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From: kanner@tymix.UUCP (Herb Kanner)
Newsgroups: net.rec.photo
Subject: Re: Is OM-4 Junk?
Message-ID: <475@tymix.UUCP>
Date: Mon, 5-Aug-85 02:13:53 EDT
Article-I.D.: tymix.475
Posted: Mon Aug  5 02:13:53 1985
Date-Received: Tue, 6-Aug-85 08:40:06 EDT
References: <1520@trwrba.UUCP> <9414@ucbvax.ARPA> <471@tymix.UUCP> <1415@peora.UUCP>
Reply-To: kanner@tymix.UUCP (Herb Kanner)
Organization: Tymnet Inc., Cupertino CA
Lines: 40
Summary: 

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.

-- 
Herb Kanner
Tymnet, Inc.
...!hplabs!oliveb!tymix!kanner