[pure-silver] Re: Agfa APX 100


-----Original Message-----
>From: Snoopy <snoopy@xxxxxxxxxx>
>Sent: Mar 31, 2008 12:40 AM
>To: pure-silver@xxxxxxxxxxxxx
>Subject: [pure-silver] Re: Agfa APX 100
>
>Hi there,
>
>this might be me again making one of my traditional mistakes but I also 
>use _agitation_ to control the contrast of my negs ?
>
>So I kind of agree with Richard saying: "Contrast is a matter of 
>development", but in this case I fiddle the agitation - more agitation 
>makes contrasts sharper and YES you can easily develop to get "blown out 
>highlights".
>
>In the days of yore, before The Rains came there are/were charts giving 
>you a "lambda" value of either .55 or 0.65/0.75 which was a contrast 
>measure. These were intended for peole with either condenser based or 
>colour head enlargers?
>
>When you had a colourt head, you needed "harder" negs and for condenser 
>you needed softer ones (9,45-0,5 lambda).
>
>Also I feel that rodinald does not only compensate "a little bit" but 
>actually manages quite a lot, at 1:150 or 1:300. But then you need time 
>for developing for an hour r so.
>
>Love
>Snoopy
>
     Kodak developing charts are usually set for the correct contrast for 
diffusion printing. Some other manufacturers do not follow this. For instance, 
Ilford states that their development charts are for a compromise contrast 
midway between that required for diffusion printing and common condenser 
printers. They also adjust the speed rating in accordance. 
     The usual difference between a diffusion printer, which includes most 
color heads, and the usual partly diffusing condenser head such as the ones 
used on Omega and Bessler enlargers, is about one paper grade. The difference 
in development time will depend on the kind of film but is about a 30% change 
for conventional emulsions and about 20% for tabular grain emulsions. 
     The old measurement for contrast was gamma. Gamma is the slope of the 
straight line portion of the film characteristic. Since some films do not 
really have a straight line portion (Tri-X professional is an extreme example) 
another method has been used. This is average gamma or G-bar. It is the slope 
of a straight line drawn from the end of the toe to some highlight density. 
Kodak has  a more systematic version of this called Contrast Index or CI. CI is 
like G-bar in that it is the slope of an artificial straight line but CI 
specifies the minimum and maximum points for the line. Gamma and either CI or 
G-bar will not in general be the same. I believe that Agfa specified gamma in 
their charts, typically around 0.65. This is probably about right for diffusion 
printing but it makes it difficult to compare the chart contrast to those 
measured for CI or G-bar. 
     FWIW, the ISO speed measuring method for B&W still films does not directly 
specify a contrast but does specify the range of densities to be produced from 
a specified range of exposure. Thus, it essentially does specify the contrast. 
This is a fairly high contrast but is about the right CI for diffusion 
printing. For this reason those using condenser printers may prefer to reduce 
deveopment time and increase exposure about 3/4 stop above that given by the 
ISO method. In fact, the ISO standard contains a chart indicating the speed 
correction when the contrast is varied from that specified. 
     The system used in conventional condenser enlargers uses a fairly large 
highly diffuse lamp which is imaged on the entrance pupil of the lens by the 
condensers. Because the light from this source is not completely collimated the 
ratio of contrast when compared to a true diffusion source is somewhat variable 
and is much less than would result from the use of a truly collimated system 
with a point source lamp. 
     The ratio of change in contrast with the amount of diffusion is called the 
Callier effect. It is mainly a function of the amount of light scattered by the 
particles making up the image. It is greatest for very coarse grain silver 
images (say IR or X-Ray film) and is nearly non-existent for the dye particles 
of color film. For the average range of pictorial films and for the average 
type of diffusion and condenser printing sources it is about one paper grade. 
     The tone rendition of the two systems is identical if either negative or 
paper contrast is adjusted to match. While diffusion sources tend to suppress 
blemishes in the negative if the same negative is printed on both kinds of 
printer the increased paper contrast required for the diffusion system will 
tend to pick the blemishes up again. 
     Very highly diluted developers, like Rodinal at 1:300, will tend to create 
a "shoulder" on the film by lowering the contrast of the highlight densities. 
While this is usually attributed to "local exhaustion" the effect is probably 
much more complex than this. Lack of agitation can produce a similar effect. 
Such "compensation" is likely to be accompanied by other effects such as 
exaggerated edge and border effects or loss of density in the center of large 
areas of relatively high density (sometimes called port-holing). It can also 
result in uneven development, and in some developers, in bromide streaks. 
     The main thing to remember is that contrast and effective film speed are 
interactive with the degree of development. That it should be possible to get 
the same contrast index and range of densities with most combinations of common 
pictorial films and developers and that noticeable variations in contrast 
between one film and another or one developer and another are probably from  
errors in development time. 
      Note that agitation is very important. Most development charts are 
written for a specific method of development and specify the kind and frequency 
of agitation for it. For instance, Kodak gives development times and agitation 
methods for small tanks, large tanks (for sheet film and multiple rolls of roll 
film), tray development, and drum development. In general, the times for each 
method will be different. As below, a consistent method of agitation is 
important if development is to be uniform, predictable, and reproducible. 
      Temperature control is also quite important. While not as critical for 
B&W as for color it is still very important if predictability and 
reproducibility are desirable. Note that the reason temperature is so critical 
in color is partially due to the requirement that the processing solutions 
penetrate the emulsion at a controlled and predictable rate so that all layers 
of the film are developed as required for correct color balance. This is not 
the case for B&W but, nonetheless errors in temperature can result in 
unexpected results so good control is desirable. 
     Tone rendition of film depends on the shape of the H&D curve. This is 
mainly a property of the emulsion and is designed into it in manufacture. While 
developers have some effect on the curve shape in general it is small compared 
to what is built-in. This, of course, excepts special purpose developers like 
the compensating ones mentioned earlier. 
     In general, I've found that Kodak's data is pretty reliable although I've 
found some obvious errors in the charts. Real control requires sensitometric 
methods. While the methods used in film research are well described and may be 
beyond the home darkroom those described for the Zone System are simpler, and, 
since the camera is usually included, are probably more useful in practice. 
     It is quite possible, BTW, to do without a densitometer by making visual 
measurements against a step wedge. The eye is quite accurate in matching 
adjacent brightnesses so, while a densitometer is a great convenience it is not 
absolutely necessary. Step wedges are inexpensive and are available over the 
web from Stouffer. 


--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
Los Angeles, CA, USA
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