today here be a fun question, that the poor folks on photrio really dont care for.
If you have a 400 iso film, and you shoot it using the "reduce by a 1/3 stop" and the meter is set to iso 325.
What would be the ISO of the negatives that came out BEST when "meter said use f/11 at 1/250," but you used 1/125 at f/11 to get the final shot.
ISO is an international standard that involves exposing the film to standardised light sources, processing using specified developer, and measuring a particular density above base+fog.
What you're asking about is EI or exposure index, which is not standardised, is more subjective, and is the reason that films like Kodak Max P3200 are ISO 800 or 1000 but can be developed to an EI of 3200.
Answering your question directly: define BEST negative. Consider also that the lens may not consistently be f11, or may have neutral density due to transmission losses from absorption in the glass, coatings and internal reflections (which is why cine lenses use T-stops which take this into account) and the shutter may not consistently be 1/250.
I’m not sure I’m following, but it seems the example is for an ISO 400 B&W (?) negative film, with the meter set to an exposure index of 320, and a comparison of two negatives, one shot at EI 320 and and one with an extra stop of exposure (or effectively an EI of 160).
Which is the better-exposed negative will depend on a variety of things: subject luminance range, meter type, how the scene was metered, film size/magnification and subjective things.
Incidentally, the ISO speed determination is based on more than a fixed density criterion (Zone System). It also specifies a gradient over an exposure range. The 1993 revision of the standard removed the requirement to use a specified developer.
No intending to be pedantic, but clarifying...Originally Posted by Vaughan
The ISO requirement for specific developers was deleted from the film speed determination standards. ISO-6 (1993), for example, now reads:
The only processing specification required in this
International Standard is that the density difference
between Points m and n shall be 0,80 (see 5.6.3 and
figure 1). No additional processing specifications are
included in recognition of the wide range of chemicals
and equipment used in processing black-and-white
films. ISO Speeds provided by film manufacturers
generally apply to films when they are processed in
accordance with their recommendations to produce
the photographic characteristics specified for the
process.
One more reason to run your own tests -- with your own chemicals and gear -- see "Controls in Black & White Photography" by Richard Henry.
I don’t think that was the point of the book.
of course its black and white film..
box label of iso 400, meter set to 320. doing a comparison between frames shot at 320 and apparently 160, the 160 ones were far superior in terms of detail, shadow and highlight then the 320 were.
If you find that you are consistently able to make better prints/scans doing things that way, then I can’t really argue against it. I’ve always said if one is going to use a different EI than the ISO speed of a film, the sound/sensible EI determination should be based on printing/digitizing experience - ie are you consistently struggling with negatives that are too dense or too thin. Anything else is arbitrary (ISO is not, by the way).
gong through the photrio place, ISO seems to be an imaginary number because youll see lots of people hung for disagreeing that tmax 100 should be shot at anything other then iso 25 or iso 50. and so forth.
Most of that is random nonsense. It’s got nothing to do with emulsion speed. Many people are also coming at EI from a Zone System perspective. A precise Zone System EI test will indicate a speed 2/3 stop lower than the ISO speed. Unfortunately it seems few people know what they are actually testing for when they do the ZS thing, so fairly often you get people saying things like the “real” or “true” speed of say TMX is 50 or whatever, as though they have discovered something hidden about the film, or that it is the speed of TMX with their own processing methods (“personal EI” bla bla bla).
One simple way of looking at it - ISO speeds before 1960 implicitly included a ~1 stop safety factor against accidental under-exposure, which made sense in the first half of the century when meters were more sketchy and film sizes were universally larger (ie image structure was a non-issue). Since modern/contemporary films are finer grained and sharper than they once were, you don’t really sacrifice anything by continuing to rate films at half box/ISO speed, so rating TMX at 50 is a perfectly fine thing to do especially if you are using large format film (although of course you are obviously going to need a slower shutter speed or larger aperture than if you rated TMX at 100).
There’s no correct/incorrect EI for a film, just correct/incorrect assumptions, test interpretations etc.
If you want good information about exposure theory and tone reproduction on Photrio, find Stephen Benskin (I assume he is still a member).
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