----- 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
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.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
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