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# Thread: ISO speed determination constants - question for Stephen, Bill etc

1. ## ISO speed determination constants - question for Stephen, Bill etc

How was the delta log H of 1.3 decided upon in the ISO "triangle"? I guess I could ask a similar question for the 0.8 delta D. Is there a relationship to the fractional gradient method that led to these numbers?

I'm also trying to understand why there was a need to move to this system versus the fractional gradient. Henry (granted, not the best discussion of speed determination methods) writes the fractional gradient method was difficult to use and also that it couldn't gain international acceptance, which led to the ISO method. Obviously this is an oversimplification, but I don't understand why the ISO method is easier to use, other than it having only one gradient. So what?

I guess the direction I'm going in here, is in theory I think I prefer a fractional gradient determination. Not sure yet. Is it possible ISO was preferable because it allows films with different toe shapes to have the same speed, which might have satisfied more manufacturers?

For me, film speed or an EI is merely a means to ensuring I have good toe contrast. So the thing that is bothering me here is that there are too many ways to satisfy the ISO method. A short and long toe film can both have the same ISO speed.

2. Michael, as you know this is one of my favorite subjects. You may come to regret asking.

3. I certainly won't regret asking , I can assure you. I find these discussions interesting and informative.

I'm trying to follow the path from fractional gradients to ISO (.1 above B+F satisfying the .8 delta D:1.3 delta log H relationship), to ZS testing etc. and figure out why ISO speeds don't "work" for me.

4. My idea - and it may not be right, it just sounds easy to comprehend...

I think it all comes down to how hard it is to "find" the tangent of the curve with a line that is 0.3 times the average gradient. If that were easy to find, we'd have picked that point long ago and been done with it.

That 0.3 Gradient point is the "real" best place to determine speed. It corresponds with the studies of people saying which print they liked best.

With everyone in the labs wanting a fixed point to look for, and where the curve meets 0.1 is easy to find, 0.1 became a popular point to look at.

So the ASA triangle was drawn up because when you hit the triangle - then there is a direct relationship between 0.1 (that is easy to find), and 0.3 Gradient (that you want but which is hard to find). So they made finding the point easy. In case there are philosophical disagreements between those who liked 0.1 and those who liked 0.3 Gradient... the argument is rendered moot because now you can look at it either way and get the same outcome. Actually a really good way to end an argument -- make everyone happy.

I think it also has something to do with being a good gradient to fit average subjects on paper.

5. Do you really get the same outcome either way though?

With the fractional gradient, you need a decent gradient right at the bottom. With ISO, you only need a gradient over a 4 1/3 stop range. So I'm thinking about two hypothetical 400 ISO speed films. One with a longer flatter toe might not satisfy the fractional gradient and/or "excellent print" criteria.

So what is the relationship, mathematical or otherwise, between the two methods (eg how did they settle on 1.3 delta log H? in the ISO method)

By the way I'm not necessarily agreeing with 0.3G either. It's too low for me. What I'm ultimately getting at (I think) is that if we're using a gradient in some way to determine speed in either case (which I agree with because I'm all about contrast), I would prefer an ISO method that had a shorter delta log H, or perhaps something higher than 0.1 net D for a speed point.

6. Michael, I’m not able to address your questions tonight to the degree in which I’d like, so this should get you started. Pretty much everything you want to know is in these two papers. Nelson and Simonds' Simple Methods for Approximating the Fractional Gradient Speeds of Photographic Materials (which is too big to upload. Contact me and I'll email it to you.) and my Delta-X Criterion (which can be uploaded).

7. Thanks, Stephen - I'm going to read the Delta X paper and see what happens.

8. Ooh this is fun.

9. Andreas, I'm definitely out of my league when it comes to discussing exposure theory, tone reproduction etc with Stephen and Bill, but I still try my best because I've learnt a lot in the Exposure forum. After all there are only so many threads I can read about magic developers and people trying to push Tri-X to EI 25,000.

10. Only someone with a good deal of knowledge on the subject is able to ask this kind of question. At it's core, this is a question about the nature of film speed.

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