A really dumb film question for Mr. Mowrey

[craig_maxwell]

Ok, here is a question about black & white film (all films for that matter) that I have

been curious about for some time now. Why does film give us different densities in

relation to the amount of light striking the emulsion. OK, let me explain, as I

understand it, black & white film is coated with an emulsion of silver halide

suspended in a gelatin mixture, probably a lot of other things in the gelatin as well,

but onward. So, if this even coating of emulsion containing individual silver grains is

exposed to light and developed, the resulting silver density will be proportional to the

amount of light that exposed the film, but why?. Since each individual grain of silver

halide is essentially "binary", it either is exposed to the light or it isn't and hence it

will change to metallic silver or not, why do we get various densities of the negative. I

have conducted my share of zone system tests so I KNOW that in fact film does

respond to various light intensities, but how. If the individual grains either are

exposed or not exposed, should we not get just either a clear film base or d-max,

what give us all the tones in between. ???? Does it have to do with the thicknes of the

emulsion and the depth that the light penetrates or.....

Thanks for answering this very basic question.

[stephen_allan]

Hi Craig,

 

Not dumb at all. It is not that the photons affect a grain of the halide <i>in- toto</i>, but that electrons generated in the grain reduce individual ions of silver to the zero-valent metallic form (the latent image). <p>So depending on the amount of light that hits an area of emulsion, the greater or lesser number of 'seed' atoms (or clusters of atoms) of silver metal there are for a silver grain to grow during development. The greater the number of metallic silver atoms in the halide grain, the faster the rest of the grain will be reduced to silver during the development, and the darker the individual grain will appear.

 

<p>So the individual photon/silver halide interactions are essentially binary in nature, but as there are a huge number of silver halide molecules in each grain, the overall effect of the process is to give a subtly graded analogue tonal appearance.

 

<p>Best wishes for the season,

Steve

[bruce_karnopp1]

First of all, there are few if any dumb questions. A few additional points. It is fortunate for us that the density of a film(within limits) is proportional to the exposure. This is shown in the famous D-H plots where the film response is plotted between the toe and the shoulder. In these two areas, a change in exposure DOES NOT result in a change in density. In between these areas, the change in density is roughly proportional to the change in exposure.

 

This is the fundamental question which occurs in all areas of instrumentation. The is a quantity to be measured and a response to indicate the change. If the plot of the quantity to the response is roughly 45 degrees, the change is one to one.

 

What is so remarkable about film is how very sensitive it can be if exposed between the toe and shoulder. And to think that the ancients figured this stuff out by 1900 is even more remarkable.

 

And when you win the lottery and go out to buy a pricey camera with a Zeiss Planar lens, think about the fact that the Planar was designed in 1896( as I recall) and it is still one of the very finest lenses one can buy.

 

The fascination of photography is that it is the marriage between mathematics, science and art. And because of the success of that marriage, we know a great deal more about our history since 1839 than we knew before that date.

 

Merry Christmas

[Troll]

One other factor: in real-world film the grains of silver halide are not perfectly identical in size, and thus respond very slightly differently to light. Incidentally, the images on the negative are not individual grains of silver halide, but clumps of grains.

[craig_maxwell]

Thanks to all who answered my apparently "not so dumb" question. The

explanation certainly sheds light on the process and has satisfied my curuosity. It is

a bit of a stretch for me to actually imagine the entire process on the molecular

level, but it actually makes such elegant sense. My parents bought me a Sears

Junior Home Developing & Printing set when I was 8, so I have been working in the

medium for 40 years now, the past 20 of which professionally as a commercial

photographer, and I confess to have taken for granted, the AMAZING phenomenon

we call Photography. Thanks to all and here is wishing you a helathy and

prosperous new year in 2005.

[rowland_mowrey]

Craig;

 

Sorry I missed the opportunity to comment on your question, but I don't need to as everyone else did an excellent job. In fact, they probably said it better than I would have.

 

I would only add that most films are blends of 2 or more emulsions of different grain sizes which also supports the above comments and is one reason for the extended latitude of most negative films.

 

Best wishes to all for a happy holiday season.

 

Ron Mowrey

[bujor_b]

Mr Maxwell Sir,

 

just to make the question even "dumber", I shall complete it (on my terms this time) in front of everybody: if some can come up with a faint and disarmingly simplified explanation for image forming mechanism (which, by the way, still remains, for the most part of it, deeply unknown at this time, after almost 200 years), where may I place the infrared film channeling (tunneling) effect, in which film is clearly directionally fogged (impressed), even when it didn't see incident radiation, or (my beloved one), considering the Kirlian effect photography, when impression is accomplished by long time vanished matter...

 

Where are the energetic "red zone" photons, or, errr, the "native" electrons, nevertheless so easy to invoke in "perpendicular" photography (activated triplet or singlet bands states)?

 

Sorry about this brutal awakening, folks!

 

Of course: Merry Christmas and a Happy New Year to everybody!

 

the rookie

[rowland_mowrey]

Bujor;

 

Accepted theory says that any energetic particle be it visible light, UV, X-ray, Infra Red, gamma radiation or cosmic rays will affect film (and also electronic imaging sensors for that matter). This radiation causes a reduction of silver ion to silver metal, and when 3 atoms have been produced (or two as in the new 2 electron sensitization) the spot is rendered developable and is called a latent image.

 

Whether it is fog caused by radiation or heat (keeping), or is an image useful to you is up to you. The source of the radiation is under the control of the user. The manufacturer only supplies a sensitized material for you to work with.

 

Grins and best wishes.

 

Ron Mowrey