Film ASA differences

[MtnMan]

I have read about and tested my film stocks thru the years. And found film speed testing results to be invaluable. I have never read why the resulting film speed test results is different for different formats. My understanding is all film is manufactured in big (wide) rolls and the various formats are cut out of this roll. So why the different film speed testing results for different formats?

[Oren Grad]

35mm, 120 and sheet film are coated on different bases, hence different master rolls. 35 and 120 are generally coated on cellulose acetate, but the 120 base in thinner. Sheet film is generally coated on polyester. Some 35 and 120 films are coated on polyester; the number has been increasing in recent years as supply-chain challenges have forced some adaptation by manufacturers.

[jp]

Sheet film is usually different film than roll film; roll film is thinner/flimsier. An exception might be a roll of 5" aerial film cut down to fit sheet film holders.

Sheet film would thus come from a different roll manufactured separately.

[Michael R]

There should be no material difference in the speed of a particular emulsion in different formats. If one finds different exposure indexes for a film across formats, it’s down to the test procedure(s).


I have read about and tested my film stocks thru the years. And found film speed testing results to be invaluable. I have never read why the resulting film speed test results is different for different formats. My understanding is all film is manufactured in big (wide) rolls and the various formats are cut out of this roll. So why the different film speed testing results for different formats?

[Drew Wiley]

There are very real practical differences like the presence of strong antihalation layers in roll film, the common necessarily use of a thinner and more flexible base than in sheet film, and therefore, not necessarily exactly the same emulsion, since that have to be coated separately anyway. Just depends. We all know it. But what are some of the implications? For one thing, the added base density of roll film, necessary to prevent light piping, often affects printing itself, especially with respect to speed and contrast. Sometimes the nature of the base seems to effect the thickness of the coating, and hence the degree it might take on something like supplemental pyro stain. There are real-world distinctions. Plus we need develop roll film somewhat differently from sheets, and some difference in time in almost inevitable in every case I can think of,
because the nature of agitation itself, and developer flow, is inherently different.

So sorry, Michael, you don't make the rules. The manufacturer decides those, and we adapt to them. And since effective speed is development-related, a degree of speed tweaking is sometimes itself necessary. It's one of the lesser variables, I suspect, but still potentially there : "Guilty Until Proven Innocent".

[Oren Grad]

There are some instances where manufacturers have recommended different developing times for the "same" film in different formats - 35 vs 120 vs sheets - to achieve a given contrast index. But I've not seen evidence that once you've adjusted development to achieve the same CI, the curve shapes and effective speeds are materially different. Not saying it's impossible, just that I haven't seen it, and I'd like to see evidence from well-controlled tests to justify any such claim.

[Michael R]

You’re confusing film speed with things like differences in processing times etc.


There are very real practical differences like the presence of strong antihalation layers in roll film, the common necessarily use of a thinner and more flexible base than in sheet film, and therefore, not necessarily exactly the same emulsion, since that have to be coated separately anyway. Just depends. We all know it. But what are some of the implications? For one thing, the added base density of roll film, necessary to prevent light piping, often affects printing itself, especially with respect to speed and contrast. Sometimes the nature of the base seems to effect the thickness of the coating, and hence the degree it might take on something like supplemental pyro stain. There are real-world distinctions. Plus we need develop roll film somewhat differently from sheets, and some difference in time in almost inevitable in every case I can think of,
because the nature of agitation itself, and developer flow, is inherently different.

So sorry, Michael, you don't make the rules. The manufacturer decides those, and we adapt to them. And since effective speed is development-related, a degree of speed tweaking is sometimes itself necessary. It's one of the lesser variables, I suspect, but still potentially there : "Guilty Until Proven Innocent".

[ic-racer]

I set my exposure index with in-camera tests, so I don't have a camera that fits two different film formats (even if the emulsion has the same name on the box) so I can't be in that argument either way.
PE on the old APUG used to post that EK added 'trimmer dyes' to bring each emulsion batch, irrespective of format, to the box ISO speed.

As this thread deteriorates into different definitions of "Film Speed," keep in mind ISO does not have a rule that will specify a developer or condition for the test. The film manufacturer can specify what ever they want for developer and development conditions for their own ISO test to get the published box speed. So, Large Format Forum members should feel free to do the same :)


No additional processing specifications are included in recognition of the wide range of chemicals and equipment used in processing black-and-white films.
The values for speed obtained using various processing procedures can differ significantly. --Determiniation of ISO Speed 1993-02-01

[MtnMan]

Using the recommended processing times and the same chemicals for the various formats result in different EI. Could the surface area of the various film formats effect the EI due to chemical exhaustion during the processing?

[Oren Grad]

Using the recommended processing times and the same chemicals for the various formats result in different EI. Could the surface area of the various film formats effect the EI due to chemical exhaustion during the processing?

Can you be specific? Which film(s), developer(s), processing times/temps, processing methods (tank/reel types, agitation, etc)? Do you end up with the same CI? How do you determine EI?

[xkaes]

Just to complicate things, film ages over time, as we know, and that varies depending on how it is stored. And that aging will diminish the EFFECTIVE film speed. Henry, in Controls in B&W Photography, suggests -- without any test results -- that film can lose up to 1/3rd f-stop of speed in a year depending on storage.
One more reason why testing to determine YOUR effective ISO is important -- and one more reason why two different formats of the same film might have different effective speeds.