[pure-silver] Re: Pure Black and Golden.

Yes, I meant the toe, sorry.

However, extra exposure does increase the overall negative contrast at
first, assuming a fairly straight line center section. With the extra
exposure, the highlight density increase is higher than the shadow density
increase, because the local gradient is lower for any point on the toe.
Hence, negative contrast goes up at first. BTW, this contrast increase is
not huge, but it can be measured and explained.





Regards



Ralph W. Lambrecht







On 2/3/05 9:21 PM, "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> wrote:

> 
> ----- 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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