calculating an ISO

[BrianShaw]

The biggest problem I see with these discussions is that too often the question is asked in hopes of a quick and easy answer but no real intent on understanding. And, right or wrong, there’s always a bevy of quick and easy answers offered.

[BrianShaw]

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.

Which film are you using?

[jnantz]

box speed / iso / asa don't change but the way you expose it for your own way of doing things may. there are no rules
if you haven't sent your shutters out for a CLA lately you probably have no idea what you exposed your film at

[Doremus Scudder]

of course it's 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.

What you are saying is that the negatives you thought best were ones you gave an extra 1 1/3-stops exposure from ISO speed (1/3-stop from 400 to your E.I. of 320 and then one more stop by overexposing one stop from the meter reading).

Several things come to mind:

First, are those negatives really "better"? Or do they just look better to you. The proof is in making a print. Maybe you could make identical, high-quality prints from both negatives. If so, then you just like the look of overexposed negatives. FWIW, overexposing black-and-white negatives by a stop from "ideal" will usually yield just as good a print as the ideal exposure, with perhaps a bit more grain (which is almost a non-issue with large-format film). So, have you made a print? If so, a "real" darkroom print or did you scan and make a print from a digital file?

Second, if indeed the "160 ones were far superior in terms of detail, shadow and highlight then the 320 were," then perhaps you are underexposing. If so, it is likely the way you are metering that is the variable factor. People underestimate just how much of a role personal metering idiosyncrasies play in determining a personal E.I. So, what meter are you using? How are you finding your base exposure? From metering a shadow value and placing it à la Zone System? Or averaging high and low values (a sure way to underexpose scenes of high SBR)? Or metering a skin tone and then just using the meter (another sure way to underexpose)? Or using an incident reading? Or maybe your meter is reading incorrectly. Have you checked it against a known reliable meter? So many variables here.

And what about other variables like equipment and compensation factors? Have you tested your shutter? Was the exposure longer than one second? Did you use a filter or extend the bellows? Was the light changing? All those things can make a difference.

There could be lots of things causing a stop of underexposure (if that is indeed the case). I'd still put my money on your metering technique.

Best,

Doremus

[Kevin Crisp]

There might be at least a dozen missing variables that would make this an answerable question.

But keep it simple...if doing what you did gives you a negative you like by whatever yardstick works for you, do it again on subsequent negatives.

[xkaes]

I don’t think that was the point of the book.

I think the title says it all -- "Controls in B&W Photography". They are for the individual photographer to use and control -- which means your own tests. His books shows how he did it and advises others to take a similar approach -- one that works for them and their gear. For example, they probably won't have a densitometer. Not only that, he advises everyone to repeat their tests periodically, since "things" can change:



He advises every six to 12 months. I think most of us would consider that a little extreme, but some people get a new computer and/or camera every six to 12 months, so why not?

[Andrew O'Neill]

Try not to get yourself banned here like you did at Photrio...
To answer your question, you're giving a stop more exposure, going from 1/250th to 1/125th.... plus a third stop if you changed from ISO 400 to EI 320. But... if you have settled on EI 320, you are only giving the film a stop more.

[paulbarden]

There might be at least a dozen missing variables that would make this an answerable question.

If you don’t say which film you’re talking about, this is immediately an unanswerable question.

[Tin Can]

I reduce all ISO if film is fresh

by 1/2

as it ages it also changes

some years ago ASA became ISO and some were advertised as

double ISO

test everything

shutters are often way off yet still in spec

I try to shoot all LF at 1/30

using strobes or flash bulbs

[Michael R]

The speeds doubled in 1960 when the extra stop safety factor (a holdover from the 1940s) was removed, which also aligned B&W speeds with colour film speeds. See Nelson (KRL), Safety Factors in Camera Exposures.

Since films are finer grained and sharper now, the benefit of the 1960 revision is marginal. Setting your EI to 1/2 ISO speed is therefore a reasonable practice. However it's good to know if you need the extra stop for a faster shutter speed or whatever, you're not losing anything by rating the film at ISO speed.

I reduce all ISO if film is fresh

by 1/2

as it ages it also changes

some years ago ASA became ISO and some were advertised as

double ISO

test everything

shutters are often way off yet still in spec

I try to shoot all LF at 1/30

using strobes or flash bulbs