Density definition ?

Hello,

Could someone say what exactly the density of an exposed BW film is ?

I mean, what is the scientific definition ? The assumption that the

Zone X corresponds to 1.44 density (for condenser enlargers) is based

on what ? Empiric ? Paper characteristics ?

 

Thanks.

Virgil

The density of film is the log10 of the attenuation. D1.44 means that the film reduces the light by a factor of 27.5.

It's important to note the density is relative to the film base and fog. Unexposed film negatives are not perfectly transparent after development, and that imperfect transmittency is the base for 0D.

The only correct way to assign a negative density value to a given tonal representation on paper is to test the exposure scale of the paper. So to answer your question the density value you expressed should have been based upon the paper characteristics. I am not sure that I agree with the value you expressed based upon my tests of several papers.

It's about the interconnection of the scene, film, processing, paper, and enlarger (among other factors). This is called Tone Reproduction. It's too big of a subject to attempt to cover on this board. There are a number of books that cover this subject well such as Photographic Materials and Processes.

D log E curves are run with calibrating the light level with no film in place. The left unexposed part of the curve then is the base fog; sometimes about 0.1 in base density. Density in D log E curves is with respect to 0.0 density being no film in the tester.<BR><BR> A density of 1.0 means the film only passes 1/10 of the light of no film.<BR><BR>A density of 2.0 means the film only passes 1/100 of the light of no film being in the tester.<BR><BR> A density of 3.0 means only 1/1000 of the light passes thru; with respect to no film in the tester. <BR><BR>To slide rule accuracy; 0.3 density means 1/2 the light; ie one stop.<BR><BR>To slide rule accuracy; 0.6 density means 1/4 the light; ie two stops.<BR><BR>To slide rule accuracy; 0.9 density means 1/8 the light; ie three stops.<BR><BR><BR>Legal film speeds; printed on film boxes requires a specific slope to the D- logE curve; The film has to be developed for a specific slope of density versus exposure. This is like saying that dirt bikes performance should measured on a standard shaped hill; not a hill that is different for each maker; to "cook the books". The speed point of B&W films is 0.1 density above base fog; with the hill/slope so there are 0.8 extra density units; for 1.3 logs of exposure. ASA is =0.8/the speed point number; in meter candle seconds. An asa of 80 has a -2 log exposure as the spped point. An asa 800 film has a -3 log exposure as the speed point. This means an exposure of 1/1000 of a meter candle second gives a density of 0.1 above base fog; with the film developed for a 0.8/1.3 slope. Over developing and under developing changes the slope; and voids the formal definition of ASA/ISO pictoral films.

Strange you should mention the slide rule. Most young people don't have the slightest notion of what it is. All through my 30 years at NASA, it was my primary calculator. Now each engineer has a desktop computer linked to the mainframe, and most of them don't know what a slide rule is, or at least how to use one.

 

A friend of mine used to demonstrate the slide rule by multiplying: "2 times 2 is 3.999...oh hell, call it 4."