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Re: [OM] Scanner DOF

Subject: Re: [OM] Scanner DOF
From: "Wayne Harridge" <wayneharridge@xxxxxxxxxxxxxxx>
Date: Sat, 18 Jul 2009 22:49:00 +1000
Thanks, that's pretty clear and also agrees with AG's .2mm for 10x8.

...Wayne

Wayne Harridge
http://lrh.structuregraphs.com/


> -----Original Message-----
> From: Chuck Norcutt [mailto:chucknorcutt@xxxxxxxxxxxxxxxx]
> Sent: Saturday, 18 July 2009 10:07 PM
> To: Olympus Camera Discussion
> Subject: Re: [OM] Scanner DOF
> 
> I think the answer I gave you is the correct one but if you want to
> understand it in terms of human vision (which is the basis for CoC)
> then
> I refer you to this wiki link which discusses that.
> <http://en.wikipedia.org/wiki/Circle_of_confusion#Circle_of_confusion_d
> iameter_limit_in_photography>
> 
> Depending on what assumptions you make you'll end up with slightly
> different answers but the article mentions 5 line pairs per millimeter
> at a 25cm viewing distance as a commonly accepted value.  That's 10
> pixels per millimeter or 254 pixels per inch.  That's also why I
> recommended 300 pixels per inch for the scan.  That's the closest,
> commonly available scanner resolution that equals or exceeds that.  But
> now that I know you have a V700 (which I also have) I know that you
> could also scan at 240 or 266 or 350 or more if you desire.  300 is
> generally considered very good for prints up to 8x10.  240 is good for
> larger prints which will be viewed from farther away.
> 
> Chuck Norcutt
> 
> 
> Wayne Harridge wrote:
> > Let me try to clarify what I am asking.
> >
> > I have 2 scanners, a cheap multifunction unit (scanner, printer,
> > copier,...), I know it has very little DOF.  I also have an Epson
> V700 which
> > has much greater DOF.  For something to be perceived as "sharp" by
> the human
> > eye, what is the size of the CoC required on a print ?  I'm sure it
> was
> > mentioned on this list at some stage but can't remember the actual
> value.
> > The answer will determine how "deep" the object on the flatbed can be
> and
> > still be perceived as sharp by the viewer of a print.
> >
> > ...Wayne
> >
> > Wayne Harridge
> > http://lrh.structuregraphs.com/
> >
> >
> >> -----Original Message-----
> >> From: Chuck Norcutt [mailto:chucknorcutt@xxxxxxxxxxxxxxxx]
> >> Sent: Saturday, 18 July 2009 12:58 PM
> >> To: Olympus Camera Discussion
> >> Subject: Re: [OM] Scanner DOF
> >>
> >> I'm really not sure I understand the question since I don't
> understand
> >> it in terms of DOF.  Some flat bed scanners have a very great DOF of
> >> maybe 6mm or more from the surface of the glass which is great for
> >> scanning 3D objects.  Others, not so much.  So, the DOF is what it
> is
> >> and you won't be able to change it.  It's not as though you can
> alter
> >> it
> >> by changing aperture or anything else except that some scanners can
> >> alter focus distance.  So with the right scanner it might be
> possible
> >> to
> >> do multiple scans and blend the layers.  But I don't know if
> scanners
> >> can really change focus distance enough to make that practical.
> >>
> >> But what I read into your question is something different.  It
> sounds
> >> more like you're asking what scanning resolution you should use to
> >> maintain a good life-size replica of whatever you're scanning.  For
> >> example, you want to scan a pencil which is 15cm long and print it
> on
> >> paper at 15cm while having it look nearly as sharp as the real thing
> >> when viewed from a distance of about 25cm.  A resolution of 300 ppi
> >> should be adequate for all but Moose.  If you want to please even
> Moose
> >> go to 600.  Even he can't see better than that.  :-)  But remember
> to
> >> sharpen the final image at print size before printing.
> >>
> >> Dr. Focus
> >>
> >>
> >>
> >> Wayne Harridge wrote:
> >>> Probably a question for Dr Focus.
> >>>
> >>> I'm wanting to scan some 3D objects on a flatbed and wondering how
> >> much
> >>> depth of field I need.  Assuming the resultant print is "life size"
> >> (i.e.
> >>> ~A4) what vale of circle-of-confusion should I be using.  This
> should
> >> be
> >>> related to the resolution of the human eye at a normal viewing
> >> distance I
> >>> think - so what is that value ?
> >>>
> >>> ...Wayne
> >>>
> >>> Wayne Harridge
> >>> http://lrh.structuregraphs.com/
> >>>
> >> --
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