[alt-photo] Re: New Platinum Prints

[etienne garbaux]

Loris wrote:

>Errr, 2000 ppi!? That's too much I think; 18 lines per mm (l/mm) is roughly
>equivalent to (only) 460 ppi. (Which is very very good in fact!)
>
>[examples snipped suggesting that 920 or 460 ppi should be 
>sufficient, depending on application]

The limits you mention are where the digineg artifacts should, in 
theory, be equal to the resolution-limiting effects of the process 
itself.  However, the eye can still resolve the artifacts at this 
level, just as the ear can hear "down into the noise," because a 
process's ability to resolve deteriorates gradually rather than 
falling off a cliff at the limit.  To make the artifacts truly 
invisible, they need to be lower than the process limit by a factor 
of 4-8 (this includes the 2-4 factor discussed below), implying a 
digineg resolution 4-8x greater (ppi) than the process limit.  If 
anything, 2000 ppi is probably conservative.  [Note for the geeky: 
yes, it is possible to produce a spatial filter with a sharp cutoff, 
but such filters look bad due to their own artifacts, so trying to 
hide quantization (pixellation) artifacts this way is not recommended 
-- baby and the bathwater.]

>OTOH, as I previously wrote in my reply to Christina, that much print
>resolution is pretty unnecessary, since even someone with perfect 20/20 eye
>sight / visual acuity won't be able to resolve something more than 13-14
>l/mm at 10" (minimum comfortable) viewing distance, w/o the aid of a loupe.
>Plus, practical viewing conditions are almost always farther away than 10",
>therefore something significantly lower than 13-14 l/mm still does the job.
>E.g. real life experience with inkjet prints / digital negatives; ~ 7-8 l/mm
>in my case. (I don't even mention lighting conditions BTW; I just assume
>perfect / ideal lighting instead - which is a whole other issue...)

For the same reason given above, the artifacts themselves don't need 
to be fully resolved by the eye to affect the viewed characteristics 
of the print -- "unable to resolve the artifacts by eye at a 
comfortable viewing distance" is not sufficient for a judgment of 
"invisible" ("can't tell the difference between an image with the 
artifacts and one without them"), so a factor of 2-4x below the level 
where artifacts are resolved fully by the eye is necessary to 
suppress them to the point where they don't detract from the image.

(In a previous professional life I designed digital audio and video 
systems, and I both did and read a lot of research into human 
perception of low-level aural and visual artifacts.  Anyone familiar 
with this literature will recognize that the discussion above is not 
rigorous, but it should provide a reasonable explanation at a basic level.)

Best regards,

etienne