Ho hum... Just more rote stereotypes about films which in fact differ with respect to how deeply down you can dig into the shadows. It can be more than -3.3, and it can certainly be less. And all of this is relative to development protocol. Stronger development (higher overall gamma) typically raises and steepens the toe.
(This is for BW negative film. Slides have a remarkably different norm)Originally Posted by Drew Wiley
Yes Drew, it can be more or less than 3.3, but if it isn't 3.3 then you are not metering with the true speed of the film/processing.
The calculation of the true speed always uses 10x the exposure in the "m" (B+F+0.1D) speed point to set the meter point. So it's 3.3 or your speed is not the true one.
Another thing is the pool of inaccuracies we have in the metering/processing...
There is sensitometry and there are recipes, both work.
In other words, if you develop exactly normal and meter with true speed then areas at -3.3 will have 0.1D over fog+base. Why? Because ISO norm says it. And a ISO norm is an ISO norm, not tales.
Beware, the BOX speed may be rounded to a standard speed, normative allows it, but then the BOX speed has a little discrepance to the true speed we are speaking, this has also to be said.
I don't give a hoot about "true film speed", as if there is only one valid way to calculate that to begin with. I do care about specific shadow distribution on the toe of the film, which is affected by several variables. And in that respect, I see no substitute to assessing one's own "personal film speed" relative to specific film, developer, and Zone placement (if Zone theory is even used). I've retained four Pentax digital Spotmeters. They all precisely match over their entire range; and if any one of them deviates, I have it serviced. My oldest one is now too beat up to bother keeping alive on life support. But one is kept virtually brand new as a reference. I'm quite confident my meter readings are accurate. My development too. For nitpicky purposes, I'm able to keep dev temp within 1/10th deg F is necessary, which is complete overkill for ordinary work, for which I have a temp compensating timer anyway. Film designers can worry about the rest. And if a batch is off, when I happen to need complete predictability, I'll spot it. But for that kind of usage, best to stick with the majors in terms of quality control - Kodak, Ilford, and Fuji. Then I have lots of densitometer plots confirming the consistency of my own protocols.
It depends... if you develop like kodak TMX dataheet says you nail the H-D curve they show with real units. I checked it and I found it exact. But if you make a creative development then your true speed can be any.
Also you may want to calibrate what true speed you have with your pocessing... in that case you'll be calculating the true speed to place the "m" at -3.3 again.
If you have an inaccuracy of a full 1 deg F you'll change the contrast, but not much the true speed.
From the analytical standpoint what makes me suspicious of any alleged industry-wide standard is just how different certain companies tend to interpret their "box speed". As much as I like Ilford products, I've found every single one of their films to be rather over-optimistic in terms of rated film speed. They seem to interpolate it too far down onto the toe, perhaps for wish-it-were-so marketing reasons? I dunno. And this seems to be the case in most developers. Then there are those cheaper EU films that are unquestionably marketed using too-good-to-be-true speed ratings. But the kind of technical lab applications I have in mind, Pere, have to be a lot more accurate than anything you're describing. General photography, no.
Yes of course... Kodak TMax P3200 is 800 true speed, and Ilford Delta 3200 is 1000... to say remarkable examples.
Foma 400 is another example of fake 400 IIRC, and CMS 20 with Adotech II... better at ISO 6.
For this reason it makes sense knowing very clear what's a true speed, because if using it you know where is the "m", and how far an spot in the scene is from m.
Many other films are well rated by the box lettering. We all know what films are...
I call it the BS coefficient. The faster that can be quantified, the better off we all are.
Yes...
I could see being suspicious of the speeds in the day of H&D but with ISO speeds I am confident that we can check them.
That's what I was told: Exposure changes the shadows and development changes the highlights.
Fomapan 400 was not giving the stated speed on the box, because shadow detail was lacking.
That is a fact that Foma acknowledge in their technical data.
The same could be said for Shanghai. Maybe 50ASA, but not sure if the 4x5 is cut from the same master rolls as the 120 version.
You have to test a film/developer combination before doing any work of importance. So you can be reasonably sure that it will meet your requirements.
Posting Permissions
Reply With Quote
Bookmarks