Archived posting to the Leica Users Group, 2000/07/19

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Subject: [Leica] lp/mm to dpi approximate conversion...
From: (Ross Bagley)
Date: Wed, 19 Jul 2000 10:51:18 -0500 (CDT)

On Wed, 19 Jul 2000 10:20:01, "Rob Studdert" <> wrote:
> On 19 Jul 2000, at 8:44, Peter Zak wrote:

>> When I examine the slides under a loupe or with a projector I see
>> much that the scanner is simply failing to pick up.  I notice this
>> most with Kodacrome 25 and Velvia - I've been advised by the local
>> Nikon service techos to turn the Nikon colour management off when
>> scanning Kodachrome.

> Don't expect any 2700 or 4000dpi scanner to come close to resolving
> all the detail contained Kodacrome 25 and Velvia, you need about
> 6800dpi to even get close.

A good approximation for conversion from film terms to dpi is 1lp/mm =
100dpi (24bit color or 16bit b&w), so to capture a "typical" film
image with 80lp/mm detail, you'd need about 8000dpi.  Of course, both
50mm 'cron and Velvia have much better resolution than this, so if you
wanted a full information scan, you'd probably need about 12000dpi.

The calculation goes like this:

1lp/mm * ~25mm/in * 2lines/lp * 2dots/line = 100dpi

The 2dots/line is a factor similar to the Nyquist ratio for digital
audio.  You need to sample at half the size of the smallest feature
you expect to capture.


- -- Ross Bagley & Associates
"We don't just write software, we help you write software better!"