise colour. The more dots that can be placed in each pixel, the
better the actual colour of that pixel can be represented. As well as
all this, Epson implement a stochastic dither process which adaptively
adjusts the precision of colour in each pixel with the detail in the
image. However don't worry too much about that at the moment - the
point is that dots (as mentioned in the manual, hence dpi) are not the
same as pixels (as mentioned by Mac and I in our responses, hence ppi).

The
manual says,

• Normal - 360 dpi
For faster printing and everyday needs

ie. one ink dot per pixel - not very good tonal fidelity at pixel level,
requiring many dots and hence many pixels to be averaged by the viewer
to get the full tonal range of an image.

• Fine -720 dpi
For high quality photos and prints

ie. four ink dots per pixel - good tonal quality at pixel level, with
only a few pixels requiring to be averaged by eye to achieve the full
tonal range of an image.

• Superfine -1440 dpi
Better than photo lab quality and

ie. 8 ink dots per pixel (1440dpi in one axis only) - excellent tonal
quality, requiring very little averaging to achieve the tonal range of
the image.

• Superphoto - 2880 dpi
For the highest quality photos and prints; provides an astonishing level
of photographic quality.

ie. 16 ink dots per pixel - incredibly fine detail reproduced with
maximum tonal fidelity at the pixel level.

 

[Mac McDougald]

Why did you and Mac write "ppi" instead of "dpi". The manual for the
Epson 7600 uses the term "dpi".

See Kennedy's excellent explanation.

In short, ppi is measurement of an image's resolution at a given output
size, dpi is measurement of printer resolution.

The terms, unfortunately, are often used synonomously, and not just by
users. Most scanner software interfaces use "dpi", even though that's
wrong. Some image editors also use "dpi" technically incorrectly also.

Ed Hamrick always argues on the side of "dpi/ppi" being same thing, for
example.

Mac

 

[Tony W.]

Thanks, Mac and Kennedy.

Tony

See Kennedy's excellent explanation.

In short, ppi is measurement of an image's resolution at a given output
size, dpi is measurement of printer resolution.

The terms, unfortunately, are often used synonomously, and not just by
users. Most scanner software interfaces use "dpi", even though that's
wrong. Some image editors also use "dpi" technically incorrectly also.

Ed Hamrick always argues on the side of "dpi/ppi" being same thing, for
example.

Mac