[pure-silver] Re: Pure Black and Golden.
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 3 Feb 2005 12:21:18 -0800
----- Original Message -----
From: "DarkroomMagic" <info@xxxxxxxxxxxxxxxx>
To: "PureSilverNew" <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, February 03, 2005 11:50 AM
Subject: [pure-silver] Re: Pure Black and Golden.
> This was discussed on another example a few weeks ago,
> where I was reminded
> that additional exposure initially increases negative
> contrast, since the
> shadow densities come off the shoulder and then decreases
> again as the
> highlight densities get onto the shoulder. So, negative
> contrast starts out
> low, increases a bit with exposure and then decreases
> again.
>
>
>
>
> Regards
>
>
>
> Ralph W. Lambrecht
>
I think you mean the toe. Increased exposure does not
increase the average contrast of the negative but does
increase the contrast of the shadows by pushing them up the
curve. This assumes a film with a reasonably straight main
section. The effect is to increase the contrast in teh
shadows while the mid tones stay about the same. Films like
Tri-X sheet film, which has a constantly rising
characteristic, will have overall contrast increased but not
the ratio of shadow to highlight contrast.
For a film with a "normal" medium length toe, the toe area
is perhaps one and a half to two stops, over wich the
contrast is gradually increasing the the value of the
straight line portion. Underexposing will result in shadow
contrast that is too low to have any detail.
Loyd Jones, et.al. in his research at Kodak Labs found
that the minimum gradient on the toe is about 0.3 of the
average gradient of the curve over a normal exposure range
(LogE about 1.8). This what he based the Kodak Speed system
on. Later research showed that for most emulsions setting
the speed point at LogD 0.1 above base and fog would result
in effectively the same speed and was much easier to
measure. Remember that these are minimum esposure values
intended to keep the negatives thin for best grain and
sharpness. If tonal rendition is the primary concern
probably greater exposure is desirable.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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