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From: briand@tekig4.UUCP (Brian Diehm)
Newsgroups: net.rec.photo
Subject: Re: Is OM-4 Junk?
Message-ID: <212@tekig4.UUCP>
Date: Mon, 12-Aug-85 20:25:52 EDT
Article-I.D.: tekig4.212
Posted: Mon Aug 12 20:25:52 1985
Date-Received: Sun, 18-Aug-85 23:47:43 EDT
References: <1520@trwrba.UUCP> <9414@ucbvax.ARPA> <471@tymix.UUCP> <1415@peora.UUCP> <475@tymix.UUCP>
Organization: Tektronix, Beaverton OR
Lines: 37

> In article <1415@peora.UUCP> jer@peora.UUCP (J. Eric Roskos) writes:
> >> If you use Ilford XP1, you can use Zone System doctrine to a limited degree
> >> by changing the ASA rating for each shot.  Exposed at ASA 100, the neg will
> >> have much lower contrast than it will when exposed at ASA 400.
> >
> >Isn't this just an effect of nonlinearities in the "toe" region of the film's
> >characteristic curve?  I.e., if you underexpose the film and develop it
> >normally, most of the exposure will end up down in the region where the
> >slope of the curve is less; which is, of course, exactly the definition of
> >low contrast (please, no flames on the definition of "contrast index," I
> >know the CI includes the nonlinear regions.).
> >
> >A disadvantage of this method, though, would be exactly that the curve
> >IS nonlinear; so differences between two lower levels of luminance would
> >be less pronounced than between two higher levels.  This would tend to
> >produce a loss of detail in the shadows.
> 
> Nope, I was not talking about working at the toe (low exposure end of the
> curve) but about the other end ( would you call that the heel :-)?)  Rate
> XP1 at ASA 100 and you will get a very dense negative, but because of the
> film's incredible dynamic range, the highlights will not block up.  You
> will, however, be on a section of the curve that has a lower gamma than the 
> section you would encounter at ASA 400.  You will also get much finer
> grain.  However, because of the density, you might not like the long
> printing time. 
> 
> Dick Delagi, in his recent article on XP1 in Popular Photography, said that
> he originally decided to shoot the stuff at ASA 200 in order to get finer
> grain (this stuff gets grainy where the negative is thin!!!), but reverted
> to  ASA 400 because of the loss of contrast and excessive density from
> over-exposure.  This implies that if your intent is to reduce contrast, you
> can achieve it by over-exposing.  The reason you can do this with XP1 and
> not with conventional film is that the high end of the H & D curve flattens
> out very gradually, whereas the curve for conventional film comes to a
> rather abrupt halt.

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