"Peter Irwin" <pirwin@[EMAIL PROTECTED]
> wrote in message
news:fg8cpp$qeq$1@[EMAIL PROTECTED]
> Richard Knoppow <dickburk@[EMAIL PROTECTED]
> wrote:
>>
>>
>> I think you are refering to log exposure. I am going
>> to
>> have to research this because I don't remember if the
>> ASA/Jones method used the same units (lux seconds) as are
>> currently used.
>
> Thanks. I think a metre-candle-second is the same as
> a lux second.
>
> I've been trying to get a handle on what the X axis
> on the H&D curves means in practical terms.
>
> Here is what I've come up with:
>
> For ISO 100, units are log lux seconds:
>
> -2.3 - Jones point (Old ASA 50)
> -2.1 - ISO B&W speed point (0.1 above base + fog)
> -1.05 - where usual light meter tries to put the average
> -0.3 - where 100% reflectance goes if meter aimed at grey
> card
> 0 - enough to fully blow highlights on typical slide
> film
>
> A difference of 0.1 is 1/3 stop.
> A difference of 0.3 is one stop.
> Thus for 400 speed film subtract 0.6 from these figures.
>
> I think this (if accurate) is a fairly useful list to
> keep in mind when looking at H&D curves.
>
> I know there are theoretical problems with trying to
> co-relate camera exposure with the figures on
> the H&D curves, but it seems to me that we are essentially
> doing just that whenever we set our light meters
> based on the ISO value. I don't think I've seen
> such a list anywhere so I've collected one together
> myself.
>
> It would be great if someone would point out errors,
> or add anything which seems useful.
>
> Peter.
> --
> pirwin@[EMAIL PROTECTED]
The ISO speed point is not actually log 0.1 density
above fog plus base density but rather the value gotten by
multiplying this by the factor 1.4. This is supposed to put
the point at about the same place as the Jones point.
Measured by the Jones method there really is not a fixed
density point since the speed is based on the relative
gradient of the toe vs: straight line of the film curve.
However, the research done by the ASA when they changed over
to the DIN method (fixed density point) showed that for the
great majority of films the two bore a fixed relation****p.
The current ISO standard is uses the same method as the
1958 ASA standard with some ammendments in detail. For
instance, the old standard specified two developers, a
general purpose one and a fine grain one, neither of which
was much like any standard commercial developer. The current
standard does not specify a developer but requires that the
published speed identify the developer used for the test.
Thus a film can have more than one speed if the manufacturer
tested it with more than one developer.
The current standard specifies that development be
carried out so that a log exposure interval of log 1.30
results in a density interval of log 0.8 measured from the
point were the density is log 0.1 above fog plus base
density. This is a gamma of about 0.62, not too far from
typical practice.
The point at log 0.1 density is called log10 Hm and
arithmetric speed is S=0.8/Hm (note that the arithmetric
value rather than the log value is used). For log speeds the
formula is S (in degrees)=1+10log10 0.80/Hm
I was in error when a stated that the correction factor is
1.5, its 1.25, the reciprocal of 0.8. Thus the ISO speed is
about 80% of the value that would be calculated directly
from the 0.1 density point. If one applies the correction
factor to the density it would be 0.125
The standard has a chart of ranges of log10 Hm to speed
where several samples are measured. I think this is what you
want. Some rounded off values are:
ISO speed Log exposure at 0.1 density point
25 -1.5
50 -1.8
100 -2.1
200 -2.4
400 -2.7
800 -3.0
Note that the exposure equivalent to the Jones point would
be about 1.25 times these values, i.e., about -2.63 for ISO
100 film.
Thanks for asking about this, its been educational to
figure out the answer.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
|
