What makes fast films less contrasty?

<p>I know that this subject has been broached before, and I found this explanation from Edward Ingold:</p>

<p><em>'The correlation between film speed and contrast is less certain. Kodak's explanation for this effect in B&W film relates to the size of the silver halide crystals. Larger crystals are more sensitive than smaller ones. A broad distribution in size yields wider dynamic range (and less contrast) thsn a more narrow distribution. Hence very fast and very slow films tend to have more contrast, presumably because the range of crystal size is smaller in order to optimize other properties (e.g., speed or fine grain).'</em></p>

<p>Still, I don't understand this phenomenon. Can anyone explain to me the relationship between film speed and contrast?</p>

<p>Thank you<br>

Rex</p>

<p>Think of the fast film / large crystals and the halftone screens used for newspapers in the old days. I used to run Tri X through Dektol to maximize grain & contrast.</p>

<p>Contrast in a photograph is the differences in brightness between light and dark areas. We say an image has high contrast when there are few if any intermediate tones between the blacks and the whites. We call this type of image “hard”. Whereas a “soft” image reveals countless intermediate tones and may not have a stark black or white. <br>

<br>

Now high-speed photographic film generally achieved using rather large crystals of light sensitive silver salts. These emulsions are faster (more- light sensitive) because a larger crystal has more surface area thus a higher probability of being hit by photons during the exposure is high. A slow film consists of a mix of sizes of silver salt crystals, the preponderance being small in size. <br>

<br>

Long scale film is constructed using a mix of large and small silver salt crystals. Such blend expands the photographic scale to 11 or 12 f-stops. We now say such a film has expanded dynamic range. An image with great dynamic rage is “soft” not “hard” thus we say its contrast is smooth or “normal”. In other words, a more faithful image may result. Plus the smaller crystal size displays less granularity “grain”. </p>

Which begs the question: which films

are long scale now? Is it only the

"old" emulsions where the crystals

aren't aligned creating varying

sensitivity by the differing

orientation of the crystals?

<p >It is my understanding that the “tabular” crystals are arranged with a flat size (stepping stone shape) facing the lens. I was unaware that the speed of the emulsion was somehow based on crystal orientation. Tabular crystals have lowered silver content because they are flattened in shape however that present a large surface area thus they have a improved chance of receiving photon hits. Should hits occur and the expose the crystal, the size of the resulting tuft of silver will be small. </p>

<p>Hmm.</p>

<p>For other than blue sensitivity, where it is adsorbed sensitizing dyes on the crystal surface, it does seem to depend on the area. For blue where AgBr is sensitive without dyes, it should depend on the volume. </p>

<p>But yes, the size of the silver grain that results depends on the volume of the crystal, not the area, so T-grain should give smaller grain for the area.</p>

<p>I'm interested in these responses. I like your definition of contrast, Alan. I've often heard it said that a contrasty image is one in which there is a significant range between the lightest and darkest tones. This, surely, is a partial understanding, and often incorrect. As you say, for an image to be contrasty requires that there be a small number of different tones. It is surely not essential that there be a large range between these tones for there to be contrast.</p>

<p>As regards inherent contrast in film, yesterday I found this in John P Schaefer's 'Basic Techniques of Photography':</p>

 

<p ><em>Fine-grained film is developed much more rapidly than course-grained film. The greater total surface area of silver halides available to the developer in a fine-grained emulsion allows swift development of the latent image, and only a short development time is required to bring a negative of slow-speed, fine-grained film to an appropriate contrast level. The amount of time it takes a film to reach normal contrast values upon development indicates its level of inherent contrast. Slow-speed films generally have a high inherent contrast, and conversely, high-speed, coarse-grained films have a low inherent contrast.</em></p>

<p>The developer is a reducing agent; it has an affinity for oxygen and is able to liberate metals from their salts. Black & white film consists of the metal silver combined with one of three halogen elements. These are iodine, chlorine, and bromine. When one of the halogens (Swedish for salt makers) combines with silver a crystal is formed. The crystals resemble table salt however they are much smaller and slightly yellowish. To make a film we glue thee silver salt crystals onto a base using a binder of animal gelatin. The gelatin silver salt mixture is called an emulsion. This is a misnomer; the mixture is actually a colloidal suspension. Too late to rename this stuff now.<br>

<br>

After exposure in the camera the silver salt undergoes a chemical change. Photon hits cause a microscopic speck of metallic silver to form. This development bud will be the trigger that the developer uses to differentiate whether the crystal has been exposed or not.<br>

<br>

When immersed in a developer solution, the developer identifies those crystals that have been light-struck. Having identified an exposed crystal, the developer reduces it to its two component parts. The metallic silver component being not soluble remains imbedded in the gelatin. The halogen component being soluble is dissolved away by the developer which is mainly water.<br>

<br>

he key ingredients, the developing agents, were all discovered between 1880 ~ 1900. These are derivatives of benzene extracted from coal tar: Pyrogallol (1850), ascorbic acid (1930), Phenidone and phenylenediamine (1940). Trade names are Hydroquinone, Elon, Phenidone, etc. <br>

<br>

Fine grain developers generally contain a silver solvent that acts to whittle down the tuft of silver that results from reduction of the crystal. Granularity is a clumping of these tufts of metallic silver. </p>

<p>Can anyone explain to me the relationship between film speed and contrast?</p>

<p>I don't believe there is one. There was once, and your quoted explanation makes some level of sense in relation to older films of the Super XX era and before. But modern films exhibit native contrast more as a function of design. That is, the designers built a specific film to have a specific set of sensitometric curves. </p>

<blockquote>

<p><em>The amount of time it takes a film to reach normal contrast values upon development indicates its level of inherent contrast. Slow-speed films generally have a high inherent contrast, and conversely, high-speed, coarse-grained films have a low inherent contrast.</em></p>

</blockquote>

<p><em> </em><br>

<em><br /></em>For all films, the contrast changes with developer and development time. Film data sheets show gamma vs. development time for different developers. As is well known by those pushing films, increasing development time increases contrast. </p>

<p>In a high speed film, one has large and small grains, with enough of each to allow for reasonably low gamma, for normal photographic uses. The control of both ISO and contrast comes in the distribution of grain sizes. </p>

<p>If there is a lower limit to grain size, the range of grain sizes for a lower speed film will necessarily be restricted more than a higher speed film. You should be able to decrease it some with more exposure (decreased EI) and shorter development. If you start with a low ISO value, people will be less interested in decreasing the EI even more.</p>

<p>The ISO method develops the film to a specific point:</p>

<p><a href="https://en.wikipedia.org/wiki/Film_speed#Determining_film_speed">1.4 Determining film speed</a></p>

<p>though not to a specific contrast. Films with an unusual shape in their D-H curve will generate an ISO value that might not be representative of the way the film should be used.</p>

<p>Bruce, if I've understood correctly, what has been said here tells us that there is a relationship between film speed and contrast. If, in general, slow films develop more quickly than fast films, then when a slow film and a fast film are developed in the same developer for the same length of time, images shot with the slower film will be more contrasty.</p>

<p>Alan, you said 'to make a film we glue...' Just out of interest, do you work in the industry?</p>

<p><em>Bruce, if I've understood correctly, what has been said here tells us that there is a relationship between film speed and contrast.</em></p>

<p>That is my reading of this thread also. But just because you read it in a forum doesn't mean it's correct. What's correct is perhaps the writings of Grant M. Haist, and in particular his two volume tome <em>Modern Photographic Processing</em>. Henry also, in his book <em>Controls in Black and White Photography</em>.</p>

<p>Some thoughts: First, the rule <em>expose for the shadows and develop for the highlights</em> still holds. Always has, always will, and this is the primary control of final contrast. For more, read up on the Zone System. Archer and Adams pretty much nailed it.</p>

<p>Second, the contrast you get depends on a host of variables, including the emulsion, developer, development time, temperature, agitation, etc., etc., etc. Different emulsions will react differently to any and all of those factors.</p>

<p>Third, if you are in fact developing two different films for the same time in the same developer and expecting to get the same results, you're in for a rude awaking. Photographic chemistry doesn't work like that. See number one above.</p>

<p>Fourth, old thick film emulsions might have been more susceptible to a speed-contast link than modern thin film multi-coated emulsions, and I'll be surprised if modern t-grain emulsions are not completely immune. But I haven't done sufficient research to know for sure. If you really want to know, you've got to do more than ask questions on forums like this. You've got to hit the books and read the papers and find out what the published science actually is.</p>

<p>I'm just sayin' that before you believe me or any of the other rabble on the forums, do some actual research.</p>

<p>Haist seems to be between $150 and about $300, but Henry is available for less than $20.</p>

<p>For some interesting discussion, notice the H-D curves for Delta 3200:</p>

<p>http://www.ilfordphoto.com/Webfiles/201071394723115.pdf</p>

<p>specifically, note that there is no straight line region, and that it gets more curvy at higher development times. The ISO speed is based on two points from the curve, but that ignores the rest of the curve! I don't know that this is a characteristic of fast films in general, maybe just Ilford's idea of how to make one.</p>

<p>As to the original question, I don't think we know yet if it is a fundamental physical property of film, or the result of design choices that are made in designing specific films. It might be true on average, but not for every pair of films available.</p>

<p> </p>