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Photo Engineer
05-20-2007, 11:09 AM
A few days ago, someone posted a table of film speeds from the 40s(?) and the 70s(?) or thereabouts. There was speculation over how much the change from Kodak or Weston scales to ASA to ISO had on these speeds and how much safety factor was sacrificed to squeeze out more speed.

Well, actually the Kodak, Weston, ASA and ISO scales are very close together and have similar safety factors. Yes, yes, I know, there are differences, but these differences amount to a quibble when you read the rest of this post.....

Back in the 50s, Kodak researchers (and others) had already known that use of sensitizing dyes changed many characteristics of films. Among the changes were included a drop in contrast and speed. So, if you had an ISO 100 blue sensitive emulsion and added a green sensitive emulsion, you would expect to see an effective 1 stop speed increase to a black and white image as a new portion of the spectrum was added to the exposing light.

What you actually got was a drop in blue speed, and then a matching green speed, such that you really kept an ISO 100 speed by having 2 50 speed portions of exposure. And, contrast would go down. (these are hypothetical idealized examples - please)

By discovering better dyes, it was found that the repression of blue speed and overall contrast could be reduced, and by discovering what are called 'supersensitizers', the inherent speed could be doubled in some cases.

So, suddenly a 100 speed emulsion sensitive to blue could be made at speeds of 200 or higher depending on sensitizing dye and supersensitizer. (of course, this is again idealized for exemplary purposes)

What did it lead to though? Well, a 100 speed emulsion could be made up to 300 speed with normal contrast. It could then be dyed back with more acutance dye to an ISO (ASA) speed of 200 and sold as an improved product. It used the same emulsion but got 1 more stop in speed at the same grain and had better sharpness. The safety factor was unchanged.

So, this is what actually took place between the 40s and the 60s and 70s in some products.

This was largely the work of the synthetic sensitizing dye group at EK, the dye scientists and emulsion engineers and ultimately led to the 2 electron sensitization which is a very efficient example of a 'supersensitizer' but in an entirely different manner than anything that went before.

What it means is that a given emulsion can be used in several products, but at different efficiency levels due to addenda that are added after precipitation of the emulsion itself or in some cases during the precipitation.

PE

Michel Hardy-Vallée
05-20-2007, 02:04 PM
Ah, that explains it! The tables I posted were respectively from ca. 1953 and ca. 1974.

Thank you for debunking the idea that it was just a change in labelling, that's something I've seen very often all over the internet and photo books, and like all photo myths, it suffers from the Elders of Zion syndrome, popping unexpectedly at random places over time as a revelation.

Ian Grant
05-20-2007, 02:49 PM
Ron has forgotten a second factor which had nothing what so ever to do with manufacturing techniques.

At some point back in the 1960's the ASA & BS changed the testing criteria that film speeds were based on and the nominal film speeds doubled. This was a response to the increasing use of accurate light meters and the removal of a built in safety factor against under-exposure.

Ian

Anscojohn
05-20-2007, 02:59 PM
All interesting, all true. I think, however, as mentioned above is the change in criteria, with minimum exposure being on a different part of the toe. Most of us will agree, in most situations, that when testing for .10 above film base plus fog, the true EI is in tune with the old ASA ratings.

Anscojohn, Mount Vernon, Virginia USA

Anscojohn
05-20-2007, 03:01 PM
As an addendum: IIRC, Plus X was at different periods, ASA 32, 50, 64, 125
Anscojohn Mount Vernon, Virginia USA

Photo Engineer
05-20-2007, 03:06 PM
Ian;

This change in ASA etc. was far less than some realize due to the reasons cited in my post. I knew that there would be contrary opinions, but having seen the data, I can say that the speeds did not, a priori, just double.

In the OP I stated that there were changes due to definitions and test methods, but they were insignificant in the face of the new technology.

I stand by what I said, and I refer you to Mees and James where you will find early discussions referring to publications in the 50s and 60s. Supersensitization will often more than double the speed of a given emulsion + sensitizing dye combination, and with the use of a proper acutance dye will yield a film of twice the speed at the same grain level and higher sharpness.

Having actually observed this, and worked with the emulsions and dyes I know this to be a fact.

PE

Ian Grant
05-20-2007, 03:43 PM
Ron, in saying there was a second factor for the increase in film speeds I wasn't disagreeing with what you stated, the changes in ASA/BS testing were additional.

The period you are talking about in the 50's and 60's was undoubtably one were many film emulsions were greatly improved and speeds increased.

So as Anscojohn says PlusX goes from ASA 32, 50, 64, 125. HP2 was 31° Schiener approx 80 ASA in the late 40's and evolved with supersensitisation into HP3 at 200ASA which was changed to 400ASA with the revised testing standards. There are plenty of other examples.

Ian

Photo Engineer
05-20-2007, 03:57 PM
Sorry Ian, I was agreeing with you but just emphasizing that the changes were smaller than the 'old myths'. I didn't want that fact to be overlooked and if there was any misunderstanding it was on my part and I apologize.

In fact, I was at Cape Canaveral during part of this and we did extensive testing of films for speed and 'reserve speed' and found that the results met the published values. In fact, there was such a close agreement to values and such good safety factor that I was somewhat against further tests as it was "IMO at this time a waste of taxpayer money". We had many lively disputes over this at the Cape and it was not until I joined EK in 1965 that I began to learn what was going on.

BTW, the first loss in safety factor came with the 400 speed color films. Kodak film was a firm 400 with no safety factor, which is why I always have used 320 for my 400 film, a force of habit from knowing this. The safety factor is now there, but I still use 320.

OTOH, the first Fuji 400 film for C41 at that time was actually a solid 320 with no safety factor at all. It was under speed and off balance. It took about 3 years IIRC, for them to fix it up. This was only in the new C41 ASA 400 film though. Their 100 speed films were spot on with safety factor. This comes from the early 80s.

PE

Ian Grant
05-20-2007, 04:18 PM
I guess we'd agree that the current ISO (BS/ASA part not the Din) tests aren't that good. Most photographers have to downrate modern B&W film if they want good tonality.

Tmax50 is a superb film, for marketing purposes and because of the ASA testing vagaries its called Tmax100 under ISO standards. APX100 was equally as good but it's ISO testing was to the German DIN testing technique and in practice is a full stop faster.

Ian

Photo Engineer
05-20-2007, 04:52 PM
I've found the current ISO ratings to give me good results with almost any film I've used when I use the exact exposure. Nevertheless, I prefer using 1/3 stop overexposure to get denser negatives.

I use D76 straight or HC110B for most of this work.

We were pretty sticky about having our sensitometers calibrated and giving exact exposures, so that our aims were right.

When I coated, I had to meet the aim or the coating set was essentially considered scrap. Some internal relative tests could be done, but otherwise a missed speed aim was reported as such.

A change of more than 0.05 log E was a bad coating. For comparison, 0.1 log E is 1/3 stop so this is 1/6 stop or a variation of about ISO 360 - 440 speed for a 400 aim in any of 3 layers in a color multilayer. Since the curve is considered a straight line in the middle, this deviation can be judged anywhere on the straight line. So, bumps count against you too when blending emulsions.

I have not done that in years, so I hope I did that correctly OTOMH.

But, some manufacturers are not as exact as we were in judging speeds. They (management) kept beating on us to make sure that the aims were met and that was the way to gain and keep the respect of the customer. So, actual release aims were on that order or tighter, and usually had a center sligthly above the aim point, such as 410 or so.

PE

billtroop
05-22-2007, 03:15 PM
Whenever the ASA standards were changed in the late 50s, the safety factor went from 2.5 to 1.25, so film speeds really did double at that point from one day to the next. I don't have a copy of my book handy but I refer to the one magazine author who has managed to get all the history right. The papers of Jones, Nelson and Simmonds from 1939 to about 1960 make clear everything one needs to know about speed evaluation, and make superb reading for anyone interested in this matter. It's some of the best and most practical research Kodak ever did. Sorry I don't have the exact citations handy at this moment. For anyone reading through those papers who is confused by the hump in the paper curve issue, Chuck Nelson cleared that up for me: it was due to an error in the measuring equipment, discovered some time after publication of the papers. The error doesn't affect the general validity of the work, which is absolutely solid. I really like those papers!

Photo Engineer
05-22-2007, 03:19 PM
Bill;

I'm glad you brought in this old but reliable information. Yes, the safety factor was still there and that is an important point. It did not vanish.

And, film speeds did double due to supersensitization and so the final camera speed brought the actual image back to the same point on the curve.

PE

eddym
05-22-2007, 04:44 PM
Bill;

I'm glad you brought in this old but reliable information. Yes, the safety factor was still there and that is an important point. It did not vanish.

And, film speeds did double due to supersensitization and so the final camera speed brought the actual image back to the same point on the curve.

PE
Wait a minute, I guess I am confused. You say that the safety factor "did not vanish" and film speeds doubled "due to supersensitization" while Bill says the safety factor "changed from 2.5 to 1.5" resulting in a one stop increase in the ASA rating. It sounds to me like you two disagree with each other.

Photo Engineer
05-22-2007, 04:57 PM
Eddy, earlier there were claims that speed doubled due to the fact that there was no safety factor. Bill has shown that there was a 2 stop safety factor that was reduced to 1 stop.

So, a 100 speed film is now 200 with a 1 stop safety factor instead of 2 stops.

Now, Kodak introduces supersensitization making it possible to have a 400 speed film (with safety factor). The previous argument would have it that the film had no safety factor.

What was actually done was this. A 50 speed film became 100 (new measurement method), then 200 (super sensitizer) and then a dye was added to reduce speed to 100 but with much greater sharpness. This was done in both color and B&W products and yielded a big improvement in grain and sharpness both. And, it could be done generally as I stated before simply by changing addenda in the film. All of this retained a generous 1 stop speed safety factor.

And actually, the supersentizer was far more potent than just 1 stop as I indicated in an earlier post.

PE

eddym
05-22-2007, 05:04 PM
What was actually done was this. A 50 speed film became 100 (new measurement method), then 200 (super sensitizer) and then a dye was added to reduce speed to 100 but with much greater sharpness. This was done in both color and B&W products and yielded a big improvement in grain and sharpness both. And, it could be done generally as I stated before simply by changing addenda in the film. All of this retained a generous 1 stop speed safety factor.
Gotcha. I remember you talking about the two stop increase coming in two steps, but this last exchange had me confused. Thanks for the clarification.