The ISO ratings of films are highly misleading. The light sensitivity of a film is determined during manufacture by the length of a curing time. In other words (and stated in a way having nothing to do with "science"), the same emulsion formula can be used to make films of different sensitivity by changing how long the emulsion chemicals interact before they dry. But those changes are fairly subtle.
|Kodak bwXX has an ostensible ISO of 200.|
What all this means for classic films, though, is that a given film emulsion has a true base sensitivity. Increasing the cure time (i.e., increasing the ISO) results in a less contrasty negative. Decreasing the cure time (i.e. decreasing the ISO) results in a more contrasty negative. To make up for the change in contrast, the film is developed for different lengths of time in particular, recommended developers (in effect, pushing or pulling it from the base sensitivity). The combination of cure time and development time results in a rated ISO film speed which should give you a negative with an acceptable contrast range. But this doesn't ever tell you what the film's true light sensitivity is, and thus you never know its honest capabilities.
|The truth teller.|
Unfortuantely, manufacturers don't tell you a film's base sensitivity. The only way to determine that is for someone to shoot a roll of film at different exposures and see which Exposure Index produces the best looking negatives. Each film is differnet, though once you know a particular film's base sensitivity, you're good to go.
I've really been enjoying shooting classic grained films. Tri-X still looks good after all these years, and it looks even better when get to know its true base sensitivity. Developed in Diafine, Tri-X looks great at an Exposure Index of 1250.** In other words, when developed completely Tri-X is actually one and half stops more sensitive to light than Kodak says on the box. The grain is fantastic, and the details in both the highlights and the shadows are great. So much for ISO 400.
|Shooting a range of exposure indexes on a roll of bwXX. 200 has blocked up highlights, while 1600 looks too thin.|
I took a roll of Double X out for a walk at Grey Towers Castle. I shot a series of images with different ISO settings on the Leica M6. All exposures were made by making Leica happy with a steady red dot and none of those arrows. I started at ISO 200 and worked up to ISO 3200. I then developed the film in Diafine, four minutes in Solution A, and three minutes in Solution B. (Temperature doesn't matter with Diafine, but I measured the soup at about 76 degrees farenheit.)
Suprisingly, all of the negatives looked good. At 200, they definitely look overexposed, and at 3200 they're too thin, but it would be possible with a lot of darkroom work to still make a print at either end of the spectrum. (Film has latitude!)
|At 640, bwXX looks very rich and detailed. I think it's a keeper.|
The final verdict won't come in until I get another box of paper from Freestyle. Scanned negatives only tell some of the story. Only a print can reveal a film's capabilites. Meanwhile, though, you can get started maximizing Double X by ignoring what it says on the box and shooting it anywhere between 400 and 800. Just be sure to develop it in Diafine!
* An Exposure Index is any speed rating assigned to a particular film which is different than what it says on the box.
** Your mileage may vary: I came to this Exposure Index based upon how my Leicas meter, and how I agitate the film during development. Others may find that 1600 works better.
*** I originally bought bwXX from CineStill. They've since sold out. However, I did find that the Film Photography Project carries it both pre-loaded and in 100 foot rolls!