Re: EI question

[George L Smyth <glsmyth@xxxxxxxxxxxxxxxxx>]

On Tue, 16 Nov 1999, Ben Kreunen wrote:

> Exposure Index = 3 more syllables than Film Speed, big words to impress 
> people, another term to confuse the hell out of amateurs.

Not really.  It's a means of relating to someone the speed at which they
expose an image.  This may or may not deviate from ISO, which is a
standard.

> As for HIE being unrated.... "We cannot give exact speed numbers for this 
> film because the ratio of infrared to visible energy varies" Which is 
> partly true and if they gave us an exact film speed then we'd all complain 
> when it wasn't right.. but essentially it's a big cop out, since 
> technically speaking a BW negative's film speed is measured from the shadow 
> detail and there is less variation between visible and infrared reflection 
> in lower light.  They also provide a table of suggested film speeds for 
> some of the main filters so they do 'rate' the film, they just say that's 
> their numbers are not legally binding  ;-)

Actually, you may notice that Kodak has moved away from the ISO standard
to which we are accustomed.  The ISO standards indicate the usage of a
specific developer (that nobody uses, it's just part of the old standard),
so it doesn't apply to "real" usage.  This is why we've always shot Tri-X
at half the rated speed.  The TMax series does not reference ISO.

Far being from a "big cop-out," Kodak states their information properly.
Note that you mention how "technically speaking" the speed is measured
from shadow detail.  Where is "shadow detail?"  To you it could mean
something entirely different than what that might mean to me (and as I
personally consider shadow detail to be less important than highlight
detail, we are looking at two very different things).  So your reference
has little to do with the technical issues.

Technically speaking, a film's speed is measured through the specific
density change in the negative over the base of the film.  HOwever, what
type of light should be used in exposing the film to measure the density
change?  What percentage of infrared radiation?  As soon as you answer
that question, you make inappropriate all other mixes.

As Kodak did not feel it appropriate to give guidance with their infrared
film, they did the proper thing, which was to give the development
information in terms of density.  No big deal, however.  Just do some film
testing, something too few people bother performing.

george

---
Handmade Photographic Images
http://www2.ari.net/glsmyth/

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