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[OM] Re: My personal Film vs. Digital tests - II

Subject: [OM] Re: My personal Film vs. Digital tests - II
From: "John A. Lind" <jalind@xxxxxxxxxxxxx>
Date: Wed, 11 May 2005 00:19:31 -0500
At 05:04 PM 5/10/2005, Moose wrote:
>More to come, I think, as I haven't answered all my own questions yet. I 
>just started looking at some Royal Gold 400 I shot on the same trip and 
>the grain in the scans is less than the Vista 200. This either means that 
>Vista is pretty grainy or that some aliasing thing is going on. The grain 
>in the RG 400 is about what I remember from scanning it in the
>past with a 2720 dpi scanner.
>
>Moose

Moose,
I don't know about the Vista 200 . . . however Kodak's premium Royal Gold 
consumer films were much finer grained than the Gold.  Some of them were 
pro films shipped green(er) and unrefrigerated (Royal Gold 25 and Ektar 25 
were the same film).

Unfortunately, Kodak uses a completely wacko "Print Grain Index" for its 
color negative films . . . pro and consumer . . . that cannot be compared 
to anything else.  As a comparator of Gold versus Royal Gold though (some 
indication of how Royal Gold stacked up against non-premium films):

Royal Gold 25:  < 25 (lowest value on Kodak's PGI scale = 25)

Gold 100:  45; Royal Gold 100:  28
Gold 200:  47; Royal Gold 200:  32 (now called High Definition 200)
Gold 400:  48; Royal Gold 400:  39 (now called High Definition 400)
Max Zoom 800:  48; Royal Gold 1000:  57

That was the really, way cool thing about Royal Gold . . . its ISO 400 beat 
out Gold 100 in resolving power.  Royal Gold 1000 . . . at 1/3rd stop 
faster than Max Zoom 800 . . . is the only one that was a disappointment in 
terms of resolution.  Kodak's Supra 100 and 400 were nearly the same as 
Royal Gold, and it's 800 was nearly the same as Max Zoom.

BTW, the diffuse RMS granularity between negative and transparency films 
cannot be directly compared.  An approximate heuristic for comparison is 
multiplying the color negative RMS numbers by 2.5 ("Photographic," 
1996).  RMS is a non-linear scale; a difference of "1" on the scale is 
twice the graininess (e.g. 10 is twice as grainy as 9).

I found the following *approximate* conversion for Kodak's wacko PGI to RMS 
granularity on Photo.net and don't know how accurate it is (or isn't) but 
it at least appears to be somewhere in the ballpark:
   RMS = (PGI / 0.5335) ^ (1 / 2.8669)
The troubling aspect of this equation is the non-linearity of the RMS scale.

The inverse of this to get an estimated PGI from RMS:
   PGI = 0.5335 * (RMS^2.8669) [Remember to approximate color neg RMS by 
dividing a chrome RMS by 2.5 first).
As a sanity check, Kodachrome 64 has a PGI of 28 using this equation.

Using this with Agfa Vista 200's RMS of 4.1 gives a PGI of 30 which is less 
than Royal Gold's 400's PGI of 39 and this doesn't pass the "sanity" test 
of Moose's observations . . . nor does it (IMHO) fail it either (possible 
ambiguities about reason; could be something else at play here).  Fuji 
Reala has a PGI of 28 (RMS = 4).

Source of the equations is Michael K. Davis and was found in some USENET 
postings, and on Photo.net in their forum.  He generated the equations from 
three known pairs of measured PGI and RMS data . . . which may not be 
enough sampling.

-- John Lind


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