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 > ============================================================================================================= > > 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. > > ============================================================================================================= 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.