Hitting ASA Triangle Does Not Mean You Got Full Film Speed
I'm consistently getting EI 250 from TMY-2. Have been for two years. The curves look great. The negatives look great (except for scratches that I complain bitterly about).
All along I thought I was getting full rated speed of 400 because I hit the ASA triangle. I made 400 my benchmark.
(Real quick: ASA triangle is 0.8 rise over 1.3 run from 0.1 over base plus fog).
I didn't question my benchmark until a roll of Panatomic-X rated extremely fast.
Reference post 146 of this thread...
I know tray processing in stacks of six sheets gives less agitation than small tanks with plenty of developer circulating around the film. But I thought hitting the triangle made up for it.
In that thread at least two estimates discredited my 400 benchmark. Now I believe my benchmark is close to 250 (may move again if new trends emerge).
Ironically, I felt clever using half box speed from the beginning. And even more ironic... after a while I felt like bumping it up to 250 because of consistently "more than enough" shadow density.
So even though I thought I was getting 400, and thought I was overexposing... I was actually using the right speed all along. That is just too scary. I could just as easily have used 400 and botched a lot of film.
I know I have insufficient agitation to meet ASA specification. I may continue to develop the way I do because the negatives meet my quality standards. I currently have no "need for speed".
But at least now there is little risk that I will overrate my film. It also emphasizes the need for photographers to determine their own effective EI. From now on, I'll be using 250 for TMY-2 and it's not just because it's "half box speed"...
You are "there."
Originally Posted by Bill Burk
I'll mention that my in-camera testing (where I can associated an exposure to a number on the camera or meter) produces similar results to what you have found (EI 200 to 250).
I'd not worry about the number. Maybe we should call it the "Exposure Index Triangle" because the results usually are used as an Exposure Index and the conditions under which the test is performed frequently do not follow all ASA or ISO 6-1993 guidelines.
Last edited by ic-racer; 07-22-2012 at 08:29 AM. Click to view previous post history.
Stupid question, way out of my league here:
Is this the same as the ASA (angle, side, angle) triangle?
“Do your work, then step back. The only path to serenity.” - Lao Tzu
This is what Bill is referencing.
I'm graphing your data to double check it. Maybe Steve can do the same.
I started with the 6min graph, just because it was first in line, though, I could see the gamma was going to be low.
My first graph has some oddball things, so maybe there are some issues that need to be addressed.
1) You indicate "calibrated grayscale" but are the values you gave your actual densitometer readings from the grayscale? It would be hard to believe your densitometer matches that gray scale exactly. I use readings from the same densitometer on both X and Y axis. If you densitometer is far from the calibrated step wedge on a lot of the range, the curve might look better if you add a correction factor to each film step checked. (unless your densitometer agrees with every calibrated step).
2) I process TMY 2 for 6 minutes and get a gamma around 0.7. In your case the gamma was only 0.3. I know you used the steeper curves to fit the ASA triangle (per protocol) and Steve posted that ISO criteria for specific developers was relaxed, I think you may be using a developer combination or process that is not giving you full speed available from the film. Not all developers give the same 0.1 point when processed to match the ASA triangle.
Which dataset did you use to match the ASA triangle? I'll graph that one next.
3) The curve just 'looks odd' I have double checked all the points and I have graphed exactly the dataset below. Perhaps there is an error in data acquisition?
We can look at these speed numbers 'for educational purposes' but since this particular sample is not processed to a gamma you are likely to ever use in practice, I'll ignore those values.
The "W" point is a mathematical estimate of 0.3G point (likely similar to Delta-X point, maybe Steve can calculate the Delta-X on this dataset)
The ISO estimate is one stop to the right of the "W Speed" point
The inertia point is where the gamma line hits the X-axis
The ASA point is not indicated because the gamma is too low to fit the ASA triangle in this sample
The Delta-X point is not indicated but I may add that functionality to my spreadsheet later.
9/9/2011 TMY-2, 6 min, B+F 0.03
Last edited by ic-racer; 07-22-2012 at 10:54 AM. Click to view previous post history.
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There's something problematic going on in the toe. In my opinion, this questions the legitimacy of the test. I don't think you can confidently conclude anything from this. Sorry.
Thanks, Stephen. I am a bit wiser now that I see the graphic.
“Do your work, then step back. The only path to serenity.” - Lao Tzu
The toe data had a transcription error: I corrected the numbers to agree with what I used for graphing and replaced the original file on web server.
9/9/2011 TMY-2, 6 min, B+F 0.03 *Corrected toe*
I have not verified the calibrated step tablet. Can't remove it from glass to clean without possibly destroying it. (First on list of things to replace). It is a 21 step tablet, and I extend its range to 25 steps by using a gelatin 0.6 ND filter. (Steps 22-25 are not really calibrated). I put two or three step wedge exposures on a sheet of film, and average the readings.
My ASA triangle typically is 13 minutes. (Jerevan, it's just a nickname I gave the ASA/ISO standard that defines two sides of a right triangle). I use D-76 1:1 and process in trays with 6 sheets at a time, tightly stacked.
I used to "gently" and "loosely" stack the film which gives these curves upswept high values. That is not a film characteristic, it is a common development anomaly in all my curves. I ignore it.
I assume upswept curves happen because edges with more access to fresh developer - develop more - and that is where the high steps are positioned, near the film sheet edges. Lately I have been "jogging" the sheets. This reduces the upswept curves... At the same time restricts the entire sheet's access to fresh chemistry.
Last edited by Bill Burk; 07-22-2012 at 04:40 PM. Click to view previous post history.
Reason: Data error again
The high end of the step wedge is where it is important to know the correct values. That is where the speed point will fall. My densitometers calibrates to a standard around 2.0, so when I measure my step wedge in the 2.0 range I know pretty well that the densities I'm using are correct.
So, with unknown high densities on wedge and the addition of extra density, you could easily be 1/3 stop off in the calculations of the light reaching the film at the 0.1 point, thus giving you the wrong speed.
That is interesting that the step wedge is between glass. On my EG&G, and the description in the manual, the plastic chamber ("gray scale box" per the manual) has a clear glass on top.
On my Wejex, the step wedge is also between glass, but the top of the platform, on which the glass resides, actually fits under the arm of my Tobias densitometer. Maybe they planned it that way because both devices are made by the same company.
Extra agitation or development should have minimum effect on the speed point, so I am suspecting errors in measurement of the step wedge density. But just speculation at this point.
With the revised numbers it still looks pretty odd at the toe. But this is not the dataset you fit to the ASA triangle, I suspect you used on of the others (which one?).