[pure-silver] Re: Acros & HC-110
- From: "Ralph W. Lambrecht" <info@xxxxxxxxxxxxxxxx>
- To: PureSilverNew <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 18 Feb 2006 14:41:42 +0100
Sorry, I admit to not fully understanding Richard's notes, but agree with
the first two paragraphs of the long note, with the exception of the typo
that the speed point is at 0.01 above base & fog. The ISO/ASA standard of
determining film speed is pretty clear on a log density of 0.1 above base &
fog to be the speed point.
The trick is that it has to be done on a film, which has been developed so
that an exposure of log 1.3 above the speed point has a density of log 0.8
above the speed point density.
In other words, the film contrast must be 0.8/1.3 or 0.62!
Once that is achieved, the exposure that produced the speed point density of
0.1 is used to calculate the film speed. Richard gave the equation in his
long note (S = 0.8/Hm), but he has another typo there, because he wrote
0.08, correcting it later in the text. The factor 0.8 is a safety factor,
moving the 'true' speed point to a 'new' minimum density, slightly above
0.1, but the actual density depends on the actual shape of the
characteristics curve for the film/developer combination in question.
Nevertheless, for all practical purposes and back to the original question,
the standard speed point has a log density of 0.1, which leads to minimum
exposure for shadows. Consequently, I have raised my speed point to 0.17,
which leads to far better shadow detail at the cost of film speed. I rate my
film around 2/3 stop below advertised speeds. The results are worth it.
Regards
Ralph W. Lambrecht
http://www.darkroomagic.com
On 2006-02-18 00:19, "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> wrote:
>
> ----- Original Message -----
> From: "Ralph W. Lambrecht" <info@xxxxxxxxxxxxxxxx>
> To: "PureSilverNew" <pure-silver@xxxxxxxxxxxxx>
> Sent: Friday, February 17, 2006 3:01 PM
> Subject: [pure-silver] Re: Acros & HC-110
>
>
>> Mike
>>
>> Custom speed-point setting is a favorite subject of mine.
>> The standard ISO
>> speed point for film is at 0.1, which I find to be far too
>> low for pictorial
>> photography. I have settled for 0.17, which is about Zone
>> I.5 for me.
>>
>> The point is, you can pick any point you want, within
>> reason, but your
>> setting of 0.15 is pretty good and will give well defined
>> shadows.
>>
>> Regards
>>
>> Ralph W. Lambrecht
>>
>> http://www.darkroomagic.com
>>
> I just posted a long piece on this. The ISO speed point
> is not really log 0.1, there is a factor of 0.8 used in the
> calculation which raises the density point for minimum
> exposure to log 1.2. However, even greater exposure may
> result in better shadow detail for some films. The ISO
> speeds for B&W still film are intended to produce the very
> minimum exposure that will result in good shadow detail.
> But, because the minimum toe gradient is not measured
> directly, the exposures given may be on the slim side. Also
> note that the speeds given by the ISO standard are valid
> only for the contrast index produced by the standard and for
> the developer used in the measurement. There is no standard
> developer and processing for a contrast index different from
> that specified (indirectly) by the standard will result in
> different minimum exposures.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>
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