Help me understand (dpi, scaling, resolution)

[barnes_jude]

I am scanning 35MM slides.

I have read up on the internet on this a bit and mostly see
recommendations to scan at highest dpi the scanner will allow when
scanning slides. However I have a colleague in the graphics dept who
says better to scan at 300dpi and "scale" it appropriately.

Just a quick example: What do I gain by scanning at 600 dpi scaled to
8 x 10, as opposed to 300dpi scaled to 8x10, when I look at the image,
I do not see much difference in detail, even when I re-sample at both
settings into a zoomed area on the slide which possess detail (such as
spokes on a bike wheel).

Or am i completely off base here and should be scanning at the max
dpi? I noticed when I do this, the scanner software does not allow
scaling. I also notice that I achieve simlar file sizes when I scale
up a lower dpi scan to a larger target size, when compared to a larger
dpi set to a smaller target size. Are they apples to apples or are
the images actually significantly different and I just can't see it on
my monitor and 8x10 test printouts I've been making?

Thanks so much in advance, I really want to get this archiving
underway for the family!

 

[Charlie Hoffpauir]

I am scanning 35MM slides.

I have read up on the internet on this a bit and mostly see
recommendations to scan at highest dpi the scanner will allow when
scanning slides. However I have a colleague in the graphics dept who
says better to scan at 300dpi and "scale" it appropriately.

Just a quick example: What do I gain by scanning at 600 dpi scaled to
8 x 10, as opposed to 300dpi scaled to 8x10, when I look at the image,
I do not see much difference in detail, even when I re-sample at both
settings into a zoomed area on the slide which possess detail (such as
spokes on a bike wheel).

Or am i completely off base here and should be scanning at the max
dpi? I noticed when I do this, the scanner software does not allow
scaling. I also notice that I achieve simlar file sizes when I scale
up a lower dpi scan to a larger target size, when compared to a larger
dpi set to a smaller target size. Are they apples to apples or are
the images actually significantly different and I just can't see it on
my monitor and 8x10 test printouts I've been making?

Thanks so much in advance, I really want to get this archiving
underway for the family!

the "best" information on the net about scanning is at
http://www.scantips.com/

Basically, if you have a scanner capable of scanning slides, then to
get the most information, you scan at the optical resolution of the
scanner. This is not necessarily the maximum scanning resolution that
the scanner will do, because some scanners increase the resolution by
stepping the scanner head, but this adds practically nothing to the
actual resolution.

The differences you are seeing in "scaling" vs scanning at a higher
resolution may be due to the scanner software you're using.... ie, by
"upscaling" the software may actually be scanning at a higher
resolution. You need to state what software and what options you're
using, then someone familiar with that software can give you better
advice.... or just read what Wayne Fulton has to say on the link given
above.

 

[tmonego]

I am scanning 35MM slides.

I have read up on the internet on this a bit and mostly see
recommendations to scan at highest dpi the scanner will allow when
scanning slides.  However I have a colleague in the graphics dept who
says better to scan at 300dpi and "scale" it appropriately.

Just a quick example: What do I gain by scanning at 600 dpi scaled to
8 x 10, as opposed to 300dpi scaled to 8x10, when I look at the image,
I do not see much difference in detail, even when I re-sample at both
settings into a zoomed area on the slide which possess detail (such as
spokes on a bike wheel).

Or am i completely off base here and should be scanning at the max
dpi? I noticed when I do this, the scanner software does not allow
scaling.  I also notice that I achieve simlar file sizes when I scale
up a lower dpi scan to a larger target size, when compared to a larger
dpi set to a smaller target size.  Are they apples to apples or are
the images actually significantly different and I just can't see it on
my monitor and 8x10 test printouts I've been making?

Thanks so much in advance, I really want to get this archiving
underway for the family!

I prefer to scan at a high res then you can scale in the scanner
software or resize in your image editor. If I am printing the slides
I will scan at 3200/4000ppi then size to the print size I want. If I
am scanning for PowerPoint, which I do a lot dropping to 8x10 x100ppi
from a 4000ppi image is asking for problems so I scan at 1200ppi.
Where your friends advice makes sense is with flatbed scanning of
prints or art work.

Tom

 

[CSM1]

The best place to learn about scanning is do a lot of reading on:
http://scantips.com/

Start reading at "START".

--
CSM1
http://www.carlmcmillan.com
--

Much of this is confusing terminology and also depends on how your
software works.

On SOME software (example: Nikon Scan on a Nikon film scanner) you just
set a dpi, and it's based on the "real" dimensions of the image. For
35mm, 2,700 dpi gives about a 10 megapixel image.

But on some other software (example: an HP flat bed document scanner with
a transparency adapter for slides), you have to establish "output
dimensions" (e.g., 4"x6" or 8"x10") and you set the dpi relative to the
OUTPUT image size, so if you set 300 dpi on an 8x10 output size (and, in
this case, that is how you set it), you will scan 2400x3000 pixels (over
the same 35mm slide).

A couple of thoughts:

Somewhere around 6 to 10 megapixels spread over the 35mm film (slide or
negative) image, you really are scanning all the detail that the TYPICAL
35mm film image has.

Another consideration, however, is "real" vs. "interpolated" pixels. If
at all possible, you want to use a resolution that will give an image
consisting entirely of "real" pixels (interpolated pixels are pixels that
exist where there is no actual image sensor element. They are
"calculated" by looking at the nearest real pixels on either side of the
interpolated pixel position and assuming a linear transition between those
points. Fine, but the actual image never bothered to study such
mathematics and that may not represent what was really in the image).
Again, you have to know your hardware and your software.

In truth, I like the Nikon approach much better ... set the actual DPI to
be used by the actual scan based only on the actual size of the 35mm image
[the image sensors in various Nikon scanners are 2,700, 2,900 and 4,000
dpi .... anything else is interpolated]. The output size is artificial
(and Nikon just uses the actual output size ... about 1" x 1.4"), and if
you want to rescale the image later, you can do that in any image editing
software (I am talking here about a rescaling in which the dpi and
dimensions are both changed such that the total number of pixels does not
change (at all) and every pixel remains an actual "real" pixel).

Re: "Or am i completely off base here and should be scanning at the max
dpi?" .... well, in most cases, the "maximum dpi" is based on interpolated
(sometimes HIGHLY (e.g. ridiculously) interpolated) pixels. Never scan at
a resolution so high that you are at more than twice the "real" resolution
of the scanning hardware (and even that is arguable, many would say to
always and only scan at exactly the maximum hardware resolution, unless
scanning at exactly half of that will meet all of your needs).

Unrelated to your question, in my mind the only acceptable hardware for
archiving 35mm images to digital are the Nikon film scanners and some
models of Epson scanners. I would not use anything else.


There is no substitute for understanding your hardware and software

I am scanning 35MM slides.

I have read up on the internet on this a bit and mostly see
recommendations to scan at highest dpi the scanner will allow when
scanning slides. However I have a colleague in the graphics dept who
says better to scan at 300dpi and "scale" it appropriately.

Just a quick example: What do I gain by scanning at 600 dpi scaled to
8 x 10, as opposed to 300dpi scaled to 8x10, when I look at the image,
I do not see much difference in detail, even when I re-sample at both
settings into a zoomed area on the slide which possess detail (such as
spokes on a bike wheel).

Or am i completely off base here and should be scanning at the max
dpi? I noticed when I do this, the scanner software does not allow
scaling. I also notice that I achieve simlar file sizes when I scale
up a lower dpi scan to a larger target size, when compared to a larger
dpi set to a smaller target size. Are they apples to apples or are
the images actually significantly different and I just can't see it on
my monitor and 8x10 test printouts I've been making?

Thanks so much in advance, I really want to get this archiving
underway for the family!

 

[Guest]

I am scanning 35MM slides.

I have read up on the internet on this a bit and mostly see
recommendations to scan at highest dpi the scanner will allow when
scanning slides. However I have a colleague in the graphics dept who
says better to scan at 300dpi and "scale" it appropriately.

Just a quick example: What do I gain by scanning at 600 dpi scaled to
8 x 10, as opposed to 300dpi scaled to 8x10, when I look at the image,
I do not see much difference in detail, even when I re-sample at both
settings into a zoomed area on the slide which possess detail (such as
spokes on a bike wheel).

Or am i completely off base here and should be scanning at the max
dpi? I noticed when I do this, the scanner software does not allow
scaling. I also notice that I achieve simlar file sizes when I scale
up a lower dpi scan to a larger target size, when compared to a larger
dpi set to a smaller target size. Are they apples to apples or are
the images actually significantly different and I just can't see it on
my monitor and 8x10 test printouts I've been making?

Thanks so much in advance, I really want to get this archiving
underway for the family!

On a flatbed scanner Set the dpi to the optical resolution of the scanner.
That will be 300, 600, 1200, 2400 or 4800 depending on the scanner, you can
also set the resolution to a value that evenly divides the resolution. Set
the scan area in pixels. On a film scanner there are also optics to help get
maximum quality from your scans.