Confusion: Is ISO fixed or variable

Sirius Glass · Feb 28, 2018

On ~~APUG~~ Photrio it has been stated that based on the film characteristic curves each film has one and only one ISO film speed and that increasing development only increases contrast, HOWEVER both Ilford Delta 3200 and Kodak P3200 state that they have an ISO of x0 but can also be used with ISO of x1,x2, x3, x4 ... using development times of t1, t2, t3, t4, ... using developer y.

Now does this mean that just like there are traditional grained films and there are tabular grained films, there are films with fixed ISOs and other films with variable ISOs?

Ted Baker · Feb 28, 2018

Actually they don't, the stated ISO with the specified conditions never changes, otherwise its not an ISO speed... The 3200 are nominal speeds, i.e. speeds that you might consider a recommended speed. i.e. they are not based on any formal test standard.

The ISO specification includes the developer used, for example TMAX 3200 developed in TMAX developer has an ISO of 1000, or at least it used to...

faberryman · Feb 28, 2018

Fixed ISO just means the manufacturer has rated it at that speed for achieving the parameters specified in the ISO standard for shadow density using a certain specified developer of its choice and processing methodology. That's why photographers have different E.I.s for the same film - they are using different developers and different processing methodology (principally agitation), and likely are not measuring the same ISO shadow density parameters (if they are measuring at all). TMax3200 doesn't have an ISO because Kodak hasn't given it one. Instead, it has given a variety of suggested speeds for different developers and development times. Yours may differ. Kodak's numbers are good ballparks, but you need to test for your own processing regime, and how you like to print. This is how it has always been.

Photo Engineer · Feb 28, 2018

ISO is fixed. All push processes only increase contrast. HOWEVER, I did include a caveat the last time I posted this. If you sharpen the toe you can sometimes see a real speed increase with no change in contrast. The threshold speed is the true speed. It is the inflection point of the curve from fog or Dmin. It is fixed.

PE

Bill Burk · Feb 28, 2018

You can develop very flat (with a POTA developer) and print on very high contrast paper, utilizing part of the toe for imagery that normally can't provide image information... and get higher speed that way.

Sirius Glass · Feb 28, 2018

Photo Engineer said:
ISO is fixed. All push processes only increase contrast. HOWEVER, I did include a caveat the last time I posted this. If you sharpen the toe you can sometimes see a real speed increase with no change in contrast. The threshold speed is the true speed. It is the inflection point of the curve from fog or Dmin. It is fixed.

PE

That is my understanding. My next question is how does one "sharpen the toe"?

trendland · Feb 28, 2018

Sirius Glass said:
On ~~APUG~~ Photrio it has been stated that based on the film characteristic curves each film has one and only one ISO film speed and that increasing development only increases contrast, HOWEVER both Ilford Delta 3200 and Kodak P3200 state that they have an ISO of x0 but can also be used with ISO of x1,x2, x3, x4 ... using development times of t1, t2, t3, t4, ... using developer y.

Now does this mean that just like there are traditional grained films and there are tabular grained films, there are films with fixed ISOs and other films with variable ISOs?

Perhaps you may notice ISO as a rating to compare different films. Within ones individual workflow it has noting more to say as (in comparison) the developing time with recomanded developers.
It is just a starting point.
The differences some noticed with ISO rating allways came from own experience and workflow.
Of course you can't rate an ISO 50 film as (individual) ISO 800 film. ...
That should be overdriven.
with regards

PS : To me ISO has noting to say at all.
Frist results within my workflow is my personal first reference.
So I am my own ISO institute. Same is with developing times.
Same is with dmax. ..same is in regard of tonals.
PPS : Never ask a technical institution about good photography - but ask them about comparison of tools.
So as you just did it here - well done by the way.

trendland · Feb 28, 2018

In other words : ISO is fixed - your workflow is variable.
If your intention is special you may notice variations in regards of reached ISO parameters. ....then (just from indication) your workflow is OUTSIDE the ISO standard. ...

with regards

ic-racer · Feb 28, 2018

Look up ISO film speed and you will have the answer.

Photo Engineer · Feb 28, 2018

Sirius Glass said:
That is my understanding. My next question is how does one "sharpen the toe"?

And that is the problem, isn't it. Each film developer affects a film differently and one must determine which is best and does the job.

PE

Bill Burk · Mar 1, 2018

The only variability to ISO speed that I know is that you are allowed to change the developer... So if you can formulate a developer that hits 0.1 over B+F with less light hitting the film and you can still hit the other parameters, you have raised its ISO speed.

You might be able to pull a stunt like discover the best keeping temperature to preserve maximum latent image during the holding period between exposure and development. I'm not sure the holding temperature is specified, near absolute zero might retain latent images better. This might raise its ISO speed.

But basically, a film's characteristic curve dives down to zero net density above Base + Fog at a certain exposure and you can't move that much to the left.

NedL · Mar 1, 2018

Bill Burk said:
.... near absolute zero might retain latent images better....

I predict something might happen to the gelatin!!!

Photo Engineer · Mar 1, 2018

To get 0.1 over B+F with less light, you increase silver laydown when coating. Of course, this will effectively have two possible effects. One is to sharpen the toe (see above) or the other is to move the inflection point of the curve, but this is somewhat less likely.

PE

Stephen Benskin · Mar 11, 2018

I guess you could say there is only one ISO speed per film / developer combination. Each film could have a dozen ISO speeds if the film manufacturers wanted to do all the necessary testing. But for any one film /developer combination, there is only one ISO speed. The ISO B&W film speed standards has strict parameters that must be followed, Including contrast parameters. If the testing is outside of the conditions stated in the standard, ISO cannot be used as the prefix. Instead EI should be used. Film speed will change based on degree of development, but not as much as most people believe. The reason for this is due to the method of speed determination. When properly determined, film speed changes little with decreased and increased development outside the ISO standard's specified contrast.

Confusion: Is ISO fixed or variable

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