Quote Originally Posted by Photo Engineer View Post
To get the true speed of a film, draw a straight line extended out from the B+F to infinity. This is done using the H&D curve. Then, note the deviation point of that H&D curve from the straight line. This is the inflection point of the emulsion and is the maximum point at which this emulsion can register an image. It is called "threshold speed".

Translate that into speed using standard methods and you get the max speed of that product.

Now, create a developer that swings the curve on that single point and you have a developer than can approach that speed point. At this time, the maximum threshold speed ever achieved is 25,000. These materials were never sold.
Thanks for this explanation---it's something I've long wondered about. If I understand correctly, this is basically the speed that follows from the principle that the most you can do is develop to completion, right?

I imagine the threshold speeds of common materials (like the one that starts with a T and ends with an X400) must be known in the industry---is there any way for us outsiders to find out what they are?

-NT