Film is a tricky business. Once you put a roll in the camera you've committed to a single ISO. Film is light sensitive, so there is no way to open the back to check to see if you got the shot you expected. You won't know what's there until after the film is developed. More importantly, because you can't check the exposure on the actual film there's no way to know whether you should adjust your development time for that particular roll. Until you repeatedly shoot the same type of film and learn to adjust the development time based on prior outcomes, developing is a matter of faith guided by the manufaturcer's recommendations. The problem is that those recommendations aren't the whole story.
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.
Film manufacturers aren't different from Taco Bell: they want to sell the highest number of products using the least number of the same ingredients. (Pintos and Cheese as a menu item, anyone?!) Playing games with development times gives the manufacturer the ability to market the same film at different ISO ratings. And I'm sure they're well-meaning in trying to meet consumer expectations for contrast and grain. Indeed, the newer T grained films definitely require more precise science stuff than the classic film emulsions, and this article isn't about T grained films. We're sticking with the classics.
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.
Using a compensating developer like Diafine bypasses much of the math and all of the games manufacturers play when creating film because Diafine develops everything that's there. Diafine works all of the emulsion's actual sensitivity, instead of the "sensitivty" created by the manufacturer's recommended development times. Using Diafine, you rate the film at an Exposure Index* which will produce appropriate contrast when the film is fully developed.
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.
But Tri-X isn't the only grain out there, and I'm very fond of the look produced by Kodak's black and white movie film, bwXX (known as "Double X").*** It captures a lot of fine detail, and has very smooth transitions between tones. For shooting in still cameras, the recommended ISO is 200. But what is Double X really capable of? Diafine answers the question.
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!)
The sweet spot is between 400 and 800. At that exposure, the images have nice contrast with plenty of detail. I will likely rate Double X in my Leicas at 640 and call it a day.
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!
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!