DX-code exposure latitude code clarification requested.
Alan Browne
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.
--
e-meil: there's no such thing as a FreeLunch.
Bill Tuthill
>
> Now the latitude of Velvia is at least two stops either way, possibly
> +/-2.5 ...
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.
> 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?
Yes, but I don't know of any cameras that actually make use of it.
Alan Browne
> Alan Browne <alan....@freelunchvideotron.ca> wrote:
>
>>Now the latitude of Velvia is at least two stops either way, possibly
>>+/-2.5 ...
>
>
> 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.
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.
>>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?
>
>
> Yes, but I don't know of any cameras that actually make use of it.
I now think that is its only purpose, eg: useless to anyone but P mode
users.