DX-code exposure latitude code clarification requested.

http://www.geocities.com/thombell/dx.html#latitude

Shows the 'code' for the exposure latitude portion of the 135 format DX
code.

On a roll of Velvia, the most pesimistic case is selected by the code,
eg, per the table in the link above +/- 1/2.

Now the latitude of Velvia is at least two stops either way, possibly
+/-2.5 ...

So does the DX code for exposure latitude have an offset base? (eg: 1.5
or 2 would seem to make sense).
eg does the +/- 1/2 really mean +2 +1/2 to -2 -1/2 (+2.5 to -2.5)?

Or is the code really meant to drive the program exposure mode s/w with
an idea of how loose or tight the s/w should interpret the scene?

One tidbit I found via Google suggests it merely helps the camera in
program mode to determine if it can keep the shutter fast enough to
avoid camera shake and still get a reasonable exposure. In this case,
only the under exp side would be needed, so seems like a partial
response at best.

All replies apreciated.

Cheers,
Alan.

 

No the dynamic range of Velvia is +2/-2 or +/-2.5 if you prefer,
but exposure latitude is not the same thing as dynamic range.
For most normal scenes with brightness ranges of 4 stops or more,
you could not expose Velvia 50 at EI 12 or EI 200 and still get
acceptable results.

That's what exposure latitude means: acceptable results at a
given number of stops over and under. Most print films can
easily handle +2/-2 and some more than that.

Yes, but I don't know of any cameras that actually make use of it.

 

No matter what tolerance "exposure latitude" they put on the DX-code,
the exposure range will not change as you change the center (18% grey)
point... , eg: if the exp. is set for 1/10 of a stop to overexpose, then
that much more detail will appear in the shaddows, and that much more
detail will be lost in the highlights. +/- 1/2 latitude means nothing.

Exp. latitude DOES mean something for negative film, as you're going to
make prints from it and you have a lot of latitude for correction. But
for reversal it seems like a meaningless term.

I now think that is its only purpose, eg: useless to anyone but P mode
users.