On Sat, Oct 16, 2010 at 6:07 PM, Nick Wright <[email protected]> wrote: > There is really only one thing I truly miss about digital ... the > ability to change ISO on the fly. > > Most of the time I prefer to shoot in lower light and I like my iso400 > film, but sometimes I find myself out shooting in bright daylight. > > Not a terrible problem except I'd rather not always be shooting at f/16. > > Then tonight something pinged in my memory. I seemed to recall having > heard you could shoot Ilford's XP2 at different ISOs on the same roll > and get it developed normally. > > So I hopped over to Ilford's Web site and sure enough, according to > them you can shoot XP2 at any ISO from 50 to 3200, even varying from > frame to frame, have it developed normally and get good results. > > Now everything I know about film developing etc is screaming at me > that this can not be true. > > I found very little talk on the net-at-large about it, so I turn to > the all-wise PDML for help. > > Has anyone ever done this? Does it actually work?
I have used XP1, XP2, and XP2 Super (as well as the similar Kodak C41 process B&W films) for *many* years, since they were first released in the 1980s. Some of my very best photographs made on film were made with these emulsions. Matter of fact, you might recall that I sold off all my 35mm film stock recently. Not quite all: I kept about a dozen rolls of XP2 Super. It's the only 35mm film I really like to work with anymore. ;-) The difference between these films and traditional process B&W film is that in these the silver is leached out of the emulsion during processing and replaced by dyes, like color negative films. In fact, these films are really single-layer color negative films, which is why they are capable of significantly better resolution than color negatives of similar sensitivity as a general rule. The notion of being able to expose them at a wide range of ISOs is true but you have to understand the chemistry a little in order to understand what to expect. The final negative image isn't silver gains but dye blobs clumping together in the emulsion. Silver grains are opaque where dye blobs allow some light to be transmitted through them. What happens when you overexpose traditional process B&W film is that the silver grains clump together into an opaque mass. No light can get through it, so the negative is blocked up solid ... the print is white in these areas. You either have to bleach the negative to reduce the blockage and let some light through or you're sunk. What happens when you underexpose traditional process film is that the silver grain clumps are too thin and the negative image lacks detail, creating a grainy look with little shadow detail in the print. What happens when you overexpose C41 process films is that the dye blobs crowd closer and closer together and create tighter edge definition, and shadow areas are filled in with more density. This creates a smoother, higher resolution, lower contrast negative image on a denser negative ... expose longer under the enlarger and you get a beautiful print. What happens when you underexpose is that the dye clumps are spread out with results similar to the traditional process image, but smoother and with good highlight values in hot spots, little blockage ... moving to a harder grade of paper and a higher contrast developer in printing can create a beautiful, high contrast image with clean definition in the highlights and deep blacks in the shadows. So what you really have is a film with which you can control the gamma curve by adjusting ISO, compensating for density range in printing with exposure, developer and paper grade. Of course, there are limits to the useful range of ISO/gamma adjustment ... ISO 50 to 3200 is a bit of an overstatement. What I've found is that, given a nominal ISO 400 film like XP2 Super, I can expose as low as ISO 100 for a flatter negative in bright contrasty light and get very nice, printable results or go as high as ISO 800 in dim circumstances. Nothing near the range of what I can get with a digital capture and raw files, but very good work. I always found that ISO 320 produced the best "normal" results. The right solution for when bright, normal contrast circumstance make it difficult to have aperture control with these films is neutral density filters, not ISO adjustment. A 3-stop ND filter brings ISO 400 down to ISO 50, 6-stop ND brings it down to ISO 6, allowing broad aperture control even in the brightest conditions. I use them still even with digital capture as ISO 100 is sometimes a bit limiting too. -- Godfrey godfreydigiorgi.posterous.com -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
