[pure-silver] Re: Densitometer for print exposure calibration

Thanks Richard.  I saved your post to review in the future when I might
hope to have a better idea what it means.  What I really want to do is
whatever it was that Shannon did, so what I was asking for was advice on
acquiring equipment that could help me do that.  That's probably what I
should have said to start out with.  I'm no longer certain I understand
what what it was Shannon did though, so I'll shelve that idea while I
assess the matter further.



Richard Knoppow wrote:
>
> ----- Original Message ----- From: "vellum" <vellum@xxxxxxxxxxxx>
> To: <pure-silver@xxxxxxxxxxxxx>
> Sent: Saturday, November 10, 2007 6:02 AM
> Subject: [pure-silver] Densitometer for print exposure calibration
>
>
>> Shannon's interesting thread about multigrade paper has made me want to
>> test my enlarger and paper to better understand it and calibrate my
>> system.  It's something I've never done before.  I'm fuzzy on some of
>> the concepts and what they really mean, like Log Exposure Range, but
>> I'll figure it out as I go.
>>
>> I have a Jobo Colorstar 3000, but I don't have the B&W density probe for
>> it.  I just have the standard color probe.
>>
>> What densitometers might be the best candidates for B&W print exposure
>> densitometry if I were shopping used on ebay?  I'd rather buy a new one
>> but I suspect new ones may be costly.  Ease of use on the enlarging
>> easel and accuracy (or consistency) would be priority.  An exposure
>> timer function would be nice, but not essential.
>>
>> Ability to use it to measure density of negatives would be nice too.  I
>> develop using both MQ and pyro developers.  Having one densitometer that
>> "does it all" would be cool, but probably there may be none that do
>> everything well.  In that case, it is the printing densitometry that
>> matters most to me.
>>
>> Skip.
>>
>
>     I think the key here is understanding that there are two media to
> be measured: negative and print. A transmission densitometer will
> allow you to measure the densities, but more important, the contrast,
> of the negative (with some qualifications). The resulting range of
> densities on the paper will also depend on the characteristics of the
> enlarger. Nearly all transmission densitometers measure diffuse visual
> density. Some enlargers use semi-specular (condenser) light sources
> which produce different effective densities than a diffuse source.
> This is called Callier effect and varies with the film. It is nearly
> absent for color film and greatest for coarse grain silver film.
>    To measure the print one needs a reflection densitometer. A number
> of these have been made and are available. Both types of densitometers
> are made for either white light or standardized colors.
>    The effective density of a negative with a stain image such as
> produced by Pyro is dependent on the color sensitivity of the printing
> material. If its printed to a panchromatic material the effect is
> nearly absent. For blue sensitive material the effect is to increase
> the contrast but not the shape of the film curve. For variable
> contrast materials the stain image may look like a masked filter
> causing the contrast to vary with the negative density. Since the
> filtering effect is strongest in the dense regions the effect is
> greatest on the print highlights. Since VC paper prints lower contrst
> for blue exposure than for green the effect will be to lower highlight
> contrast in about the same way a shoulder in the film characteristic
> would do. The amount of this effect willd depend on the relative
> density of the stain versus the silver density, the spectral
> characteristics of the paper, and the light source.
>    Beacause the paper characteristics affect the resulting curve it is
> difficult to measure using a densitometer. You would have to have
> filters at the correct colors to match the two components of the
> printing paper.  I don't think the blue and green filters used for
> color work are at the right wavelength.
>    One could presumably print a step tablet onto the film, process it,
> and make prints from which a reflection densitometer could measure the
> resulting density curve. I am pretty sure I have seen published
> results of such tests but can't cite a source.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
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