[pure-silver] Re: Simple Step Wedge Testing
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 14 Mar 2005 11:16:31 -0800
----- Original Message -----
From: "Ryuji Suzuki" <rs@xxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, March 14, 2005 5:38 AM
Subject: [pure-silver] Re: Simple Step Wedge Testing
> From: "Breukel, C. (HKG)" <C.Breukel@xxxxxxx>
> Subject: [pure-silver] Simple Step Wedge Testing
> Date: Mon, 14 Mar 2005 13:31:31 +0100
>
>> The only point is I do not exactly = know waht to plot on
>> the X
>> axis: just the Stouffer step numbers, or 3.0 = (Zone X)
>> minus the
>> exact densities of the Stouffer steps, or..?..
>
> I didn't exactly follow the procedure but the abscissa is
> usually log
> exposure in lux-seconds, and the ordinate is optical
> density (or log
> transmission-to-incident ratio). For a 100 speed film, the
> line should
> break and make a toe at exposure of about -2. But the
> amount of light
> incident to the film plane in camera is probably difficult
> to measure.
>
> Quite frankly, I don't understand why people are so
> energetic about
> determining film speed, etc. If I were shooting
> commercially available
> films and papers, a couple of "clip tests" and eyeballing
> would get me
> the normal contrast which prints perfectly with grade 2.5.
> Then I give
> rather generous exposure to secure shadow unless flare is
> a concern.
> There's not a lot of meaning in standardizing the film
> density at
> certain point unless you are printing many negtives shot
> in the same
> way, and printing with the same exposure. But this is just
> my opinion.
>
> --
> Ryuji Suzuki
I agree with this and point out that ISO speeds are
intended to give the _minimum_ exposure consistent with good
shadow detail. The idea is that grain and image spread from
irradiation increase with density. So, for minimum grain and
sharpest images its wise to keep negatives as thin as
possible consistent with good tonal rendition. However, this
is not so critical an issue as it was in the past when
emulsions were grainier and thicker than they are now, and
is also not much of an issue for formats larger than 35mm.
Tone rendition is not much affected over a tremendous range
of exposure greater than that given by the ISO speed. In
some cases its improved.
The current ISO testing method was derived from the DIN
method, which establishes a minimum density above fog and
base density as the starting point. The standard adopted the
old ASA method developed by L.A. Jones of Kodak insofar as
setting a standard range of densities from a standard range
of exposure, essentially a standard contrast index (not
gamma) for the test. Jones determined the speed point from
the place of the toe of the characteristic where the
gradient was one third of the average gradient (bar G). This
proved very hard to measure in practice. The ASA determined
through testing of a great many films that there was a
pretty constant relation between the Jones minimum gradient
point and the point at which density was 0.1 above fog and
support density. This ratio was used in the new standard to
calculate the speed.
I often see the term "true speed" used in Usenet and
mailing list posts. It has no meaning other than the poster
has probably discovered that increasing exposure improves
his/her results.
The original ASA standard and earlier versions of the ISO
standard specified a developer, and later two developers,
for testing. The current standard allows the use of any
developer but requires it to be specified along with the
results. In both cases, the user may find that the speed
given by the standard does not yield the best negatives.
Also, the ISO standard is such that the contrast index is
fairly high, probably suitable for diffusion printing. When
the contrast of the negative is adjusted by changing
development time the _effective_ speed also changes. Since
there is very little underexpsure latitude with ISO speeds,
and since the effective speed is reduced when development is
reduced to reduce contrast index, the exposure must be
increased, otherwise there may be a loss of shadow detail.
Testing can be interesting but a rule of thumb will be close
enough, for a one paper grand reduction in negative contrast
increase exposure by about a stop. Development time change
varies with the film but is usualy indicated in its
literature.
I have been doing some informal testing lately of
exposure vs: tonal rendition, mostly using Plus-X roll film.
Not enough data yet to say much. This will be slow because I
am limited in the time I have for this project.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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