What are the actual film differences in regard to ISO?

William Crow · Apr 15, 2018

If all black and white film is made with a silver halide crystal coating what are the differences coatings that determine film speed? Is it the size of the crystal, the thickness of the coating? Or is there something else in the emulsion that allows more or less might reach the silver. What other variables might there be?

I just read a post here from a guy that shot 3200 at 100 and was asking about how to develop it. One response was from someone, I think that shot it at 50.

If its in regard to grain size, I think about standard emulsions - 400 speed, for instance - and there is the larger grain when compared to the Tmax style.

Or is it just about development?

Gerald C Koch · Apr 15, 2018

The sensitization dye(s) play a big part.

E. von Hoegh · Apr 15, 2018

T-Max (and the Ilford equivalent) has a tabular grain form.
Grain size is the greatest and most apparent difference between fast and slow films.

Bill Burk · Apr 15, 2018

Hi William Crow,

Gerald C Koch and E. von Hoegh gave answers that hint where film speed differences come from.

The manufacture of the film.

Larger grains of silver are more sensitive to light. "Tabular" grains (triangles and hexagons) have always been with us, but manufacturers "recently" found a way to make them settle flat, making film more sensitive.

So with the film being more sensitive to light, you can get images on a negative with less exposure.

Most film has a very long straight line, a region of exposure which gives about the same proportion of change in density to change in exposure... So once you give the minimum exposure for a good picture, more exposure continues to give a good picture even if you give 1000 times the minimum exposure... (A great picture only comes from a small range of exposures above about at the minimum, but you don't destroy the image until much higher exposures begin to affect the graininess and halation)...

tedr1 · Apr 15, 2018

Regarding development, normal development yields normal speed of course. Extended development causes the dense regions of the negative (highlights) to become denser faster than the thin regions of the negative (shadows) and the result is not much change in speed but a significant increase in contrast.

The wikipedia entry on film speed might be helpful on this.

E. von Hoegh · Apr 15, 2018

Bill Burk said:
Hi William Crow,

Gerald C Koch and E. von Hoegh gave answers that hint where film speed differences come from.

The manufacture of the film.

Larger grains of silver are more sensitive to light. "Tabular" grains (triangles and hexagons) have always been with us, but manufacturers "recently" found a way to make them settle flat, making film more sensitive.

So with the film being more sensitive to light, you can get images on a negative with less exposure.

Most film has a very long straight line, a region of exposure which gives about the same proportion of change in density to change in exposure... So once you give the minimum exposure for a good picture, more exposure continues to give a good picture even if you give 1000 times the minimum exposure... (A great picture only comes from a small range of exposures above about at the minimum, but you don't destroy the image until much higher exposures begin to affect the graininess and halation)...

Tabular grains present more area per grain volume, giving higher sensitivity (and finer grain for the same sensitivity), IF the grains lay flat as Mr. Burke mentions. T-Max 400 iso has grain comparable to 125 iso "standard grain" films.
The developer used has some effect on speed, some developers give less than rated speed, some more, but the difference is usually around one stop, (with some exceptions).

Alan Johnson · Apr 15, 2018

The intrinsic sensitivity is proportional to the grain volume V up to some limiting value [ about 1000 ISO] , beyond this the sensitivity increases more slowly and eventually falls off due to loss reactions in latent image formation.
Karlheinz Keller-Photography -p28.

Craig · Apr 15, 2018

If you're interested, take a look on the emulsion making forum here, https://www.photrio.com/forum/forums/silver-gelatin-based-emulsion-making-coating.93/ and look at the stickied threads on Photo System Engineering. It will explain a lot.

Pioneer · Apr 15, 2018

The way I understand it is that it has to do with the size of silver grains in the film. The larger the grains the less exposure to light is required to generate a latent image tha can be developed.

Tabular (or flat) grain was just a way for Kodak to reduce the cost of making the film while raising the cost of buying it. Worked out great. At one time Kodak management was relatively intelligent and knew how to run a company. I wonder what happened?

Since silver is a very costly component of film manufacture it wouldn't surprise me one bit to find out that the manufacturers have been quietly converting all their "traditional" grain films to tabular grain.

Adrian Bacon · Apr 15, 2018

Grain surface area and sensitation dyes are the two biggest factors. The type of grain will affect surface area for a given grain size.

Other factors that affect film speed is what developer you use and what contrast you develop to.

If we’re talking ISO conditions, that specifies a standard contrast of approximately 0.615 (more commonly referred to as 0.60 or 0.62). If you pull your development time in to get less contrast for a condenser enlarger, with many developers, to a degree, you generally need to add exposure as less dev time results in less effective film speed. Going the other way, if you add dev time to develop more contrast, to a degree you will see a higher effective film speed.

You can have similar effects with your choice of developer. A super fine grain developer has a tendency to wipe film speed off for a given emulsion, and a speed increasing developer has a tendency to add film speed, sometimes while keeping contrast to manageable levels, and sometimes with contrast that is completely out of control.

All that being said, many emulsions tend to be very close to the manufacturers rated ISO film speed and it tends to be pretty difficult to effect the effective film speed in either direction by more than 1/2 to full stop without some type of compromise.

Bill Burk · Apr 15, 2018

Pioneer said:
The way I understand it is that it has to do with the size of silver grains in the film. The larger the grains the less exposure to light is required to generate a latent image that can be developed.

Tabular (or flat) grain was just a way for Kodak to reduce the cost of making the film while raising the cost of buying it. Worked out great. At one time Kodak management was relatively intelligent and knew how to run a company. I wonder what happened?

Since silver is a very costly component of film manufacture it wouldn't surprise me one bit to find out that the manufacturers have been quietly converting all their "traditional" grain films to tabular grain.

I don't believe the "less silver" conspiracy... It doesn't take as much silver true, but I think the goal was to make a finer-grained and more-sensitive film.

Bill Burk · Apr 15, 2018

Adrian Bacon said:
If we’re talking ISO conditions, that specifies a standard contrast of approximately 0.615 (more commonly referred to as 0.60 or 0.62).

That's the approximate contrast for determining ISO, but is not the standard development aim contrast. I learned that trivia detail last year.

Most people actually develop less, for example to 0.5 or 0.55. In practice, you can still use the ISO speed and still get the same outcome, even with this slightly reduced development. The benefit of slightly reduced development is slightly improved graininess and resolution. The "Delta-X" criterion can be used to explain why the speed doesn't change in this scenario. (Short version - you'll use a slightly higher contrast of paper to print which will show detail slightly further down the toe of the film - same result with same exposure and less development).

E. von Hoegh · Apr 15, 2018

Bill Burk said:
I don't believe the "less silver" conspiracy... It doesn't take as much silver true, but I think the goal was to make a finer-grained and more-sensitive film.

At $17/troy ounce the cost of the silver per roll is pennies, pretty well swamped by production costs, markups, etc. etc..

Adrian Bacon · Apr 15, 2018

Bill Burk said:
That's the approximate contrast for determining ISO, but is not the standard development aim contrast. I learned that trivia detail last year.

Most people actually develop less, for example to 0.5 or 0.55. In practice, you can still use the ISO speed and still get the same outcome, even with this slightly reduced development. The benefit of slightly reduced development is slightly improved graininess and resolution. The "Delta-X" criterion can be used to explain why the speed doesn't change in this scenario. (Short version - you'll use a slightly higher contrast of paper to print which will show detail slightly further down the toe of the film - same result with same exposure and less development).

Very true. How contrast is calculated can take many forms, and the ISO way isn’t the only way to get great images.

Mr Bill · Apr 15, 2018

Bill Burk said:
I don't believe the "less silver" conspiracy... It doesn't take as much silver true, but I think the goal was to make a finer-grained and more-sensitive film.

Hi, I dunno how long you've been around the photo business, but back around 1980 there was a bit of panic when the Hunt brothers had attempted to "corner the silver market." In short order the market price for silver went from around $6 per TO up to near $50. There is little doubt that manufacturers were on a mission to investigate ways to reduce silver loading after that event. Eventually things settled back down but I would imagine that the fear of a possible repeat stuck in people's minds.

In later years, I imagine that investigation into things like "antenna dyes" and "fragmentable electron donors" (2 electron sensitization) were largely motivated by the desire to improve sensitivity, as well as overall efficiency of the photographic effect on film.

Over the past 20 years or so, Tadaaki Tani of Fuji has given periodic talks on how much potential existed in "future" efficiency improvements in silver halide photography; these might be interesting to some of those in this thread. At least some were published by IS&T; see also older things by T. H. James and Muenter [sp?] of Kodak.

Sirius Glass · Apr 15, 2018

For me the biggest differences between two films of the same ISO is traditional grain versus tabular grain.

Pioneer · Apr 15, 2018

Production costs almost always "swamp" material costs.

Labor is usually the guerilla in the room however energy is also a big player. However I know that reducing "all" costs in manufacturing is a good thing. Silver costs may be relatively low today but that hasn't always been true and likely won't remain true in the future.

Pioneer · Apr 15, 2018

Some may find this article of passing interest.

https://www.bullionvault.com/gold-news/silver-bullion-photographic-demand-062120133

What are the actual film differences in regard to ISO?

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