Hi, This information is quite valuable. Same was needed when I have switched from condenser type to diffusion type. I needed a grade more to achieve what I was getting from condenser. I know that I need approx. 30% increase in development time but these will cure my future films. For my old ones I have to use my old enlarger. I have another question. Will the general contast of the film increases when printing say at f/22 compared to f/8? Regards Ibrahim Pamuk -----Original Message----- From: pure-silver-bounce@xxxxxxxxxxxxx [mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of Richard Knoppow Sent: 31 Mart 2008 Pazartesi 15:29 To: pure-silver@xxxxxxxxxxxxx Subject: [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 ============================================================================================================= To unsubscribe from this list, go to www.freelists.org and logon to your account (the same e-mail address and password you set-up when you subscribed,) and unsubscribe from there.