I always find it interesting, and it seems to happen every time this topic comes up, that someone will bring up a change in the standard from over 60 years ago. While the change is a true fact, does anyone really think that an international group of industry experts made a mistake way back then and failed to correct it in the subsequent revisions? There are much better explanations for choosing a "personal EI" than that!
Whether you were buying Tri-X the day before it was ISO 400, or the day after it was ISO 200, some simple tests should be done -- even though 99.9% of the users never did.
Test for what, though? And what test?Originally Posted by xkaes
You test for the ISO and C.I. for any film, with the chemicals and method and gear that you have. The chemicals and method and gear that the film manufacturer uses is completely different from the chemicals and methods and gear that you have, so you will of course get a different ISO and C.I. than what the manufacturer got -- and suggests.
Exposure (ISO) controls density, while development controls contrast, and the two interact. The biggest part of the problem is many photographers don't know what a good negative looks like.
"I love my Verito lens, but I always have to sharpen everything in Photoshop..."
The ISO rating for a film is set by... ISO, the French standards organization. It's a property of the film. What you do with the film, how you expose it, develop it, etc. does not and can not change the ISO rating.
While you can not change "box speed", you can change how box speed relates to you and your uses of the film in the box. Most LF photographers tend to use Personal Exposure Index (PEI, or often just EI) for this duty. Most of the Zone System books (and similar film/exposure/development control systems) cover how to do testing for PEI. But basically you use your own camera, your own lenses and shutters, your own darkroom techniques (including developer and developer dilutions) to establish an exposure index that works for you, personally. Your PEI will probably not be accurate for me, even with the same film, same developer, same processing, etc. simply because my stuff is different from your stuff, and the way I do something is probably different from the way you do it.
I've done PEI testing for a few different films and developer combinations and it's interesting how different the numbers can be. For example, I found that I had a PEI of 250 for my use of 5x4 Tri-X developed in HC110-H in a Jobo 3010 tank, but a PEI of 500 for 5x4 TMY developed with XTOL 1:3 in the same Jobo 3010 tank. The difference in emulsions is part of it, the difference in developers is part of it, but the Jobo system use was as exactly the same as I could make it.
All of this can be reduced in the old saying: "Expose for the shadows, develop for the highlights." Because that's what the Zone System and almost all the other systems out there are telling you in the end. These systems' strength is that they provide a methodology to help you accomplish that goal, repeatably.
The other thing I can tell you about modern B&W films (cubic-grained or tabular-grained) is that film density (aka highlights) are created by converting exposed silver halides to metallic silver during the development process (yes, I know you already know this, but stay with me here...). The metallic silver is opaque to light giving rise to the Callier Effect (light scatter) when you project light through the film (enlargers, scanners, are all effected by this). The general result is that the more density your film uses to capture your highlights, the more compressed the highlight tones become in your final print. And all this brings me to this concept: I typically want my B&W negatives to use the least amount of highlight density required to print with minimal trouble on a #2 darkroom paper. Because I think it gives me better looking highlights.
I hope at least some of this helps.
Bruce Watson
"ISO, the French standards organization"
Actually, ISO is a nongovernmental international standards organization with headquarters in Geneva, Switzerland. It is a network of national standards bodies. The French national standardization body is Association française de normalisation (AFNOR). French, however, is one of the three official languages used by ISO.
“Personal EI” based on anything other than experience is really little more than gobbledygook.
True -- I should not have gone from my (clearly faulty) memory without looking that up first. My bad.
Bruce Watson
Once you are out in the field so much of this becomes an issue of experience and judgement rather than science.
I usually rate films at box speed or just under and meter for where I want shadow detail and place that on zone III, then I check the highlights aren't too bright and make my exposure. Another photographer might reach the same settings as me by rating the film at 1 stop over and using an ambient meter. So I would say I rate at box speed and another photographer might say he halves box speed, but out in the field we are using the same settings.
That is why it is important to test and reach your own conclusions.
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