From: Liam Lawless (liam.lawless@blueyonder.co.uk)
Date: 01/31/03-09:58:47 PM Z
>POP papers have an uncharacteristic curve "that results in
>greater shadow contrast and lower contrast in the highlights." What
>gives??
>
>Mike
As far as the shadow end is concerned, perhaps he means not that POP is
inherently contrasty in the shadows, but that if shadow detail resists
blocking up [because of self-masking?] then the shadows will display more
contrast than if they were blocked. If so, wouldn't the "uncharacteristic"
thing be a flatter than usual shoulder?
Liam
I agree with most of what Barnier says about the characteristics of
POP but that particular sentence is confusing and inadequately
describes the situation. Given the type of negative I described, i.e.
one where the density range of the negatives exceeds the exposure
scale of the paper, the shadows of any printing out process,
including salted paper and albumen, will lose separation and become
somewhat murky by the time adequate density is reached in the
highlights.
On the other hand, assuming a perfect negative in which the density
range of the negative exactly matches the exposure scale of the
paper, there should be good shadow contrast with POP processes. And
there will also be good shadow contrast in situations where the
density range of the negative is less than that of the exposure scale
of the process, though in this case the print will lose detail in the
shadows.
Sandy King
>
>
>
>>From: Sandy King <sanking@clemson.edu>
>>Reply-To: alt-photo-process-l@sask.usask.ca
>>To: alt-photo-process-l@sask.usask.ca
>>Subject: Re: self masking and POP
>>Date: Fri, 31 Jan 2003 11:42:40 -0500
>>
>>Chris wrote:
>>
>>>I've come across a couple books that conflict on self masking. One book
>>>says cyanotype is not self masking because the purple/blue/grey color
does
>>>not hold back actinic light and shadows block up, but that VDB, platinum,
>>>and palladium are all self masking (Van Keuren). Another book says
>>>kallitypes and VDB are not self masking but salt is, and hence shadows
don't
>>>get as deep (Crawford). Another says VDB is not self masking, too
(James).
>>>Who is correct? I think all people agree that platinum is self masking,
but
>>>how about:
>>>palladium
>>>kallitype
>>>brownprint
>>>van dyke
>>>cyanotype
>>>salt?
>>>And furthermore, which of these are POP? Cyano, VDB, salt??
>>>Chris
>>
>>The confusion and/or inaccuracies that exist in the literature may
>>result from the fact that in addition to processes that are POP and
>>those that are DOP, there are also processes that are POP to
>>varying degrees. In another response to your message Richard
>>Knoppow notes that there are two mechanisms for self masking: the
>>increasing density of the photolytic silver holding back
>>further light, and desensitization with increasing exposure.
>>
>>There are two another mechanisms which may also result in some
>>self-masking. In colloid photography, the conversion of dichromate
>>to chromate will produce a printed out image, even in the absence
>>of any pigment in the colloid layer. And in iron processes that do
>>not use silver the iron itself will produce a slight printed out
>>image.
>>
>>From the above one can assume that there is some self-masking going
>>on with virtually all of the processes you mention. However, for me
>>a true POP process is one where the image appears completely, or
>>almost so, with exposure. With this understanding salted paper and
>>albumen are perhaps the best examples of POP processes. Vandyke is
>>mostly a POP process but the image goes through some
>>intensification with further treatment. Ziatype is also mostly POP.
>>
>>With most forms of kallitype, as well as platinum and palladium,
>>exposure forms only a whisper of an image, much too faint to cause
>>much self-masking. The same would appear to be true of cyanotype.
>>And, even though there may be some masking caused by the printed
>>out image that results from the conversion of dichromate to
>>chromate in gum and carbon this does not appear to result in any
>>significant self-masking.
>>
>>Finally, look for the proof in the pudding. In a real POP process
>>one will often see a very long toe that results in low contrast and
>>reduced separation in the shadows. This will happen to some extent
>>with any POP process where the negative is of greater density range
>>than the exposure scale of the process.
>>
>>Sandy King
>
>
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