Michael I enjoy these threats and post of you Stephen and Bill including a few others. I have learnt a lot more, get confused very often. This certainly helps to understand all this better.
Exposure is the most important single factor in photography in my opionion and it is discussed too little.
Am I correct or incorrect in say that the Delta-X criterion is more similar to th CI from Kodak. Looking at curves near the foot it seems to make sense regarding film speed. Now am I over ambitious in saying when I get my EI there is no need to change the EI when pushing or pulling?
The lower gradient part of CI uses the same basic theory as the fractional gradient method. Long toed films are where the influence is greatest.
Originally Posted by AndreasT
Film speeds don't change much with fraction gradient / Delta-X. The pre 1960 standard didn't have an aim contrast for film development (it did have a minimum). While the Delta-X Criterion uses a fixed density point, the Delta-X point moves in relation to the fixed point as the average contrast changes. This tends to compensate for the shifting of the fixed density point.
I am excited to see the beginning of a (I hope) another illuminating chapter in the saga of the variables of the tone reproduction cycle and their relationships. Count me in to lurk over you. Stephen, Bill, Michael, and others—thank you.
Here's a comparison of speeds derived from the fixed density method and the Delta-X Criterion.
Except this is not as much a comparison between two methods as it is between an accurate method, Delta-X, and the erroneous and inaccurate use of another.
This is why the Delta-X speeds change little with changes in development.
As the gradient changes with development and the fixed density point shifts left and right, the fractional gradient point's relationship to the fixed density point changes but it's log-H remains relatively constant.
Whoa, you can't start a thread about fractional gradient approximations without discussing "W speed"...:)
Delta-X was adopted for the ISO standard. W speed is more of a historic footnote for being the runner up. As the paper says, W speed "is of particular interest in the mathematical theory of the shape of the sensitometric curve and its relation to speed." It's worth knowing for that, but it has never been used as a film speed method. Seriously, film speed can be a confusing topic and W speed will only add to the confusion unnecessarily.
Originally Posted by ic-racer
My take-away from the W speed article that these different speed ratings are compared based on how well they correlate to the "Picture Tests".
Then the writer makes an outlandish statement that I will challenge: "It is, of course, too time-consuming to use the print-judgment method."
"Of course"? "Too time-consuming"?
Look at how much time we spend talking about film speed testing.
It might be fair to say that is more time-consuming than making some prints and showing them to your friends.
I'm not suggesting to throw out the tests... Just providing some necessary perspective.
I still like to test to ASA triangle and 0.1 density. Then I accept Delta-X Criterion as justification that I can use whatever speed I discover when I get as close as I can to ASA parameters... And I use that speed for all my varied development times (N-1, N and N+1 etc.), because it is easier to pick one speed than use a sliding EI... and because it is probably correct.
The ring-around test can probably be thought of as a very stripped down personalized version of the first excellent print test.
Originally Posted by Bill Burk
I have to agree with Nelson. It took 18 months to produce the negatives for the first excellent print test.